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Routine calls in Arm

Japan Dave
Note: This was originally posted on 19th July 2010 at http://forums.arm.com

I have a quetion about mechanism used while making nested routine calls in ARM based products.

Say suppose at an instant my core is executing the routine A and routine B is invoked by encountering the instruction.

As it tells, the address of the immidiate instruction of routine A from where the execution turned its stream towards B is stoed on list register. Uptill this it is clear like crystal.

But what if now while routine B is executing any interrupt occurs or any call is made to say routine C . . .
Now we need to store the address of the next instruction of the B routine . .
If we store it onto the list register then how can we preserve its previous value which points to immediate instruction of routine A.

Pls eloberate on this.


Thanks in advance,
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  • Arm Employee Badge
    Note: This was originally posted on 19th July 2010 at http://forums.arm.com

    Ignoring specific optimisations, the general solution is to have a "stack" in memory, and to push the old link-register value onto the stack before making another call. Arbitrary depth calls can then be made until memory is exhausted. Example code might be:

    myfunc
      PUSH {LR} ; store old return address onto the stack
      BL myfunc ; call myfunc again / another procedure
      POP {LR}  ; restore old return address from stack
      BX LR  ; return from myfunc

    hth
    s.
Reply
  • Arm Employee Badge
    Note: This was originally posted on 19th July 2010 at http://forums.arm.com

    Ignoring specific optimisations, the general solution is to have a "stack" in memory, and to push the old link-register value onto the stack before making another call. Arbitrary depth calls can then be made until memory is exhausted. Example code might be:

    myfunc
      PUSH {LR} ; store old return address onto the stack
      BL myfunc ; call myfunc again / another procedure
      POP {LR}  ; restore old return address from stack
      BX LR  ; return from myfunc

    hth
    s.
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