Intel 80286, 80287 manual IRET-Interrupt Return, Opcode Instruction Clock Description, Iret

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THE 80286 INSTRUCTION SET

IRET-Interrupt Return

Opcode	Instruction	Clock.	Description
CF	IRET	17,pm=31	Interrupt return (far return and pop flags)
CF	IRET	55	Interrupt return, lesser privilege
CF	IRET	169	Interrupt return, different task (NT=1)

"Add one clock for each byte in the next instruction executed.

FLAGS MODIFIED

Entire flags register popped from stack

FLAGS UNDEFINED

None

OPERATION

In real address mode, IRET pops IP, CS, and FLAGS from the stack in that order, and resumes the interrupted routine.

In protected mode, the action of IRET depends on the setting of the Nested Task Flag (NT) bit in the flag register. When popping the new flag image from the stack, note that the IOPL bits in the flag register are changed only when CPL=O.

If NT=O, IRET returns from an interrupt procedure without a task switch. The code returned to must be equally or less privileged than the interrupt routine as indicated by the RPL bits of the CS selector popped from the stack. If the destination code is of less privilege, IRET then also pops SP and SS from the stack.

If NT= 1, IRET reverses Jhe operation of a CALL or INT that caused a task switch. The task execut- ing IRET has its updated state saved in its Task State Segment. This means that. if the task is re- entered, the code that follows IRET will be executed.

The exact checks and actions performed by IRET in protected mode are given on the following page.

INTERRUPT RETURN:

If Nested Task Flag=1'then

RETURN FROM NESTED TASK:

Examine Back Link Selector in TSS addressed by the current Task Register: Must specify global in the local/global bit else #TS (new TSS selector) Index must be within GDT limits else #TS (new TSS selector)

AR byte must specify TSS else #TS (new TSS selector) New TSS must be busy else #TS (new TSS selector)

Task State Segment must be PRESENT else #NP (new TSS selector) SWITCH_TASKS without nesting to TSS specified by back link selector Mark the task just abandoned as NOT BUSY

IP must be in code segment limit else #GP(O)

B-51