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SETSYSP mn     Return From User Interrupt     Rabbit 4000/5000 Instruction

Opcode	Instruction	Operation
ED B1 n m	SETSYSP mn	SU={SU[1:0],SU[7:2]} tmplow = (SP) tmphigh = (SP + 1) SP = SP + 2 if {tmp != mn} System Violation

Clocks	8-Bit Access	16-Bit Unaligned	16-Bit Aligned
Rabbit 2000/3000/4000	12	n/a	n/a
Rabbit 5000	12	10	8

Flags				ALTD			IOI/IOE
S	Z	L/V	C	F	R	SP	S	D
-	-	-	-

Description

This instruction is used to handle user mode interrupts in system/user mode. It sets the current processor mode to the previous processor mode by rotating two places to the right the bits of SU. Bits 1 and 0 are moved to bit positions 7 and 6 respectively. The System/User Mode Register, SU, is an 8-bit register that forms a stack of the current processor mode and the previous 3 modes.

This is a chained-atomic instruction, meaning that an interrupt cannot take place between this instruction and the instruction following it.