User's Manual

inter
THE 80286 INSTRUCTION SET
IRET -Interrupt Return
Opcode
CF
CF
CF
Instruction
IRET
IRET
IRET
Clock.
17,pm=31
55
169
Description
Interrupt return (far return and pop flags)
Interrupt
return, lesser privilege
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