CODEWITHSUNDEEP
assemblyoperating-systeminterrupt-handling
Amit Singh Tomar
Amit Singh Tomar

Reputation: 8608

Difference between return from interrupt(RTI) and return from subroutine(RTS)

I would like to know what is difference between return from interrupt(RTI) and return from subroutine(RTS). Are both are the same or there is any difference between these two?

Upvotes: 7

Views: 22458

Answers (7)

Skar
Skar

Reputation: 642

RTE or RTI takes minimal context from top of the stack, while RTS takes return address (from top of the stack)

Upvotes: 0

Kapil
Kapil

Reputation: 71

RET:  pop ip ; its "brother" is *near* CALL - to the same segment 

RETF: pop ip ; *far* CALL - there's return path of segment and offset on the 
      pop cs ;  stack 

IRET: pop ip ; INT - this instruction PUSHes *three* registers on stack: 
      pop cs ; IP, CS, and FLags 
      pop fl ; well, in fact as for the stack operation INT is similiar 
             ; to far CALL but with the FL put on the stack

Upvotes: 0

abhilash
abhilash

Reputation: 11

RET will just pop the two bytes to PC (Addr low and Addr High) RETI will reset the interrupt enable flipflop and two bytes will be poped from the stack

Upvotes: 1

Olof Forshell
Olof Forshell

Reputation: 3284

When a hardware interrupt occurs on an x86 the flags and return code segment+offset are pushed onto the stack. Then interrupts are disabled. This is to set the stage for the interrupt routine to service the interrupt: switch stacks or whatever it wants to do before either re-enabling interrupts and processing some more before/or returning from the interrupt. The iret instruction pops the previously saved flags (including the interrupt flag which was originally enabled) and the return location so that the interrupted routine can continue processing none the wiser.

Upvotes: 3

Curd
Curd

Reputation: 12457

The fact that a processor is handling an interrupt is marked in one or more flags in the status register of a CPU.

This is needed, e.g. to mask other potential interrupts.

In order to tell the processor that interrupt handling is over and the flag can be reset, the RTI instruction is used instead of just RTS.

Of course the details depend on the particular CPU.

Upvotes: 1

old_timer
old_timer

Reputation: 71606

Usually return from interrupt restores the flags so that the interrupted code can continue to execute properly. Return from subroutine does not need to do that instruction is used intentionally in that flow of code and known that the flags are or are not destroyed depending on the architecture. In architectures that use a stack for the return address it is very apparent. A return from interrupt will pop the flags then the return address where a return from subroutine will pop only the return address.

Upvotes: 4

BlackBear
BlackBear

Reputation: 22989

Well, interrupts are stored in a different code segment, so RTI are called with a far-call, while subroutines usually stays in the same code segment as the main program, so they're called with a near-call. The difference is that far-calls push the segment and the offset as return address, while near-calls push only the offset.

Upvotes: 1

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