If successful, open() returns a non-negative
integer, termed a file descriptor. It returns -1 on failure,
and sets errno to indicate the error.
The assembly language programmer new to Unix and FreeBSD will
immediately ask the puzzling question: Where is
errno and how do I get to it?
The information presented in the man pages applies
to C programs. The assembly language programmer needs additional
information.
Where Are the Return Values?
Unfortunately, it depends... For most system calls it is
in EAX, but not for all.
A good rule of thumb,
when working with a system call for
the first time, is to look for
the return value in EAX.
If it is not there, you
need further research.
I am aware of one system call that returns the value in
EDX: SYS_fork. All others
I have worked with use EAX.
But I have not worked with them all yet.
If you cannot find the answer here or anywhere else,
study libc source code and see how it
interfaces with the kernel.
Where Is <varname>errno</varname>?
Actually, nowhere...
errno is part of the C language, not the
Unix kernel. When accessing kernel services directly, the
error code is returned in EAX,
the same register the proper
return value generally ends up in.
This makes perfect sense. If there is no error, there is
no error code. If there is an error, there is no return
value. One register can contain either.
Determining an Error Occurred
When using the standard FreeBSD calling convention,
the carry flag is cleared upon success,
set upon failure.
When using the Linux emulation mode, the signed
value in EAX is non-negative upon success,
and contains the return value. In case of an error, the value
is negative, i.e., -errno.
Creating Portable Code
Portability is generally not one of the strengths of assembly language.
Yet, writing assembly language programs for different platforms is
possible, especially with nasm. I have written
assembly language libraries that can be assembled for such different
operating systems as Windows and FreeBSD.
It is all the more possible when you want your code to run
on two platforms which, while different, are based on
similar architectures.
For example, FreeBSD is Unix, Linux is Unix-like. I only
mentioned three differences between them (from an assembly language
programmer's perspective): The calling convention, the
function numbers, and the way of returning values.
Dealing with Function Numbers
In many cases the function numbers are the same. However,
even when they are not, the problem is easy to deal with:
Instead of using numbers in your code, use constants which
you have declared differently depending on the target
architecture:
%ifdef LINUX
%define SYS_execve 11
%else
%define SYS_execve 59
%endif
Dealing with Conventions
Both, the calling convention, and the return value (the
errno problem) can be resolved with macros:
%ifdef LINUX
%macro system 0
call kernel
%endmacro
align 4
kernel:
push ebx
push ecx
push edx
push esi
push edi
push ebp
mov ebx, [esp+32]
mov ecx, [esp+36]
mov edx, [esp+40]
mov esi, [esp+44]
mov ebp, [esp+48]
int 80h
pop ebp
pop edi
pop esi
pop edx
pop ecx
pop ebx
or eax, eax
js .errno
clc
ret
.errno:
neg eax
stc
ret
%else
%macro system 0
int 80h
%endmacro
%endif
Dealing with Other Portability Issues
The above solutions can handle most cases of writing code
portable between FreeBSD and Linux. Nevertheless, with some
kernel services the differences are deeper.
In that case, you need to write two different handlers
for those particular system calls, and use conditional
assembly. Luckily, most of your code does something other
than calling the kernel, so usually you will only need
a few such conditional sections in your code.
Using a Library
You can avoid portability issues in your main code altogether
by writing a library of system calls. Create a separate library
for FreeBSD, a different one for Linux, and yet other libraries
for more operating systems.
In your library, write a separate function (or procedure, if
you prefer the traditional assembly language terminology) for each system
call. Use the C calling convention of passing parameters.
But still use EAX to pass the call number in.
In that case, your FreeBSD library can be very simple, as
many seemingly different functions can be just labels to
the same code:
sys.open:
sys.close:
[etc...]
int 80h
ret
Your Linux library will require more different functions.
But even here you can group system calls using the same
number of parameters:
sys.exit:
sys.close:
[etc... one-parameter functions]
push ebx
mov ebx, [esp+12]
int 80h
pop ebx
jmp sys.return
...
sys.return:
or eax, eax
js sys.err
clc
ret
sys.err:
neg eax
stc
ret
The library approach may seem inconvenient at first because
it requires you to produce a separate file your code depends
on. But it has many advantages: For one, you only need to
write it once and can use it for all your programs. You can
even let other assembly language programmers use it, or perhaps use
one written by someone else. But perhaps the greatest
advantage of the library is that your code can be ported
to other systems, even by other programmers, by simply
writing a new library without any changes to your code.
If you do not like the idea of having a library, you can
at least place all your system calls in a separate assembly language file
and link it with your main program. Here, again, all porters
have to do is create a new object file to link with your
main program.
Using an Include File
If you are releasing your software as (or with)
source code, you can use macros and place them
in a separate file, which you include in your
code.
Porters of your software will simply write a new
include file. No library or external object file
is necessary, yet your code is portable without any
need to edit the code.
This is the approach we will use throughout this chapter.
We will name our include file system.inc, and
add to it whenever we deal with a new system call.
We can start our system.inc by declaring the
standard file descriptors:
%define stdin 0
%define stdout 1
%define stderr 2
Next, we create a symbolic name for each system call:
%define SYS_nosys 0
%define SYS_exit 1
%define SYS_fork 2
%define SYS_read 3
%define SYS_write 4
; [etc...]
We add a short, non-global procedure with a long name,
so we do not accidentally reuse the name in our code:
section .text
align 4
access.the.bsd.kernel:
int 80h
ret
We create a macro which takes one argument, the syscall number:
%macro system 1
mov eax, %1
call access.the.bsd.kernel
%endmacro
Finally, we create macros for each syscall. These macros take
no arguments.
%macro sys.exit 0
system SYS_exit
%endmacro
%macro sys.fork 0
system SYS_fork
%endmacro
%macro sys.read 0
system SYS_read
%endmacro
%macro sys.write 0
system SYS_write
%endmacro
; [etc...]
Go ahead, enter it into your editor and save it as
system.inc. We will add more to it as we
discuss more syscalls.
Our First Program
We are now ready for our first program, the mandatory
Hello, World!
1: %include 'system.inc'
2:
3: section .data
4: hello db 'Hello, World!', 0Ah
5: hbytes equ $-hello
6:
7: section .text
8: global _start
9: _start:
10: push dword hbytes
11: push dword hello
12: push dword stdout
13: sys.write
14:
15: push dword 0
16: sys.exit
Here is what it does: Line 1 includes the defines, the macros,
and the code from system.inc.
Lines 3-5 are the data: Line 3 starts the data section/segment.
Line 4 contains the string "Hello, World!" followed by a new
line (0Ah). Line 5 creates a constant that contains
the length of the string from line 4 in bytes.
Lines 7-16 contain the code. Note that FreeBSD uses the elf
file format for its executables, which requires every
program to start at the point labeled _start (or, more
precisely, the linker expects that). This label has to be
global.
Lines 10-13 ask the system to write hbytes bytes
of the hello string to stdout.
Lines 15-16 ask the system to end the program with the return
value of 0. The SYS_exit syscall never
returns, so the code ends there.
If you have come to Unix from MS DOS
assembly language background, you may be used to writing directly
to the video hardware. You will never have to worry about
this in FreeBSD, or any other flavor of Unix. As far as
you are concerned, you are writing to a file known as
stdout. This can be the video screen, or
a telnet terminal, or an actual file,
or even the input of another program. Which one it is,
is for the system to figure out.
Assembling the Code
Type the code (except the line numbers) in an editor, and save
it in a file named hello.asm. You need
nasm to assemble it.
Installing <application>nasm</application>
If you do not have nasm, type:
&prompt.user; su
Password:your root password
&prompt.root; cd /usr/ports/devel/nasm
&prompt.root; make install
&prompt.root; exit
&prompt.user;
You may type make install clean instead of just
make install if you do not want to keep
nasm source code.
Either way, FreeBSD will automatically download
nasm from the Internet,
compile it, and install it on your system.
If your system is not FreeBSD, you need to get
nasm from its
home
page. You can still use it to assemble FreeBSD code.
Now you can assemble, link, and run the code:
&prompt.user; nasm -f elf hello.asm
&prompt.user; ld -s -o hello hello.o
&prompt.user; ./hello
Hello, World!
&prompt.user;Writing Unix Filters
A common type of Unix application is a filter—a program
that reads data from the stdin, processes it
somehow, then writes the result to stdout.
In this chapter, we shall develop a simple filter, and
learn how to read from stdin and write to
stdout. This filter will convert each byte
of its input into a hexadecimal number followed by a
blank space.
%include 'system.inc'
section .data
hex db '0123456789ABCDEF'
buffer db 0, 0, ' '
section .text
global _start
_start:
; read a byte from stdin
push dword 1
push dword buffer
push dword stdin
sys.read
add esp, byte 12
or eax, eax
je .done
; convert it to hex
movzx eax, byte [buffer]
mov edx, eax
shr dl, 4
mov dl, [hex+edx]
mov [buffer], dl
and al, 0Fh
mov al, [hex+eax]
mov [buffer+1], al
; print it
push dword 3
push dword buffer
push dword stdout
sys.write
add esp, byte 12
jmp short _start
.done:
push dword 0
sys.exit
In the data section we create an array called hex.
It contains the 16 hexadecimal digits in ascending order.
The array is followed by a buffer which we will use for
both input and output. The first two bytes of the buffer
are initially set to 0. This is where we will write
the two hexadecimal digits (the first byte also is
where we will read the input). The third byte is a
space.
The code section consists of four parts: Reading the byte,
converting it to a hexadecimal number, writing the result,
and eventually exiting the program.
To read the byte, we ask the system to read one byte
from stdin, and store it in the first byte
of the buffer. The system returns the number
of bytes read in EAX. This will be 1
while data is coming, or 0, when no more input
data is available. Therefore, we check the value of
EAX. If it is 0,
we jump to .done, otherwise we continue.
For simplicity sake, we are ignoring the possibility
of an error condition at this time.
The hexadecimal conversion reads the byte from the
buffer into EAX, or actually just
AL, while clearing the remaining bits of
EAX to zeros. We also copy the byte to
EDX because we need to convert the upper
four bits (nibble) separately from the lower
four bits. We store the result in the first two
bytes of the buffer.
Next, we ask the system to write the three bytes
of the buffer, i.e., the two hexadecimal digits and
the blank space, to stdout. We then
jump back to the beginning of the program and
process the next byte.
Once there is no more input left, we ask the system
to exit our program, returning a zero, which is
the traditional value meaning the program was
successful.
Go ahead, and save the code in a file named hex.asm,
then type the following (the ^D means press the
control key and type D while holding the
control key down):
&prompt.user; nasm -f elf hex.asm
&prompt.user; ld -s -o hex hex.o
&prompt.user; ./hexHello, World!
48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A Here I come!
48 65 72 65 20 49 20 63 6F 6D 65 21 0A ^D &prompt.user;
If you are migrating to Unix from MS DOS,
you may be wondering why each line ends with 0A
instead of 0D 0A.
This is because Unix does not use the cr/lf convention, but
a "new line" convention, which is 0A in hexadecimal.
Can we improve this? Well, for one, it is a bit confusing because
once we have converted a line of text, our input no longer
starts at the beginning of the line. We can modify it to print
a new line instead of a space after each 0A:
%include 'system.inc'
section .data
hex db '0123456789ABCDEF'
buffer db 0, 0, ' '
section .text
global _start
_start:
mov cl, ' '
.loop:
; read a byte from stdin
push dword 1
push dword buffer
push dword stdin
sys.read
add esp, byte 12
or eax, eax
je .done
; convert it to hex
movzx eax, byte [buffer]
mov [buffer+2], cl
cmp al, 0Ah
jne .hex
mov [buffer+2], al
.hex:
mov edx, eax
shr dl, 4
mov dl, [hex+edx]
mov [buffer], dl
and al, 0Fh
mov al, [hex+eax]
mov [buffer+1], al
; print it
push dword 3
push dword buffer
push dword stdout
sys.write
add esp, byte 12
jmp short .loop
.done:
push dword 0
sys.exit
We have stored the space in the CL register. We can
do this safely because, unlike Microsoft Windows, Unix system
calls do not modify the value of any register they do not use
to return a value in.
