Asembler Question...

[johngb]

I want to be execute a Call (or GoSub) from my code where the label I want to call is held as a Code address in RAM. To try and explain further... I have an array holding the addresses of a number of routines. I want to call these routines so that when the routines complete they will return to the calling routine.

I cannot see how I can do this so what am doing at present is jumping to the routine using the following code. Note Addr is the label of the routine I want to call.

                Movff Addr + 1, PCLATH          ; Set PCH via PCLATH
                Movff Addr, WREG                ; Can't write direct to PCL with movff
                Movwf PCL                       ; Writing to PCL will also load PCLath
RtnLabel:

The called routines at present finish with a GoTo back to Rtnlabel but I would prefer they just did a Return. Is it possible to manipulate the stack so that when the routine finishes with a Return the prgram returns to RtnLabel?

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[fanie]

Hi John,

What's the difference between what you want to do and just calling subroutines in a sequence ?

[johngb]

What's the difference between what you want to do and just calling subroutines in a sequence ?

Everything, I am developing a real time operating system for PDS. This means that the user can write code in the form of tasks and the OS will run those tasks at appropriate times. To do this I keep a list of all the tasks (subroutines) in a table and then when the right conditions are met I run the appropriate task which means calling that task.

Its a bit more complicated than that but that's the gist of it. I would rather the tasks return to the OS rather than jumping to a fixed label as I have implemented at present.

[wastrix]

The called routines at present finish with a GoTo back to Rtnlabel but I would prefer they just did a Return. Is it possible to manipulate the stack so that when the routine finishes with a Return the prgram returns to RtnLabel?

Something like this should work:

main:
    'etc.
    Asm-
    ;setup return address
    Push
    Movlw   ReturnLabel & 255
    Movwf   TOSL
    Movlw   (ReturnLabel >> 8) & 255
    Movwf   TOSH
    Movlw   (ReturnLabel >> 16) & 255   ;writing upper byte not necessary with < 64K program memory
    Movwf   TOSU
    Endasm-
    ;branch to subroutine
    GoTo subroutine
ReturnLabel:
    'etc.
GoTo main


subroutine:
    'etc.
    Return

It's a bit messy, I don't know if this is the best way to do this, but we must manipulate the top of stack. The push instruction is there basically to increment the stack pointer, the actual value placed on the stack by push is overwritten by the correct return address before jumping to the subroutine. The return instruction in the subroutine will pop our modified value off the TOS and branch back to the return label. The stack pointer at this point is decremented back to where it was before and ready for the next call.

[johngb]

Thanks - that's what I needed