That means we only need to set CL once. We have, therefore,
added a new label .loop and jump to it for the next byte
instead of jumping at _start. We have also added the
.hex label so we can either have a blank space or a
new line as the third byte of the buffer.
Once you have changed hex.asm to reflect
these changes, type:
&prompt.user; nasm -f elf hex.asm
&prompt.user; ld -s -o hex hex.o
&prompt.user; ./hexHello, World!
48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A
Here I come!
48 65 72 65 20 49 20 63 6F 6D 65 21 0A
^D &prompt.user;
That looks better. But this code is quite inefficient! We
are making a system call for every single byte twice (once
to read it, another time to write the output).
Buffered Input and Output
We can improve the efficiency of our code by buffering our
input and output. We create an input buffer and read a whole
sequence of bytes at one time. Then we fetch them one by one
from the buffer.
We also create an output buffer. We store our output in it until
it is full. At that time we ask the kernel to write the contents
of the buffer to stdout.
The program ends when there is no more input. But we still need
to ask the kernel to write the contents of our output buffer
to stdout one last time, otherwise some of our output
would make it to the output buffer, but never be sent out.
Do not forget that, or you will be wondering why some of your
output is missing.
%include 'system.inc'
%define BUFSIZE 2048
section .data
hex db '0123456789ABCDEF'
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
global _start
_start:
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
.loop:
; read a byte from stdin
call getchar
; convert it to hex
mov dl, al
shr al, 4
mov al, [hex+eax]
call putchar
mov al, dl
and al, 0Fh
mov al, [hex+eax]
call putchar
mov al, ' '
cmp dl, 0Ah
jne .put
mov al, dl
.put:
call putchar
jmp short .loop
align 4
getchar:
or ebx, ebx
jne .fetch
call read
.fetch:
lodsb
dec ebx
ret
read:
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword stdin
sys.read
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
ret
align 4
.done:
call write ; flush output buffer
push dword 0
sys.exit
align 4
putchar:
stosb
inc ecx
cmp ecx, BUFSIZE
je write
ret
align 4
write:
sub edi, ecx ; start of buffer
push ecx
push edi
push dword stdout
sys.write
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
ret
We now have a third section in the source code, named
.bss. This section is not included in our
executable file, and, therefore, cannot be initialized. We use
resb instead of db.
It simply reserves the requested size of uninitialized memory
for our use.
We take advantage of the fact that the system does not modify the
registers: We use registers for what, otherwise, would have to be
global variables stored in the .data section. This is
also why the Unix convention of passing parameters to system calls
on the stack is superior to the Microsoft convention of passing
them in the registers: We can keep the registers for our own use.
We use EDI and ESI as pointers to the next byte
to be read from or written to. We use EBX and
ECX to keep count of the number of bytes in the
two buffers, so we know when to dump the output to, or read more
input from, the system.
Let us see how it works now:
&prompt.user; nasm -f elf hex.asm
&prompt.user; ld -s -o hex hex.o
&prompt.user; ./hexHello, World!Here I come!
48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A
48 65 72 65 20 49 20 63 6F 6D 65 21 0A
^D &prompt.user;
Not what you expected? The program did not print the output
until we pressed ^D. That is easy to fix by
inserting three lines of code to write the output every time
we have converted a new line to 0A. I have marked
the three lines with > (do not copy the > in your
hex.asm).
%include 'system.inc'
%define BUFSIZE 2048
section .data
hex db '0123456789ABCDEF'
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
global _start
_start:
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
.loop:
; read a byte from stdin
call getchar
; convert it to hex
mov dl, al
shr al, 4
mov al, [hex+eax]
call putchar
mov al, dl
and al, 0Fh
mov al, [hex+eax]
call putchar
mov al, ' '
cmp dl, 0Ah
jne .put
mov al, dl
.put:
call putchar
> cmp al, 0Ah
> jne .loop
> call write
jmp short .loop
align 4
getchar:
or ebx, ebx
jne .fetch
call read
.fetch:
lodsb
dec ebx
ret
read:
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword stdin
sys.read
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
ret
align 4
.done:
call write ; flush output buffer
push dword 0
sys.exit
align 4
putchar:
stosb
inc ecx
cmp ecx, BUFSIZE
je write
ret
align 4
write:
sub edi, ecx ; start of buffer
push ecx
push edi
push dword stdout
sys.write
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
ret
Now, let us see how it works:
&prompt.user; nasm -f elf hex.asm
&prompt.user; ld -s -o hex hex.o
&prompt.user; ./hexHello, World!
48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A
Here I come!
48 65 72 65 20 49 20 63 6F 6D 65 21 0A
^D &prompt.user;
Not bad for a 644-byte executable, is it!
This approach to buffered input/output still
contains a hidden danger. I will discuss—and
fix—it later, when I talk about the
dark
side of buffering.How to Unread a Character
This may be a somewhat advanced topic, mostly of interest to
programmers familiar with the theory of compilers. If you wish,
you may skip to the next
section, and perhaps read this later.
While our sample program does not require it, more sophisticated
filters often need to look ahead. In other words, they may need
to see what the next character is (or even several characters).
If the next character is of a certain value, it is part of the
token currently being processed. Otherwise, it is not.
For example, you may be parsing the input stream for a textual
string (e.g., when implementing a language compiler): If a
character is followed by another character, or perhaps a digit,
it is part of the token you are processing. If it is followed by
white space, or some other value, then it is not part of the
current token.
This presents an interesting problem: How to return the next
character back to the input stream, so it can be read again
later?
One possible solution is to store it in a character variable,
then set a flag. We can modify getchar to check the flag,
and if it is set, fetch the byte from that variable instead of the
input buffer, and reset the flag. But, of course, that slows us
down.
The C language has an ungetc() function, just for that
purpose. Is there a quick way to implement it in our code?
I would like you to scroll back up and take a look at the
getchar procedure and see if you can find a nice and
fast solution before reading the next paragraph. Then come back
here and see my own solution.
The key to returning a character back to the stream is in how
we are getting the characters to start with:
First we check if the buffer is empty by testing the value
of EBX. If it is zero, we call the
read procedure.
If we do have a character available, we use lodsb, then
decrease the value of EBX. The lodsb
instruction is effectively identical to:
mov al, [esi]
inc esi
The byte we have fetched remains in the buffer until the next
time read is called. We do not know when that happens,
but we do know it will not happen until the next call to
getchar. Hence, to "return" the last-read byte back
to the stream, all we have to do is decrease the value of
ESI and increase the value of EBX:
ungetc:
dec esi
inc ebx
ret
But, be careful! We are perfectly safe doing this if our look-ahead
is at most one character at a time. If we are examining more than
one upcoming character and call ungetc several times
in a row, it will work most of the time, but not all the time
(and will be tough to debug). Why?
Because as long as getchar does not have to call
read, all of the pre-read bytes are still in the buffer,
and our ungetc works without a glitch. But the moment
getchar calls read,
the contents of the buffer change.
We can always rely on ungetc working properly on the last
character we have read with getchar, but not on anything
we have read before that.
If your program reads more than one byte ahead, you have at least
two choices:
If possible, modify the program so it only reads one byte ahead.
This is the simplest solution.
If that option is not available, first of all determine the maximum
number of characters your program needs to return to the input
stream at one time. Increase that number slightly, just to be
sure, preferably to a multiple of 16—so it aligns nicely.
Then modify the .bss section of your code, and create
a small "spare" buffer right before your input buffer,
something like this:
section .bss
resb 16 ; or whatever the value you came up with
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
You also need to modify your ungetc to pass the value
of the byte to unget in AL:
ungetc:
dec esi
inc ebx
mov [esi], al
ret
With this modification, you can call ungetc
up to 17 times in a row safely (the first call will still
be within the buffer, the remaining 16 may be either within
the buffer or within the "spare").
Command Line Arguments
Our hex program will be more useful if it can
read the names of an input and output file from its command
line, i.e., if it can process the command line arguments.
But... Where are they?
Before a Unix system starts a program, it pushes some
data on the stack, then jumps at the _start
label of the program. Yes, I said jumps, not calls. That means the
data can be accessed by reading [esp+offset],
or by simply popping it.
The value at the top of the stack contains the number of
command line arguments. It is traditionally called
argc, for "argument count."
Command line arguments follow next, all argc of them.
These are typically referred to as argv, for
"argument value(s)." That is, we get argv[0],
argv[1], ...,
argv[argc-1]. These are not the actual
arguments, but pointers to arguments, i.e., memory addresses of
the actual arguments. The arguments themselves are
NUL-terminated character strings.
The argv list is followed by a NULL pointer,
which is simply a 0. There is more, but this is
enough for our purposes right now.
If you have come from the MS DOS programming
environment, the main difference is that each argument is in
a separate string. The second difference is that there is no
practical limit on how many arguments there can be.
Armed with this knowledge, we are almost ready for the next
version of hex.asm. First, however, we need to
add a few lines to system.inc:
First, we need to add two new entries to our list of system
call numbers:
%define SYS_open 5
%define SYS_close 6
Then we add two new macros at the end of the file:
%macro sys.open 0
system SYS_open
%endmacro
%macro sys.close 0
system SYS_close
%endmacro
Here, then, is our modified source code:
%include 'system.inc'
%define BUFSIZE 2048
section .data
fd.in dd stdin
fd.out dd stdout
hex db '0123456789ABCDEF'
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
align 4
err:
push dword 1 ; return failure
sys.exit
align 4
global _start
_start:
add esp, byte 8 ; discard argc and argv[0]
pop ecx
jecxz .init ; no more arguments
; ECX contains the path to input file
push dword 0 ; O_RDONLY
push ecx
sys.open
jc err ; open failed
add esp, byte 8
mov [fd.in], eax
pop ecx
jecxz .init ; no more arguments
; ECX contains the path to output file
push dword 420 ; file mode (644 octal)
push dword 0200h | 0400h | 01h
; O_CREAT | O_TRUNC | O_WRONLY
push ecx
sys.open
jc err
add esp, byte 12
mov [fd.out], eax
.init:
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
.loop:
; read a byte from input file or stdin
call getchar
; convert it to hex
mov dl, al
shr al, 4
mov al, [hex+eax]
call putchar
mov al, dl
and al, 0Fh
mov al, [hex+eax]
call putchar
mov al, ' '
cmp dl, 0Ah
jne .put
mov al, dl
.put:
call putchar
cmp al, dl
jne .loop
call write
jmp short .loop
align 4
getchar:
or ebx, ebx
jne .fetch
call read
.fetch:
lodsb
dec ebx
ret
read:
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword [fd.in]
sys.read
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
ret
align 4
.done:
call write ; flush output buffer
; close files
push dword [fd.in]
sys.close
push dword [fd.out]
sys.close
; return success
push dword 0
sys.exit
align 4
putchar:
stosb
inc ecx
cmp ecx, BUFSIZE
je write
ret
align 4
write:
sub edi, ecx ; start of buffer
push ecx
push edi
push dword [fd.out]
sys.write
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
ret
In our .data section we now have two new variables,
fd.in and fd.out. We store the input and
output file descriptors here.
In the .text section we have replaced the references
to stdin and stdout with
[fd.in] and [fd.out].
The .text section now starts with a simple error
handler, which does nothing but exit the program with a return
value of 1.
The error handler is before _start so we are
within a short distance from where the errors occur.
Naturally, the program execution still begins at _start.
First, we remove argc and argv[0] from the
stack: They are of no interest to us (in this program, that is).
We pop argv[1] to ECX. This
register is particularly suited for pointers, as we can handle
NULL pointers with jecxz. If argv[1]
is not NULL, we try to open the file named in the first
argument. Otherwise, we continue the program as before: Reading
from stdin, writing to stdout.
If we fail to open the input file (e.g., it does not exist),
we jump to the error handler and quit.
If all went well, we now check for the second argument. If
it is there, we open the output file. Otherwise, we send
the output to stdout. If we fail to open the output
file (e.g., it exists and we do not have the write permission),
we, again, jump to the error handler.
The rest of the code is the same as before, except we close
the input and output files before exiting, and, as mentioned,
we use [fd.in] and [fd.out].
Our executable is now a whopping 768 bytes long.
Can we still improve it? Of course! Every program can be improved.
Here are a few ideas of what we could do:
Have our error handler print a message to
stderr.
Add error handlers to the read
and write functions.
Close stdin when we open an input file,
stdout when we open an output file.
Add command line switches, such as -i
and -o, so we can list the input and
output files in any order, or perhaps read from
stdin and write to a file.
Print a usage message if command line arguments are incorrect.
I shall leave these enhancements as an exercise to the reader:
You already know everything you need to know to implement them.
Unix Environment
An important Unix concept is the environment, which is defined by
environment variables. Some are set by the system, others
by you, yet others by the shell, or any program
that loads another program.
How to Find Environment Variables
I said earlier that when a program starts executing, the stack
contains argc followed by the NULL-terminated
argv array, followed by something else. The
"something else" is the environment, or,
to be more precise, a NULL-terminated array of pointers to
environment variables. This is often referred
to as env.
The structure of env is the same as that of
argv, a list of memory addresses followed by a
NULL (0). In this case, there is no
"envc"—we figure out where the array ends
by searching for the final NULL.
The variables usually come in the name=value
format, but sometimes the =value part
may be missing. We need to account for that possibility.
webvars
I could just show you some code that prints the environment
the same way the Unix env command does. But
I thought it would be more interesting to write a simple
assembly language CGI utility.
CGI: A Quick Overview
I have a
detailed
CGI tutorial on my web site,
but here is a very quick overview of CGI:
The web server communicates with the CGI
program by setting environment variables.
The CGI program
sends its output to stdout.
The web server reads it from there.
It must start with an HTTP
header followed by two blank lines.
It then prints the HTML
code, or whatever other type of data it is producing.
While certain environment variables use
standard names, others vary, depending on the web server. That
makes webvars
quite a useful diagnostic tool.
The Code
Our webvars program, then, must send out
the HTTP header followed by some
HTML mark-up. It then must read
the environment variables one by one
and send them out as part of the
HTML page.
The code follows. I placed comments and explanations
right inside the code:
;;;;;;; webvars.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
; Copyright (c) 2000 G. Adam Stanislav
; All rights reserved.
;
; Redistribution and use in source and binary forms, with or without
; modification, are permitted provided that the following conditions
; are met:
; 1. Redistributions of source code must retain the above copyright
; notice, this list of conditions and the following disclaimer.
; 2. Redistributions in binary form must reproduce the above copyright
; notice, this list of conditions and the following disclaimer in the
; documentation and/or other materials provided with the distribution.
;
; THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
; ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
; IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
; ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
; FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
; DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
; OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
; HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
; LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
; OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
; SUCH DAMAGE.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
; Version 1.0
;
; Started: 8-Dec-2000
; Updated: 8-Dec-2000
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
%include 'system.inc'
section .data
http db 'Content-type: text/html', 0Ah, 0Ah
db '<?xml version="1.0" encoding="UTF-8"?>', 0Ah
db '<!DOCTYPE html PUBLIC "-//W3C/DTD XHTML Strict//EN" '
db '"DTD/xhtml1-strict.dtd">', 0Ah
db '<html xmlns="http://www.w3.org/1999/xhtml" '
db 'xml.lang="en" lang="en">', 0Ah
db '<head>', 0Ah
db '<title>Web Environment</title>', 0Ah
db '<meta name="author" content="G. Adam Stanislav" />', 0Ah
db '</head>', 0Ah, 0Ah
db '<body bgcolor="#ffffff" text="#000000" link="#0000ff" '
db 'vlink="#840084" alink="#0000ff">', 0Ah
db '<div class="webvars">', 0Ah
db '<h1>Web Environment</h1>', 0Ah
db '<p>The following <b>environment variables</b> are defined '
db 'on this web server:</p>', 0Ah, 0Ah
db '<table align="center" width="80" border="0" cellpadding="10" '
db 'cellspacing="0" class="webvars">', 0Ah
httplen equ $-http
left db '<tr>', 0Ah
db '<td class="name"><tt>'
leftlen equ $-left
middle db '</tt></td>', 0Ah
db '<td class="value"><tt><b>'
midlen equ $-middle
undef db '<i>(undefined)</i>'
undeflen equ $-undef
right db '</b></tt></td>', 0Ah
db '</tr>', 0Ah
rightlen equ $-right
wrap db '</table>', 0Ah
db '</div>', 0Ah
db '</body>', 0Ah
db '</html>', 0Ah, 0Ah
wraplen equ $-wrap
section .text
global _start
_start:
; First, send out all the http and xhtml stuff that is
; needed before we start showing the environment
push dword httplen
push dword http
push dword stdout
sys.write
; Now find how far on the stack the environment pointers
; are. We have 12 bytes we have pushed before "argc"
mov eax, [esp+12]
; We need to remove the following from the stack:
;
; The 12 bytes we pushed for sys.write
; The 4 bytes of argc
; The EAX*4 bytes of argv
; The 4 bytes of the NULL after argv
;
; Total:
; 20 + eax * 4
;
; Because stack grows down, we need to ADD that many bytes
; to ESP.
lea esp, [esp+20+eax*4]
cld ; This should already be the case, but let's be sure.
; Loop through the environment, printing it out
.loop:
pop edi
or edi, edi ; Done yet?
je near .wrap
; Print the left part of HTML
push dword leftlen
push dword left
push dword stdout
sys.write
; It may be tempting to search for the '=' in the env string next.
; But it is possible there is no '=', so we search for the
; terminating NUL first.
mov esi, edi ; Save start of string
sub ecx, ecx
not ecx ; ECX = FFFFFFFF
sub eax, eax
repne scasb
not ecx ; ECX = string length + 1
mov ebx, ecx ; Save it in EBX
; Now is the time to find '='
mov edi, esi ; Start of string
mov al, '='
repne scasb
not ecx
add ecx, ebx ; Length of name
push ecx
push esi
push dword stdout
sys.write
; Print the middle part of HTML table code
push dword midlen
push dword middle
push dword stdout
sys.write
; Find the length of the value
not ecx
lea ebx, [ebx+ecx-1]
; Print "undefined" if 0
or ebx, ebx
jne .value
mov ebx, undeflen
mov edi, undef
.value:
push ebx
push edi
push dword stdout
sys.write
; Print the right part of the table row
push dword rightlen
push dword right
push dword stdout
sys.write
; Get rid of the 60 bytes we have pushed
add esp, byte 60
; Get the next variable
jmp .loop
.wrap:
; Print the rest of HTML
push dword wraplen
push dword wrap
push dword stdout
sys.write
; Return success
push dword 0
sys.exit
This code produces a 1,396-byte executable. Most of it is data,
i.e., the HTML mark-up we need to send out.
Assemble and link it as usual:
&prompt.user; nasm -f elf webvars.asm
&prompt.user; ld -s -o webvars webvars.o
To use it, you need to upload webvars to your
web server. Depending on how your web server is set up, you
may have to store it in a special cgi-bin directory,
or perhaps rename it with a .cgi extension.
Then you need to use your browser to view its output.
To see its output on my web server, please go to
http://www.int80h.org/webvars/.
If curious about the additional environment variables
present in a password protected web directory, go to
http://www.int80h.org/private/,
using the name asm and password
programmer.
Working with Files
We have already done some basic file work: We know how
to open and close them, how to read and write them using
buffers. But Unix offers much more functionality when it
comes to files. We will examine some of it in this section,
and end up with a nice file conversion utility.
Indeed, let us start at the end, that is, with the file
conversion utility. It always makes programming easier
when we know from the start what the end product is
supposed to do.
One of the first programs I wrote for Unix was
tuc,
a text-to-Unix file converter. It converts a text
file from other operating systems to a Unix text file.
In other words, it changes from different kind of line endings
to the newline convention of Unix. It saves the output
in a different file. Optionally, it converts a Unix text
file to a DOS text file.
I have used tuc extensively, but always
only to convert from some other OS
to Unix, never the other way. I have always wished
it would just overwrite the file instead of me having
to send the output to a different file. Most of the time,
I end up using it like this:
&prompt.user; tuc myfile tempfile
&prompt.user; mv tempfile myfile
It would be nice to have a ftuc,
i.e., fast tuc, and use it like this:
&prompt.user; ftuc myfile
In this chapter, then, we will write
ftuc in assembly language
(the original tuc
is in C), and study various
file-oriented kernel services in the process.
At first sight, such a file conversion is very
simple: All you have to do is strip the carriage
returns, right?
If you answered yes, think again: That approach will
work most of the time (at least with MS
DOS text files), but will fail occasionally.
The problem is that not all non-Unix text files end their
line with the carriage return / line feed sequence. Some
use carriage returns without line feeds. Others combine several
blank lines into a single carriage return followed by several
line feeds. And so on.
A text file converter, then, must be able to handle
any possible line endings:
carriage return / line feed
carriage return
line feed / carriage return
line feed
It should also handle files that use some kind of a
combination of the above (e.g., carriage return followed
by several line feeds).
Finite State Machine
The problem is easily solved by the use of a technique
called finite state machine, originally developed
by the designers of digital electronic circuits. A
finite state machine is a digital circuit
whose output is dependent not only on its input but on
its previous input, i.e., on its state. The microprocessor
is an example of a finite state machine: Our
assembly language code is assembled to machine language in which
some assembly language code produces a single byte
of machine language, while others produce several bytes.
As the microprocessor fetches the bytes from the memory
one by one, some of them simply change its state rather than
produce some output. When all the bytes of the op code are
fetched, the microprocessor produces some output, or changes
the value of a register, etc.
Because of that, all software is essentially a sequence of state
instructions for the microprocessor. Nevertheless, the concept
of finite state machine is useful in software design as well.
Our text file converter can be designed as a finite state machine with three
possible states. We could call them states 0-2,
but it will make our life easier if we give them symbolic names:
ordinary
cr
lf
Our program will start in the ordinary
state. During this state, the program action depends on
its input as follows:
If the input is anything other than a carriage return
or line feed, the input is simply passed on to the output. The
state remains unchanged.
If the input is a carriage return, the state is changed
to cr. The input is then discarded, i.e.,
no output is made.
If the input is a line feed, the state is changed to
lf. The input is then discarded.
Whenever we are in the cr state, it is
because the last input was a carriage return, which was
unprocessed. What our software does in this state again
depends on the current input:
If the input is anything other than a carriage return
or line feed, output a line feed, then output the input, then
change the state to ordinary.
If the input is a carriage return, we have received
two (or more) carriage returns in a row. We discard the
input, we output a line feed, and leave the state unchanged.
If the input is a line feed, we output the line feed
and change the state to ordinary. Note that
this is not the same as the first case above – if we tried
to combine them, we would be outputting two line feeds
instead of one.
Finally, we are in the lf state after
we have received a line feed that was not preceded by a
carriage return. This will happen when our file already is
in Unix format, or whenever several lines in a row are
expressed by a single carriage return followed by several
line feeds, or when line ends with a line feed /
carriage return sequence. Here is how we need to handle
our input in this state:
If the input is anything other than a carriage return or
line feed, we output a line feed, then output the input, then
change the state to ordinary. This is exactly
the same action as in the cr state upon
receiving the same kind of input.
If the input is a carriage return, we discard the input,
we output a line feed, then change the state to ordinary.
If the input is a line feed, we output the line feed,
and leave the state unchanged.
The Final State
The above finite state machine works for the entire file, but leaves
the possibility that the final line end will be ignored. That will
happen whenever the file ends with a single carriage return or
a single line feed. I did not think of it when I wrote
tuc, just to discover that
occasionally it strips the last line ending.
This problem is easily fixed by checking the state after the
entire file was processed. If the state is not
ordinary, we simply
need to output one last line feed.
Now that we have expressed our algorithm as a finite state machine,
we could easily design a dedicated digital electronic
circuit (a "chip") to do the conversion for us. Of course,
doing so would be considerably more expensive than writing
an assembly language program.
The Output Counter
Because our file conversion program may be combining two
characters into one, we need to use an output counter. We
initialize it to 0, and increase it
every time we send a character to the output. At the end of
the program, the counter will tell us what size we need
to set the file to.
Implementing FSM in Software
The hardest part of working with a finite state machine
is analyzing the problem and expressing it as a
finite state machine. That accomplished,
the software almost writes itself.
In a high-level language, such as C, there are several main
approaches. One is to use a switch statement
which chooses what function should be run. For example,
switch (state) {
default:
case REGULAR:
regular(inputchar);
break;
case CR:
cr(inputchar);
break;
case LF:
lf(inputchar);
break;
}
Another approach is by using an array of function pointers,
something like this:
(output[state])(inputchar);
Yet another is to have state be a
function pointer, set to point at the appropriate function:
(*state)(inputchar);
This is the approach we will use in our program because it is very easy to do in assembly language, and very fast, too. We will simply keep the address of the right procedure in EBX, and then just issue:
call ebx
This is possibly faster than hardcoding the address in the code
because the microprocessor does not have to fetch the address from
the memory—it is already stored in one of its registers. I said
possibly because with the caching modern
microprocessors do, either way may be equally fast.
Memory Mapped Files
Because our program works on a single file, we cannot use the
approach that worked for us before, i.e., to read from an input
file and to write to an output file.
Unix allows us to map a file, or a section of a file,
into memory. To do that, we first need to open the file with the
appropriate read/write flags. Then we use the mmap
system call to map it into the memory. One nice thing about
mmap is that it automatically works with
virtual memory: We can map more of the file into the memory than
we have physical memory available, yet still access it through
regular memory op codes, such as mov,
lods, and stos.
Whatever changes we make to the memory image of the file will be
written to the file by the system. We do not even have to keep
the file open: As long as it stays mapped, we can
read from it and write to it.
The 32-bit Intel microprocessors can access up to four
gigabytes of memory – physical or virtual. The FreeBSD system
allows us to use up to a half of it for file mapping.
For simplicity sake, in this tutorial we will only convert files
that can be mapped into the memory in their entirety. There are
probably not too many text files that exceed two gigabytes in size.
If our program encounters one, it will simply display a message
suggesting we use the original
tuc instead.
If you examine your copy of syscalls.master,
you will find two separate syscalls named mmap.
This is because of evolution of Unix: There was the traditional
BSD mmap,
syscall 71. That one was superceded by the POSIX mmap,
syscall 197. The FreeBSD system supports both because
older programs were written by using the original BSD
version. But new software uses the POSIX version,
which is what we will use.
The syscalls.master file lists
the POSIX version like this:
197 STD BSD { caddr_t mmap(caddr_t addr, size_t len, int prot, \
int flags, int fd, long pad, off_t pos); }
This differs slightly from what
mmap2
says. That is because
mmap2
describes the C version.
The difference is in the long pad argument, which is not present in the C version. However, the FreeBSD syscalls add a 32-bit pad after pushing a 64-bit argument. In this case, off_t is a 64-bit value.
When we are finished working with a memory-mapped file,
we unmap it with the munmap syscall:
For an in-depth treatment of mmap, see
W. Richard Stevens'
Unix
Network Programming, Volume 2, Chapter 12.
Determining File Size
Because we need to tell mmap how many bytes
of the file to map into the memory, and because we want to map
the entire file, we need to determine the size of the file.
We can use the fstat syscall to get all
the information about an open file that the system can give us.
That includes the file size.
Again, syscalls.master lists two versions
of fstat, a traditional one
(syscall 62), and a POSIX one
(syscall 189). Naturally, we will use the
POSIX version:
189 STD POSIX { int fstat(int fd, struct stat *sb); }
This is a very straightforward call: We pass to it the address
of a stat structure and the descriptor
of an open file. It will fill out the contents of the
stat structure.
I do, however, have to say that I tried to declare the
stat structure in the
.bss section, and
fstat did not like it: It set the carry
flag indicating an error. After I changed the code to allocate
the structure on the stack, everything was working fine.
Changing the File Size
Because our program may combine carriage return / line feed
sequences into straight line feeds, our output may be smaller
than our input. However, since we are placing our output into
the same file we read the input from, we may have to change the
size of the file.
The ftruncate system call allows us to do
just that. Despite its somewhat misleading name, the
ftruncate system call can be used to both
truncate the file (make it smaller) and to grow it.
And yes, we will find two versions of ftruncate
in syscalls.master, an older one
(130), and a newer one (201). We will use
the newer one:
201 STD BSD { int ftruncate(int fd, int pad, off_t length); }
Please note that this one contains a int pad again.
ftuc
We now know everything we need to write ftuc.
We start by adding some new lines in system.inc.
First, we define some constants and structures, somewhere at
or near the beginning of the file:
;;;;;;; open flags
%define O_RDONLY 0
%define O_WRONLY 1
%define O_RDWR 2
;;;;;;; mmap flags
%define PROT_NONE 0
%define PROT_READ 1
%define PROT_WRITE 2
%define PROT_EXEC 4
;;
%define MAP_SHARED 0001h
%define MAP_PRIVATE 0002h
;;;;;;; stat structure
struc stat
st_dev resd 1 ; = 0
st_ino resd 1 ; = 4
st_mode resw 1 ; = 8, size is 16 bits
st_nlink resw 1 ; = 10, ditto
st_uid resd 1 ; = 12
st_gid resd 1 ; = 16
st_rdev resd 1 ; = 20
st_atime resd 1 ; = 24
st_atimensec resd 1 ; = 28
st_mtime resd 1 ; = 32
st_mtimensec resd 1 ; = 36
st_ctime resd 1 ; = 40
st_ctimensec resd 1 ; = 44
st_size resd 2 ; = 48, size is 64 bits
st_blocks resd 2 ; = 56, ditto
st_blksize resd 1 ; = 64
st_flags resd 1 ; = 68
st_gen resd 1 ; = 72
st_lspare resd 1 ; = 76
st_qspare resd 4 ; = 80
endstruc
We define the new syscalls:
%define SYS_mmap 197
%define SYS_munmap 73
%define SYS_fstat 189
%define SYS_ftruncate 201
We add the macros for their use:
%macro sys.mmap 0
system SYS_mmap
%endmacro
%macro sys.munmap 0
system SYS_munmap
%endmacro
%macro sys.ftruncate 0
system SYS_ftruncate
%endmacro
%macro sys.fstat 0
system SYS_fstat
%endmacro
And here is our code:
;;;;;;; Fast Text-to-Unix Conversion (ftuc.asm) ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;
;; Started: 21-Dec-2000
;; Updated: 22-Dec-2000
;;
;; Copyright 2000 G. Adam Stanislav.
;; All rights reserved.
;;
;;;;;;; v.1 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
%include 'system.inc'
section .data
db 'Copyright 2000 G. Adam Stanislav.', 0Ah
db 'All rights reserved.', 0Ah
usg db 'Usage: ftuc filename', 0Ah
usglen equ $-usg
co db "ftuc: Can't open file.", 0Ah
colen equ $-co
fae db 'ftuc: File access error.', 0Ah
faelen equ $-fae
ftl db 'ftuc: File too long, use regular tuc instead.', 0Ah
ftllen equ $-ftl
mae db 'ftuc: Memory allocation error.', 0Ah
maelen equ $-mae
section .text
align 4
memerr:
push dword maelen
push dword mae
jmp short error
align 4
toolong:
push dword ftllen
push dword ftl
jmp short error
align 4
facerr:
push dword faelen
push dword fae
jmp short error
align 4
cantopen:
push dword colen
push dword co
jmp short error
align 4
usage:
push dword usglen
push dword usg
error:
push dword stderr
sys.write
push dword 1
sys.exit
align 4
global _start
_start:
pop eax ; argc
pop eax ; program name
pop ecx ; file to convert
jecxz usage
pop eax
or eax, eax ; Too many arguments?
jne usage
; Open the file
push dword O_RDWR
push ecx
sys.open
jc cantopen
mov ebp, eax ; Save fd
sub esp, byte stat_size
mov ebx, esp
; Find file size
push ebx
push ebp ; fd
sys.fstat
jc facerr
mov edx, [ebx + st_size + 4]
; File is too long if EDX != 0 ...
or edx, edx
jne near toolong
mov ecx, [ebx + st_size]
; ... or if it is above 2 GB
or ecx, ecx
js near toolong
; Do nothing if the file is 0 bytes in size
jecxz .quit
; Map the entire file in memory
push edx
push edx ; starting at offset 0
push edx ; pad
push ebp ; fd
push dword MAP_SHARED
push dword PROT_READ | PROT_WRITE
push ecx ; entire file size
push edx ; let system decide on the address
sys.mmap
jc near memerr
mov edi, eax
mov esi, eax
push ecx ; for SYS_munmap
push edi
; Use EBX for state machine
mov ebx, ordinary
mov ah, 0Ah
cld
.loop:
lodsb
call ebx
loop .loop
cmp ebx, ordinary
je .filesize
; Output final lf
mov al, ah
stosb
inc edx
.filesize:
; truncate file to new size
push dword 0 ; high dword
push edx ; low dword
push eax ; pad
push ebp
sys.ftruncate
; close it (ebp still pushed)
sys.close
add esp, byte 16
sys.munmap
.quit:
push dword 0
sys.exit
align 4
ordinary:
cmp al, 0Dh
je .cr
cmp al, ah
je .lf
stosb
inc edx
ret
align 4
.cr:
mov ebx, cr
ret
align 4
.lf:
mov ebx, lf
ret
align 4
cr:
cmp al, 0Dh
je .cr
cmp al, ah
je .lf
xchg al, ah
stosb
inc edx
xchg al, ah
; fall through
.lf:
stosb
inc edx
mov ebx, ordinary
ret
align 4
.cr:
mov al, ah
stosb
inc edx
ret
align 4
lf:
cmp al, ah
je .lf
cmp al, 0Dh
je .cr
xchg al, ah
stosb
inc edx
xchg al, ah
stosb
inc edx
mov ebx, ordinary
ret
align 4
.cr:
mov ebx, ordinary
mov al, ah
; fall through
.lf:
stosb
inc edx
ret
Do not use this program on files stored on a disk formated
by MS DOS or Windows. There seems to be a
subtle bug in the FreeBSD code when using mmap
on these drives mounted under FreeBSD: If the file is over
a certain size, mmap will just fill the memory
with zeros, and then copy them to the file overwriting
its contents.
One-Pointed Mind
As a student of Zen, I like the idea of a one-pointed mind:
Do one thing at a time, and do it well.
This, indeed, is very much how Unix works as well. While
a typical Windows application is attempting to do everything
imaginable (and is, therefore, riddled with bugs), a
typical Unix program does only one thing, and it does it
well.
The typical Unix user then essentially assembles his own
applications by writing a shell script which combines the
various existing programs by piping the output of one
program to the input of another.
When writing your own Unix software, it is generally a
good idea to see what parts of the problem you need to
solve can be handled by existing programs, and only
write your own programs for that part of the problem
that you do not have an existing solution for.
CSV
I will illustrate this principle with a specific real-life
example I was faced with recently:
I needed to extract the 11th field of each record from a
database I downloaded from a web site. The database was a
CSV file, i.e., a list of
comma-separated values. That is quite
a standard format for sharing data among people who may be
using different database software.
The first line of the file contains the list of various fields
separated by commas. The rest of the file contains the data
listed line by line, with values separated by commas.
I tried awk, using the comma as a separator.
But because several lines contained a quoted comma,
awk was extracting the wrong field
from those lines.
Therefore, I needed to write my own software to extract the 11th
field from the CSV file. However, going with the Unix
spirit, I only needed to write a simple filter that would do the
following:
Remove the first line from the file;
Change all unquoted commas to a different character;
Remove all quotation marks.
Strictly speaking, I could use sed to remove
the first line from the file, but doing so in my own program
was very easy, so I decided to do it and reduce the size of
the pipeline.
At any rate, writing a program like this took me about
20 minutes. Writing a program that extracts the 11th field
from the CSV file would take a lot longer,
and I could not reuse it to extract some other field from some
other database.
This time I decided to let it do a little more work than
a typical tutorial program would:
It parses its command line for options;
It displays proper usage if it finds wrong arguments;
It produces meaningful error messages.
Here is its usage message:
Usage: csv [-t<delim>] [-c<comma>] [-p] [-o <outfile>] [-i <infile>]
All parameters are optional, and can appear in any order.
The -t parameter declares what to replace
the commas with. The tab is the default here.
For example, -t; will replace all unquoted
commas with semicolons.
I did not need the -c option, but it may
come in handy in the future. It lets me declare that I want a
character other than a comma replaced with something else.
For example, -c@ will replace all at signs
(useful if you want to split a list of email addresses
to their user names and domains).
The -p option preserves the first line, i.e.,
it does not delete it. By default, we delete the first
line because in a CSV file it contains the field
names rather than data.
The -i and -o
options let me specify the input and the output files. Defaults
are stdin and stdout,
so this is a regular Unix filter.
I made sure that both -i filename and
-ifilename are accepted. I also made
sure that only one input and one output files may be
specified.
To get the 11th field of each record, I can now do:
&prompt.user; csv '-t;' data.csv | awk '-F;' '{print $11}'
The code stores the options (except for the file descriptors)
in EDX: The comma in DH, the new
separator in DL, and the flag for
the -p option in the highest bit of
EDX, so a check for its sign will give us a
quick decision what to do.
Here is the code:
;;;;;;; csv.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
; Convert a comma-separated file to a something-else separated file.
;
; Started: 31-May-2001
; Updated: 1-Jun-2001
;
; Copyright (c) 2001 G. Adam Stanislav
; All rights reserved.
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
%include 'system.inc'
%define BUFSIZE 2048
section .data
fd.in dd stdin
fd.out dd stdout
usg db 'Usage: csv [-t<delim>] [-c<comma>] [-p] [-o <outfile>] [-i <infile>]', 0Ah
usglen equ $-usg
iemsg db "csv: Can't open input file", 0Ah
iemlen equ $-iemsg
oemsg db "csv: Can't create output file", 0Ah
oemlen equ $-oemsg
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
align 4
ierr:
push dword iemlen
push dword iemsg
push dword stderr
sys.write
push dword 1 ; return failure
sys.exit
align 4
oerr:
push dword oemlen
push dword oemsg
push dword stderr
sys.write
push dword 2
sys.exit
align 4
usage:
push dword usglen
push dword usg
push dword stderr
sys.write
push dword 3
sys.exit
align 4
global _start
_start:
add esp, byte 8 ; discard argc and argv[0]
mov edx, (',' << 8) | 9
.arg:
pop ecx
or ecx, ecx
je near .init ; no more arguments
; ECX contains the pointer to an argument
cmp byte [ecx], '-'
jne usage
inc ecx
mov ax, [ecx]
.o:
cmp al, 'o'
jne .i
; Make sure we are not asked for the output file twice
cmp dword [fd.out], stdout
jne usage
; Find the path to output file - it is either at [ECX+1],
; i.e., -ofile --
; or in the next argument,
; i.e., -o file
inc ecx
or ah, ah
jne .openoutput
pop ecx
jecxz usage
.openoutput:
push dword 420 ; file mode (644 octal)
push dword 0200h | 0400h | 01h
; O_CREAT | O_TRUNC | O_WRONLY
push ecx
sys.open
jc near oerr
add esp, byte 12
mov [fd.out], eax
jmp short .arg
.i:
cmp al, 'i'
jne .p
; Make sure we are not asked twice
cmp dword [fd.in], stdin
jne near usage
; Find the path to the input file
inc ecx
or ah, ah
jne .openinput
pop ecx
or ecx, ecx
je near usage
.openinput:
push dword 0 ; O_RDONLY
push ecx
sys.open
jc near ierr ; open failed
add esp, byte 8
mov [fd.in], eax
jmp .arg
.p:
cmp al, 'p'
jne .t
or ah, ah
jne near usage
or edx, 1 << 31
jmp .arg
.t:
cmp al, 't' ; redefine output delimiter
jne .c
or ah, ah
je near usage
mov dl, ah
jmp .arg
.c:
cmp al, 'c'
jne near usage
or ah, ah
je near usage
mov dh, ah
jmp .arg
align 4
.init:
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
; See if we are to preserve the first line
or edx, edx
js .loop
.firstline:
; get rid of the first line
call getchar
cmp al, 0Ah
jne .firstline
.loop:
; read a byte from stdin
call getchar
; is it a comma (or whatever the user asked for)?
cmp al, dh
jne .quote
; Replace the comma with a tab (or whatever the user wants)
mov al, dl
.put:
call putchar
jmp short .loop
.quote:
cmp al, '"'
jne .put
; Print everything until you get another quote or EOL. If it
; is a quote, skip it. If it is EOL, print it.
.qloop:
call getchar
cmp al, '"'
je .loop
cmp al, 0Ah
je .put
call putchar
jmp short .qloop
align 4
getchar:
or ebx, ebx
jne .fetch
call read
.fetch:
lodsb
dec ebx
ret
read:
jecxz .read
call write
.read:
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword [fd.in]
sys.read
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
ret
align 4
.done:
call write ; flush output buffer
; close files
push dword [fd.in]
sys.close
push dword [fd.out]
sys.close
; return success
push dword 0
sys.exit
align 4
putchar:
stosb
inc ecx
cmp ecx, BUFSIZE
je write
ret
align 4
write:
jecxz .ret ; nothing to write
sub edi, ecx ; start of buffer
push ecx
push edi
push dword [fd.out]
sys.write
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
.ret:
ret
Much of it is taken from hex.asm above. But there
is one important difference: I no longer call write
whenever I am outputting a line feed. Yet, the code can be
used interactively.
I have found a better solution for the interactive problem
since I first started writing this chapter. I wanted to
make sure each line is printed out separately only when needed.
After all, there is no need to flush out every line when used
non-interactively.
The new solution I use now is to call write every
time I find the input buffer empty. That way, when running in
the interactive mode, the program reads one line from the user's
keyboard, processes it, and sees its input buffer is empty. It
flushes its output and reads the next line.
The Dark Side of Buffering
This change prevents a mysterious lockup
in a very specific case. I refer to it as the
dark side of buffering, mostly
because it presents a danger that is not
quite obvious.
It is unlikely to happen with a program like the
csv above, so let us consider yet
another filter: In this case we expect our input
to be raw data representing color values, such as
the red, green, and
blue intensities of a pixel. Our
output will be the negative of our input.
Such a filter would be very simple to write.
Most of it would look just like all the other
filters we have written so far, so I am only
going to show you its inner loop:
.loop:
call getchar
not al ; Create a negative
call putchar
jmp short .loop
Because this filter works with raw data,
it is unlikely to be used interactively.
But it could be called by image manipulation software.
And, unless it calls write before each call
to read, chances are it will lock up.
Here is what might happen:
The image editor will load our filter using the
C function popen().
It will read the first row of pixels from
a bitmap or pixmap.
It will write the first row of pixels to
the pipe leading to
the fd.in of our filter.
Our filter will read each pixel
from its input, turn it to a negative,
and write it to its output buffer.
Our filter will call getchar
to fetch the next pixel.
getchar will find an empty
input buffer, so it will call
read.
read will call the
SYS_read system call.
The kernel will suspend
our filter until the image editor
sends more data to the pipe.
The image editor will read from the
other pipe, connected to the
fd.out of our filter so it can set the first row of the
output image before
it sends us the second row of the input.
The kernel suspends
the image editor until it receives
some output from our filter, so it
can pass it on to the image editor.
At this point our filter waits for the image
editor to send it more data to process, while
the image editor is waiting for our filter
to send it the result of the processing
of the first row. But the result sits in
our output buffer.
The filter and the image editor will continue
waiting for each other forever (or, at least,
until they are killed). Our software has just
entered a
race condition.
This problem does not exist if our filter flushes
its output buffer before asking the
kernel for more input data.
Using the <acronym>FPU</acronym>
Strangely enough, most of assembly language literature does not
even mention the existence of the FPU,
or floating point unit, let alone discuss
programming it.
Yet, never does assembly language shine more than when
we create highly optimized FPU
code by doing things that can be done only in assembly language.Organization of the <acronym>FPU</acronym>
The FPU consists of 8 80–bit floating–point registers.
These are organized in a stack fashion—you can
push a value on TOS
(top of stack) and you can
pop it.
That said, the assembly language op codes are not push
and pop because those are already taken.
You can push a value on TOS
by using fld, fild,
and fbld. Several other op codes
let you push many common
constants—such as pi—on
the TOS.
Similarly, you can pop a value by
using fst, fstp,
fist, fistp, and
fbstp. Actually, only the op
codes that end with a p will
literally pop the value,
the rest will store it
somewhere else without removing it from
the TOS.
We can transfer the data between the
TOS and the computer memory either as
a 32–bit, 64–bit, or 80–bit real,
a 16–bit, 32–bit, or 64–bit integer,
or an 80–bit packed decimal.
The 80–bit packed decimal is
a special case of binary coded
decimal which is very convenient when
converting between the ASCII
representation of data and the internal
data of the FPU. It allows us to use
18 significant digits.
No matter how we represent data in the memory,
the FPU always stores it in the 80–bit
real format in its registers.
Its internal precision is at least 19 decimal
digits, so even if we choose to display results
as ASCII in the full
18–digit precision, we are still showing
correct results.
We can perform mathematical operations on the
TOS: We can calculate its
sine, we can scale it
(i.e., we can multiply or divide it by a power
of 2), we can calculate its base–2
logarithm, and many other things.
We can also multiply or
divide it by, add
it to, or subtract it from,
any of the FPU registers (including
itself).
The official Intel op code for the
TOS is st, and
for the registersst(0)–st(7).
st and st(0), then,
refer to the same register.
For whatever reasons, the original author of
nasm has decided to use
different op codes, namely
st0–st7.
In other words, there are no parentheses,
and the TOS is always
st0, never just st.
The Packed Decimal Format
The packed decimal format
uses 10 bytes (80 bits) of
memory to represent 18 digits. The
number represented there is always an
integer.
You can use it to get decimal places
by multiplying the TOS
by a power of 10 first.
The highest bit of the highest byte
(byte 9) is the sign bit:
If it is set, the number is negative,
otherwise, it is positive.
The rest of the bits of this byte are unused/ignored.
The remaining 9 bytes store the 18 digits
of the number: 2 digits per byte.
The more significant digit is
stored in the high nibble
(4 bits), the less significant
digit in the low nibble.
That said, you might think that -1234567
would be stored in the memory like this (using
hexadecimal notation):
80 00 00 00 00 00 01 23 45 67
Alas it is not! As with everything else of Intel make,
even the packed decimal is
little–endian.
That means our -1234567
is stored like this:
67 45 23 01 00 00 00 00 00 80
Remember that, or you will be pulling your hair out
in desperation!
The book to read—if you can find it—is Richard Startz'
8087/80287/80387
for the IBM PC & Compatibles.
Though it does seem to take the fact about the
little–endian storage of the packed
decimal for granted. I kid you not about the
desperation of trying to figure out what was wrong
with the filter I show below before
it occurred to me I should try the
little–endian order even for this type of data.
Excursion to Pinhole Photography
To write meaningful software, we must not only
understand our programming tools, but also the
field we are creating software for.
Our next filter will help us whenever we want
to build a pinhole camera,
so, we need some background in pinhole
photography before we can continue.
The Camera
The easiest way to describe any camera ever built
is as some empty space enclosed in some
lightproof material, with a small hole in the
enclosure.
The enclosure is usually sturdy (e.g., a box),
though sometimes it is flexible (the bellows).
It is quite dark inside the camera. However, the
hole lets light rays in through a single point
(though in some cases there may be several).
These light rays form an image, a representation
of whatever is outside the camera, in front of the
hole.
If some light sensitive material (such as film)
is placed inside the camera, it can capture the
image.
The hole often contains a lens, or
a lens assembly, often called the objective.
The Pinhole
But, strictly speaking, the lens is not necessary:
The original cameras did not use a lens but a
pinhole. Even today, pinholes
are used, both as a tool to study how cameras
work, and to achieve a special kind of image.
The image produced by the pinhole
is all equally sharp. Or blurred.
There is an ideal size for a pinhole: If it is
either larger or smaller, the image loses its
sharpness.Focal Length
This ideal pinhole diameter is a function
of the square root of focal
length, which is the distance of the
pinhole from the film.
D = PC * sqrt(FL)
In here, D is the
ideal diameter of the pinhole,
FL is the focal length,
and PC is a pinhole
constant. According to Jay Bender,
its value is 0.04, while
Kenneth Connors has determined it to
be 0.037. Others have
proposed other values. Plus, this
value is for the daylight only: Other types
of light will require a different constant,
whose value can only be determined by
experimentation.
The F–Number
The f–number is a very useful measure of
how much light reaches the film. A light
meter can determine that, for example,
to expose a film of specific sensitivity
with f5.6 may require the exposure to last
1/1000 sec.
It does not matter whether it is a 35–mm
camera, or a 6x9cm camera, etc.
As long as we know the f–number, we can determine
the proper exposure.
The f–number is easy to calculate:
F = FL / D
In other words, the f–number equals the focal
length divided by the diameter of the pinhole.
It also means a higher f–number either implies
a smaller pinhole or a larger focal distance,
or both. That, in turn, implies, the higher
the f–number, the longer the exposure has to be.
Furthermore, while pinhole diameter and focal
distance are one–dimensional measurements,
both, the film and the pinhole, are two–dimensional.
That means that
if you have measured the exposure at f–number
A as t, then the exposure
at f–number B is:
t * (B / A)²
Normalized F–Number
While many modern cameras can change the diameter
of their pinhole, and thus their f–number, quite
smoothly and gradually, such was not always the case.
To allow for different f–numbers, cameras typically
contained a metal plate with several holes of
different sizes drilled to them.
Their sizes were chosen according to the above
formula in such a way that the resultant f–number
was one of standard f–numbers used on all cameras
everywhere. For example, a very old Kodak Duaflex IV
camera in my possession has three such holes for
f–numbers 8, 11, and 16.
A more recently made camera may offer f–numbers of
2.8, 4, 5.6, 8, 11,
16, 22, and 32 (as well as others).
These numbers were not chosen arbitrarily: They all are
powers of the square root of 2, though they may
be rounded somewhat.
The F–Stop
A typical camera is designed in such a way that setting
any of the normalized f–numbers changes the feel of the
dial. It will naturally stop in that
position. Because of that, these positions of the dial
are called f–stops.
Since the f–numbers at each stop are powers of the
square root of 2, moving the dial by 1
stop will double the amount of light required for
proper exposure. Moving it by 2 stops will
quadruple the required exposure. Moving the dial by
3 stops will require the increase in exposure
8 times, etc.
Designing the Pinhole Software
We are now ready to decide what exactly we want our
pinhole software to do.
Processing Program Input
Since its main purpose is to help us design a working
pinhole camera, we will use the focal
length as the input to the program. This is something
we can determine without software: Proper focal length
is determined by the size of the film and by the need
to shoot "regular" pictures, wide angle pictures, or
telephoto pictures.
Most of the programs we have written so far worked with
individual characters, or bytes, as their input: The
hex program converted individual bytes
into a hexadecimal number, the csv
program either let a character through, or deleted it,
or changed it to a different character, etc.
One program, ftuc used the state machine
to consider at most two input bytes at a time.
But our pinhole program cannot just
work with individual characters, it has to deal with
larger syntactic units.
For example, if we want the program to calculate the
pinhole diameter (and other values we will discuss
later) at the focal lengths of 100 mm,
150 mm, and 210 mm, we may want
to enter something like this:100, 150, 210
Our program needs to consider more than a single byte of
input at a time. When it sees the first 1,
it must understand it is seeing the first digit of a
decimal number. When it sees the 0 and
the other 0, it must know it is seeing
more digits of the same number.
When it encounters the first comma, it must know it is
no longer receiving the digits of the first number.
It must be able to convert the digits of the first number
into the value of 100. And the digits of the
second number into the value of 150. And,
of course, the digits of the third number into the
numeric value of 210.
We need to decide what delimiters to accept: Do the
input numbers have to be separated by a comma? If so,
how do we treat two numbers separated by something else?
Personally, I like to keep it simple. Something either
is a number, so I process it. Or it is not a number,
so I discard it. I do not like the computer complaining
about me typing in an extra character when it is
obvious that it is an extra character. Duh!
Plus, it allows me to break up the monotony of computing
and type in a query instead of just a number:
What is the best pinhole diameter for the focal length of 150?
There is no reason for the computer to spit out
a number of complaints:
Syntax error: What
Syntax error: is
Syntax error: the
Syntax error: best
Et cetera, et cetera, et cetera.
Secondly, I like the # character to denote
the start of a comment which extends to the end of the
line. This does not take too much effort to code, and
lets me treat input files for my software as executable
scripts.
In our case, we also need to decide what units the
input should come in: We choose millimeters
because that is how most photographers measure
the focus length.
Finally, we need to decide whether to allow the use
of the decimal point (in which case we must also
consider the fact that much of the world uses a
decimal comma).
In our case allowing for the decimal point/comma
would offer a false sense of precision: There is
little if any noticeable difference between the
focus lengths of 50 and 51,
so allowing the user to input something like
50.5 is not a good idea. This is
my opinion, mind you, but I am the one writing
this program. You can make other choices in yours,
of course.
Offering Options
The most important thing we need to know when building
a pinhole camera is the diameter of the pinhole. Since
we want to shoot sharp images, we will use the above
formula to calculate the pinhole diameter from focal length.
As experts are offering several different values for the
PC constant, we will need to have the choice.
It is traditional in Unix programming to have two main ways
of choosing program parameters, plus to have a default for
the time the user does not make a choice.
Why have two ways of choosing?
One is to allow a (relatively) permanent
choice that applies automatically each time the
software is run without us having to tell it over and
over what we want it to do.
The permanent choices may be stored in a configuration
file, typically found in the user's home directory.
The file usually has the same name as the application
but is started with a dot. Often "rc"
is added to the file name. So, ours could be
~/.pinhole or ~/.pinholerc.
(The ~/ means current user's
home directory.)
The configuration file is used mostly by programs
that have many configurable parameters. Those
that have only one (or a few) often use a different
method: They expect to find the parameter in an
environment variable. In our case,
we might look at an environment variable named
PINHOLE.
Usually, a program uses one or the other of the
above methods. Otherwise, if a configuration
file said one thing, but an environment variable
another, the program might get confused (or just
too complicated).
Because we only need to choose one
such parameter, we will go with the second method
and search the environment for a variable named
PINHOLE.
The other way allows us to make ad hoc
decisions: "Though I usually want
you to use 0.039, this time I want 0.03872."
In other words, it allows us to override
the permanent choice.
This type of choice is usually done with command
line parameters.
Finally, a program always needs a
default. The user may not make
any choices. Perhaps he does not know what
to choose. Perhaps he is "just browsing."
Preferably, the default will be the value
most users would choose anyway. That way
they do not need to choose. Or, rather, they
can choose the default without an additional
effort.
Given this system, the program may find conflicting
options, and handle them this way:
If it finds an ad hoc choice
(e.g., command line parameter), it should
accept that choice. It must ignore any permanent
choice and any default.
Otherwise, if it finds
a permanent option (e.g., an environment
variable), it should accept it, and ignore
the default.Otherwise, it should use
the default.
We also need to decide what format
our PC option should have.
At first site, it seems obvious to use the
PINHOLE=0.04 format for the
environment variable, and -p0.04
for the command line.
Allowing that is actually a security risk.
The PC constant is a very small
number. Naturally, we will test our software
using various small values of PC.
But what will happen if someone runs the program
choosing a huge value?
It may crash the program because we have not
designed it to handle huge numbers.
Or, we may spend more time on the program so
it can handle huge numbers. We might do that
if we were writing commercial software for
computer illiterate audience.
Or, we might say, "Tough!
The user should know better.""
Or, we just may make it impossible for the user
to enter a huge number. This is the approach we
will take: We will use an implied 0.
prefix.
In other words, if the user wants 0.04,
we will expect him to type -p04,
or set PINHOLE=04 in his environment.
So, if he says -p9999999, we will
interpret it as 0.9999999—still
ridiculous but at least safer.
Secondly, many users will just want to go with either
Bender's constant or Connors' constant.
To make it easier on them, we will interpret
-b as identical to -p04,
and -c as identical to -p037.
The Output
We need to decide what we want our software to
send to the output, and in what format.
Since our input allows for an unspecified number
of focal length entries, it makes sense to use
a traditional database–style output of showing
the result of the calculation for each
focal length on a separate line, while
separating all values on one line by a
tab character.
Optionally, we should also allow the user
to specify the use of the CSV
format we have studied earlier. In this case,
we will print out a line of comma–separated
names describing each field of every line,
then show our results as before, but substituting
a comma for the tab.
We need a command line option for the CSV
format. We cannot use -c because
that already means use Connors' constant.
For some strange reason, many web sites refer to
CSV files as "Excel
spreadsheet" (though the CSV
format predates Excel). We will, therefore, use
the -e switch to inform our software
we want the output in the CSV format.
We will start each line of the output with the
focal length. This may sound repetitious at first,
especially in the interactive mode: The user
types in the focal length, and we are repeating it.
But the user can type several focal lengths on one
line. The input can also come in from a file or
from the output of another program. In that case
the user does not see the input at all.
By the same token, the output can go to a file
which we will want to examine later, or it could
go to the printer, or become the input of another
program.
So, it makes perfect sense to start each line with
the focal length as entered by the user.
No, wait! Not as entered by the user. What if the user
types in something like this:00000000150
Clearly, we need to strip those leading zeros.
So, we might consider reading the user input as is,
converting it to binary inside the FPU,
and printing it out from there.
But...
What if the user types something like this:
17459765723452353453534535353530530534563507309676764423
Ha! The packed decimal FPU format
lets us input 18–digit numbers. But the
user has entered more than 18 digits. How
do we handle that?
Well, we could modify our code to read
the first 18 digits, enter it to the FPU,
then read more, multiply what we already have on the
TOS by 10 raised to the number
of additional digits, then add to it.
Yes, we could do that. But in this
program it would be ridiculous (in a different one it may be just the thing to do): Even the circumference of the Earth expressed in
millimeters only takes 11 digits. Clearly,
we cannot build a camera that large (not yet,
anyway).
So, if the user enters such a huge number, he is
either bored, or testing us, or trying to break
into the system, or playing games—doing
anything but designing a pinhole camera.
What will we do?
We will slap him in the face, in a manner of speaking:17459765723452353453534535353530530534563507309676764423 ??? ??? ??? ??? ???
To achieve that, we will simply ignore any leading zeros.
Once we find a non–zero digit, we will initialize a
counter to 0 and start taking three steps:
Send the digit to the output.
Append the digit to a buffer we will use later to
produce the packed decimal we can send to the
FPU.
Increase the counter.
Now, while we are taking these three steps,
we also need to watch out for one of two
conditions:
If the counter grows above 18,
we stop appending to the buffer. We
continue reading the digits and sending
them to the output.
If, or rather when,
the next input character is not
a digit, we are done inputting
for now.
Incidentally, we can simply
discard the non–digit, unless it
is a #, which we must
return to the input stream. It
starts a comment, so we must see it
after we are done producing output
and start looking for more input.
That still leaves one possibility
uncovered: If all the user enters
is a zero (or several zeros), we
will never find a non–zero to
display.
We can determine this has happened
whenever our counter stays at 0.
In that case we need to send 0
to the output, and perform another
"slap in the face":
0 ??? ??? ??? ??? ???
Once we have displayed the focal
length and determined it is valid
(greater than 0
but not exceeding 18 digits),
we can calculate the pinhole diameter.
It is not by coincidence that pinhole
contains the word pin. Indeed,
many a pinhole literally is a pin
hole, a hole carefully punched with the
tip of a pin.
That is because a typical pinhole is very
small. Our formula gets the result in
millimeters. We will multiply it by 1000,
so we can output the result in microns.
At this point we have yet another trap to face:
Too much precision.
Yes, the FPU was designed
for high precision mathematics. But we
are not dealing with high precision
mathematics. We are dealing with physics
(optics, specifically).
Suppose we want to convert a truck into
a pinhole camera (we would not be the
first ones to do that!). Suppose its box is
12
meters long, so we have the focal length
of 12000. Well, using Bender's constant, it gives us square root of
12000 multiplied by 0.04,
which is 4.381780460 millimeters,
or 4381.780460 microns.
Put either way, the result is absurdly precise.
Our truck is not exactly12000
millimeters long. We did not measure its length
with such a precision, so stating we need a pinhole
with the diameter of 4.381780460
millimeters is, well, deceiving. 4.4
millimeters would do just fine.
I "only" used ten digits in the above example.
Imagine the absurdity of going for all 18!
We need to limit the number of significant
digits of our result. One way of doing it
is by using an integer representing microns.
So, our truck would need a pinhole with the diameter
of 4382 microns. Looking at that number, we still decide that 4400 microns,
or 4.4 millimeters is close enough.
Additionally, we can decide that no matter how
big a result we get, we only want to display four
significant digits (or any other number
of them, of course). Alas, the FPU
does not offer rounding to a specific number
of digits (after all, it does not view the
numbers as decimal but as binary).
We, therefore, must devise an algorithm to reduce
the number of significant digits.
Here is mine (I think it is awkward—if
you know a better one, please, let me know):
Initialize a counter to 0.
While the number is greater than or equal to
10000, divide it by
10 and increase the counter.
Output the result.
While the counter is greater than 0,
output 0 and decrease the counter.
The 10000 is only good if you want
four significant digits. For any other
number of significant digits, replace
10000 with 10
raised to the number of significant digits.
We will, then, output the pinhole diameter
in microns, rounded off to four significant
digits.
At this point, we know the focal
length and the pinhole
diameter. That means we have enough
information to also calculate the
f–number.
We will display the f–number, rounded to
four significant digits. Chances are the
f–number will tell us very little. To make
it more meaningful, we can find the nearest
normalized f–number, i.e.,
the nearest power of the square root
of 2.
We do that by multiplying the actual f–number
by itself, which, of course, will give us
its square. We will then calculate
its base–2 logarithm, which is much
easier to do than calculating the
base–square–root–of–2 logarithm!
We will round the result to the nearest integer.
Next, we will raise 2 to the result. Actually,
the FPU gives us a good shortcut
to do that: We can use the fscale
op code to "scale" 1, which is
analogous to shifting an
integer left. Finally, we calculate the square
root of it all, and we have the nearest
normalized f–number.
If all that sounds overwhelming—or too much
work, perhaps—it may become much clearer
if you see the code. It takes 9 op
codes altogether:
fmul st0, st0
fld1
fld st1
fyl2x
frndint
fld1
fscale
fsqrt
fstp st1
The first line, fmul st0, st0, squares
the contents of the TOS
(top of the stack, same as st,
called st0 by nasm).
The fld1 pushes 1
on the TOS.
The next line, fld st1, pushes
the square back to the TOS.
At this point the square is both in st
and st(2) (it will become
clear why we leave a second copy on the stack
in a moment). st(1) contains
1.
Next, fyl2x calculates base–2
logarithm of st multiplied by
st(1). That is why we placed 1 on st(1) before.
At this point, st contains
the logarithm we have just calculated,
st(1) contains the square
of the actual f–number we saved for later.
frndint rounds the TOS
to the nearest integer. fld1 pushes
a 1. fscale shifts the
1 we have on the TOS
by the value in st(1),
effectively raising 2 to st(1).
Finally, fsqrt calculates
the square root of the result, i.e.,
the nearest normalized f–number.
We now have the nearest normalized
f–number on the TOS,
the base–2 logarithm rounded to the
nearest integer in st(1),
and the square of the actual f–number
in st(2). We are saving
the value in st(2) for later.
But we do not need the contents of
st(1) anymore. The last
line, fstp st1, places the
contents of st to
st(1), and pops. As a
result, what was st(1)
is now st, what was st(2)
is now st(1), etc.
The new st contains the
normalized f–number. The new
st(1) contains the square
of the actual f–number we have
stored there for posterity.
At this point, we are ready to output
the normalized f–number. Because it is
normalized, we will not round it off to
four significant digits, but will
send it out in its full precision.
The normalized f-number is useful as long
as it is reasonably small and can be found
on our light meter. Otherwise we need a
different method of determining proper
exposure.
Earlier we have figured out the formula
of calculating proper exposure at an arbitrary
f–number from that measured at a different
f–number.
Every light meter I have ever seen can determine
proper exposure at f5.6. We will, therefore,
calculate an "f5.6 multiplier,"
i.e., by how much we need to multiply the exposure measured
at f5.6 to determine the proper exposure
for our pinhole camera.
From the above formula we know this factor can be
calculated by dividing our f–number (the
actual one, not the normalized one) by
5.6, and squaring the result.
Mathematically, dividing the square of our
f–number by the square of 5.6
will give us the same result.
Computationally, we do not want to square
two numbers when we can only square one.
So, the first solution seems better at first.
But...5.6 is a constant.
We do not have to have our FPU
waste precious cycles. We can just tell it
to divide the square of the f–number by
whatever 5.6² equals to.
Or we can divide the f–number by 5.6,
and then square the result. The two ways
now seem equal.
But, they are not!
Having studied the principles of photography
above, we remember that the 5.6
is actually square root of 2 raised to
the fifth power. An irrational
number. The square of this number is
exactly32.
Not only is 32 an integer,
it is a power of 2. We do not need
to divide the square of the f–number by
32. We only need to use
fscale to shift it right by
five positions. In the FPU
lingo it means we will fscale it
with st(1) equal to
-5. That is much
faster than a division.
So, now it has become clear why we have
saved the square of the f–number on the
top of the FPU stack.
The calculation of the f5.6 multiplier
is the easiest calculation of this
entire program! We will output it rounded
to four significant digits.
There is one more useful number we can calculate:
The number of stops our f–number is from f5.6.
This may help us if our f–number is just outside
the range of our light meter, but we have
a shutter which lets us set various speeds,
and this shutter uses stops.
Say, our f–number is 5 stops from
f5.6, and the light meter says
we should use 1/1000 sec.
Then we can set our shutter speed to 1/1000
first, then move the dial by 5 stops.
This calculation is quite easy as well. All
we have to do is to calculate the base-2
logarithm of the f5.6 multiplier
we had just calculated (though we need its
value from before we rounded it off). We then
output the result rounded to the nearest integer.
We do not need to worry about having more than
four significant digits in this one: The result
is most likely to have only one or two digits
anyway.FPU Optimizations
In assembly language we can optimize the FPU
code in ways impossible in high languages,
including C.
Whenever a C function needs to calculate
a floating–point value, it loads all necessary
variables and constants into FPU
registers. It then does whatever calculation is
required to get the correct result. Good C
compilers can optimize that part of the code really
well.
It "returns" the value by leaving
the result on the TOS.
However, before it returns, it cleans up.
Any variables and constants it used in its
calculation are now gone from the FPU.
It cannot do what we just did above: We calculated
the square of the f–number and kept it on the
stack for later use by another function.
We knew we would need that value
later on. We also knew we had enough room on the
stack (which only has room for 8 numbers)
to store it there.
A C compiler has no way of knowing
that a value it has on the stack will be
required again in the very near future.
Of course, the C programmer may know it.
But the only recourse he has is to store the
value in a memory variable.
That means, for one, the value will be changed
from the 80-bit precision used internally
by the FPU to a C double
(64 bits) or even single (32
bits).
That also means that the value must be moved
from the TOS into the memory,
and then back again. Alas, of all FPU
operations, the ones that access the computer
memory are the slowest.
So, whenever programming the FPU
in assembly language, look for the ways of keeping
intermediate results on the FPU
stack.
We can take that idea even further! In our
program we are using a constant
(the one we named PC).
It does not matter how many pinhole diameters
we are calculating: 1, 10, 20,
1000, we are always using the same constant.
Therefore, we can optimize our program by keeping
the constant on the stack all the time.
Early on in our program, we are calculating the
value of the above constant. We need to divide
our input by 10 for every digit in the
constant.
It is much faster to multiply than to divide.
So, at the start of our program, we divide 10
into 1 to obtain 0.1, which we
then keep on the stack: Instead of dividing the
input by 10 for every digit,
we multiply it by 0.1.
By the way, we do not input 0.1 directly,
even though we could. We have a reason for that:
While 0.1 can be expressed with just one
decimal place, we do not know how many binary
places it takes. We, therefore, let the FPU
calculate its binary value to its own high precision.
We are using other constants: We multiply the pinhole
diameter by 1000 to convert it from
millimeters to microns. We compare numbers to
10000 when we are rounding them off to
four significant digits. So, we keep both, 1000
and 10000, on the stack. And, of course,
we reuse the 0.1 when rounding off numbers
to four digits.
Last but not least, we keep -5 on the stack.
We need it to scale the square of the f–number,
instead of dividing it by 32. It is not
by coincidence we load this constant last. That makes
it the top of the stack when only the constants
are on it. So, when the square of the f–number is
being scaled, the -5 is at st(1),
precisely where fscale expects it to be.
It is common to create certain constants from
scratch instead of loading them from the memory.
That is what we are doing with -5:
fld1 ; TOS = 1
fadd st0, st0 ; TOS = 2
fadd st0, st0 ; TOS = 4
fld1 ; TOS = 1
faddp st1, st0 ; TOS = 5
fchs ; TOS = -5
We can generalize all these optimizations into one rule:
Keep repeat values on the stack!PostScript is a stack–oriented
programming language. There are many more books
available about PostScript than about the
FPU assembly language: Mastering
PostScript will help you master the FPU.
<application>pinhole</application>—The Code
;;;;;;; pinhole.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
; Find various parameters of a pinhole camera construction and use
;
; Started: 9-Jun-2001
; Updated: 10-Jun-2001
;
; Copyright (c) 2001 G. Adam Stanislav
; All rights reserved.
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
%include 'system.inc'
%define BUFSIZE 2048
section .data
align 4
ten dd 10
thousand dd 1000
tthou dd 10000
fd.in dd stdin
fd.out dd stdout
envar db 'PINHOLE=' ; Exactly 8 bytes, or 2 dwords long
pinhole db '04,', ; Bender's constant (0.04)
connors db '037', 0Ah ; Connors' constant
usg db 'Usage: pinhole [-b] [-c] [-e] [-p <value>] [-o <outfile>] [-i <infile>]', 0Ah
usglen equ $-usg
iemsg db "pinhole: Can't open input file", 0Ah
iemlen equ $-iemsg
oemsg db "pinhole: Can't create output file", 0Ah
oemlen equ $-oemsg
pinmsg db "pinhole: The PINHOLE constant must not be 0", 0Ah
pinlen equ $-pinmsg
toobig db "pinhole: The PINHOLE constant may not exceed 18 decimal places", 0Ah
biglen equ $-toobig
huhmsg db 9, '???'
separ db 9, '???'
sep2 db 9, '???'
sep3 db 9, '???'
sep4 db 9, '???', 0Ah
huhlen equ $-huhmsg
header db 'focal length in millimeters,pinhole diameter in microns,'
db 'F-number,normalized F-number,F-5.6 multiplier,stops '
db 'from F-5.6', 0Ah
headlen equ $-header
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
dbuffer resb 20 ; decimal input buffer
bbuffer resb 10 ; BCD buffer
section .text
align 4
huh:
call write
push dword huhlen
push dword huhmsg
push dword [fd.out]
sys.write
add esp, byte 12
ret
align 4
perr:
push dword pinlen
push dword pinmsg
push dword stderr
sys.write
push dword 4 ; return failure
sys.exit
align 4
consttoobig:
push dword biglen
push dword toobig
push dword stderr
sys.write
push dword 5 ; return failure
sys.exit
align 4
ierr:
push dword iemlen
push dword iemsg
push dword stderr
sys.write
push dword 1 ; return failure
sys.exit
align 4
oerr:
push dword oemlen
push dword oemsg
push dword stderr
sys.write
push dword 2
sys.exit
align 4
usage:
push dword usglen
push dword usg
push dword stderr
sys.write
push dword 3
sys.exit
align 4
global _start
_start:
add esp, byte 8 ; discard argc and argv[0]
sub esi, esi
.arg:
pop ecx
or ecx, ecx
je near .getenv ; no more arguments
; ECX contains the pointer to an argument
cmp byte [ecx], '-'
jne usage
inc ecx
mov ax, [ecx]
inc ecx
.o:
cmp al, 'o'
jne .i
; Make sure we are not asked for the output file twice
cmp dword [fd.out], stdout
jne usage
; Find the path to output file - it is either at [ECX+1],
; i.e., -ofile --
; or in the next argument,
; i.e., -o file
or ah, ah
jne .openoutput
pop ecx
jecxz usage
.openoutput:
push dword 420 ; file mode (644 octal)
push dword 0200h | 0400h | 01h
; O_CREAT | O_TRUNC | O_WRONLY
push ecx
sys.open
jc near oerr
add esp, byte 12
mov [fd.out], eax
jmp short .arg
.i:
cmp al, 'i'
jne .p
; Make sure we are not asked twice
cmp dword [fd.in], stdin
jne near usage
; Find the path to the input file
or ah, ah
jne .openinput
pop ecx
or ecx, ecx
je near usage
.openinput:
push dword 0 ; O_RDONLY
push ecx
sys.open
jc near ierr ; open failed
add esp, byte 8
mov [fd.in], eax
jmp .arg
.p:
cmp al, 'p'
jne .c
or ah, ah
jne .pcheck
pop ecx
or ecx, ecx
je near usage
mov ah, [ecx]
.pcheck:
cmp ah, '0'
jl near usage
cmp ah, '9'
ja near usage
mov esi, ecx
jmp .arg
.c:
cmp al, 'c'
jne .b
or ah, ah
jne near usage
mov esi, connors
jmp .arg
.b:
cmp al, 'b'
jne .e
or ah, ah
jne near usage
mov esi, pinhole
jmp .arg
.e:
cmp al, 'e'
jne near usage
or ah, ah
jne near usage
mov al, ','
mov [huhmsg], al
mov [separ], al
mov [sep2], al
mov [sep3], al
mov [sep4], al
jmp .arg
align 4
.getenv:
; If ESI = 0, we did not have a -p argument,
; and need to check the environment for "PINHOLE="
or esi, esi
jne .init
sub ecx, ecx
.nextenv:
pop esi
or esi, esi
je .default ; no PINHOLE envar found
; check if this envar starts with 'PINHOLE='
mov edi, envar
mov cl, 2 ; 'PINHOLE=' is 2 dwords long
rep cmpsd
jne .nextenv
; Check if it is followed by a digit
mov al, [esi]
cmp al, '0'
jl .default
cmp al, '9'
jbe .init
; fall through
align 4
.default:
; We got here because we had no -p argument,
; and did not find the PINHOLE envar.
mov esi, pinhole
; fall through
align 4
.init:
sub eax, eax
sub ebx, ebx
sub ecx, ecx
sub edx, edx
mov edi, dbuffer+1
mov byte [dbuffer], '0'
; Convert the pinhole constant to real
.constloop:
lodsb
cmp al, '9'
ja .setconst
cmp al, '0'
je .processconst
jb .setconst
inc dl
.processconst:
inc cl
cmp cl, 18
ja near consttoobig
stosb
jmp short .constloop
align 4
.setconst:
or dl, dl
je near perr
finit
fild dword [tthou]
fld1
fild dword [ten]
fdivp st1, st0
fild dword [thousand]
mov edi, obuffer
mov ebp, ecx
call bcdload
.constdiv:
fmul st0, st2
loop .constdiv
fld1
fadd st0, st0
fadd st0, st0
fld1
faddp st1, st0
fchs
; If we are creating a CSV file,
; print header
cmp byte [separ], ','
jne .bigloop
push dword headlen
push dword header
push dword [fd.out]
sys.write
.bigloop:
call getchar
jc near done
; Skip to the end of the line if you got '#'
cmp al, '#'
jne .num
call skiptoeol
jmp short .bigloop
.num:
; See if you got a number
cmp al, '0'
jl .bigloop
cmp al, '9'
ja .bigloop
; Yes, we have a number
sub ebp, ebp
sub edx, edx
.number:
cmp al, '0'
je .number0
mov dl, 1
.number0:
or dl, dl ; Skip leading 0's
je .nextnumber
push eax
call putchar
pop eax
inc ebp
cmp ebp, 19
jae .nextnumber
mov [dbuffer+ebp], al
.nextnumber:
call getchar
jc .work
cmp al, '#'
je .ungetc
cmp al, '0'
jl .work
cmp al, '9'
ja .work
jmp short .number
.ungetc:
dec esi
inc ebx
.work:
; Now, do all the work
or dl, dl
je near .work0
cmp ebp, 19
jae near .toobig
call bcdload
; Calculate pinhole diameter
fld st0 ; save it
fsqrt
fmul st0, st3
fld st0
fmul st5
sub ebp, ebp
; Round off to 4 significant digits
.diameter:
fcom st0, st7
fstsw ax
sahf
jb .printdiameter
fmul st0, st6
inc ebp
jmp short .diameter
.printdiameter:
call printnumber ; pinhole diameter
; Calculate F-number
fdivp st1, st0
fld st0
sub ebp, ebp
.fnumber:
fcom st0, st6
fstsw ax
sahf
jb .printfnumber
fmul st0, st5
inc ebp
jmp short .fnumber
.printfnumber:
call printnumber ; F number
; Calculate normalized F-number
fmul st0, st0
fld1
fld st1
fyl2x
frndint
fld1
fscale
fsqrt
fstp st1
sub ebp, ebp
call printnumber
; Calculate time multiplier from F-5.6
fscale
fld st0
; Round off to 4 significant digits
.fmul:
fcom st0, st6
fstsw ax
sahf
jb .printfmul
inc ebp
fmul st0, st5
jmp short .fmul
.printfmul:
call printnumber ; F multiplier
; Calculate F-stops from 5.6
fld1
fxch st1
fyl2x
sub ebp, ebp
call printnumber
mov al, 0Ah
call putchar
jmp .bigloop
.work0:
mov al, '0'
call putchar
align 4
.toobig:
call huh
jmp .bigloop
align 4
done:
call write ; flush output buffer
; close files
push dword [fd.in]
sys.close
push dword [fd.out]
sys.close
finit
; return success
push dword 0
sys.exit
align 4
skiptoeol:
; Keep reading until you come to cr, lf, or eof
call getchar
jc done
cmp al, 0Ah
jne .cr
ret
.cr:
cmp al, 0Dh
jne skiptoeol
ret
align 4
getchar:
or ebx, ebx
jne .fetch
call read
.fetch:
lodsb
dec ebx
clc
ret
read:
jecxz .read
call write
.read:
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword [fd.in]
sys.read
add esp, byte 12
mov ebx, eax
or eax, eax
je .empty
sub eax, eax
ret
align 4
.empty:
add esp, byte 4
stc
ret
align 4
putchar:
stosb
inc ecx
cmp ecx, BUFSIZE
je write
ret
align 4
write:
jecxz .ret ; nothing to write
sub edi, ecx ; start of buffer
push ecx
push edi
push dword [fd.out]
sys.write
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
.ret:
ret
align 4
bcdload:
; EBP contains the number of chars in dbuffer
push ecx
push esi
push edi
lea ecx, [ebp+1]
lea esi, [dbuffer+ebp-1]
shr ecx, 1
std
mov edi, bbuffer
sub eax, eax
mov [edi], eax
mov [edi+4], eax
mov [edi+2], ax
.loop:
lodsw
sub ax, 3030h
shl al, 4
or al, ah
mov [edi], al
inc edi
loop .loop
fbld [bbuffer]
cld
pop edi
pop esi
pop ecx
sub eax, eax
ret
align 4
printnumber:
push ebp
mov al, [separ]
call putchar
; Print the integer at the TOS
mov ebp, bbuffer+9
fbstp [bbuffer]
; Check the sign
mov al, [ebp]
dec ebp
or al, al
jns .leading
; We got a negative number (should never happen)
mov al, '-'
call putchar
.leading:
; Skip leading zeros
mov al, [ebp]
dec ebp
or al, al
jne .first
cmp ebp, bbuffer
jae .leading
; We are here because the result was 0.
; Print '0' and return
mov al, '0'
jmp putchar
.first:
; We have found the first non-zero.
; But it is still packed
test al, 0F0h
jz .second
push eax
shr al, 4
add al, '0'
call putchar
pop eax
and al, 0Fh
.second:
add al, '0'
call putchar
.next:
cmp ebp, bbuffer
jb .done
mov al, [ebp]
push eax
shr al, 4
add al, '0'
call putchar
pop eax
and al, 0Fh
add al, '0'
call putchar
dec ebp
jmp short .next
.done:
pop ebp
or ebp, ebp
je .ret
.zeros:
mov al, '0'
call putchar
dec ebp
jne .zeros
.ret:
ret
The code follows the same format as all the other
filters we have seen before, with one subtle
exception:
We are no longer assuming that the end of input
implies the end of things to do, something we
took for granted in the character–oriented
filters.
This filter does not process characters. It
processes a language
(albeit a very simple
one, consisting only of numbers).
When we have no more input, it can mean one
of two things:
We are done and can quit. This is the
same as before.
The last character we have read was a digit.
We have stored it at the end of our
ASCII–to–float conversion
buffer. We now need to convert
the contents of that buffer into a
number and write the last line of our
output.
For that reason, we have modified our getchar
and our read routines to return with
the carry flagclear whenever we are
fetching another character from the input, or the
carry flagset whenever there is no more
input.
Of course, we are still using assembly language magic
to do that! Take a good look at getchar.
It always returns with the
carry flagclear.
Yet, our main code relies on the carry
flag to tell it when to quit—and it works.
The magic is in read. Whenever it
receives more input from the system, it just
returns to getchar, which
fetches a character from the input buffer,
clears the carry flag
and returns.
But when read receives no more
input from the system, it does not
return to getchar at all.
Instead, the add esp, byte 4
op code adds 4 to ESP,
sets the carry
flag, and returns.
So, where does it return to? Whenever a
program uses the call op code,
the microprocessor pushes the
return address, i.e., it stores it on
the top of the stack (not the FPU
stack, the system stack, which is in the memory).
When a program uses the ret
op code, the microprocessor pops
the return value from the stack, and jumps
to the address that was stored there.
But since we added 4 to
ESP (which is the stack
pointer register), we have effectively
given the microprocessor a minor case
of amnesia: It no longer
remembers it was getchar
that called read.
And since getchar never
pushed anything before
calling read,
the top of the stack now contains the
return address to whatever or whoever
called getchar.
As far as that caller is concerned,
he called getchar,
which returned with the
carry flag set!