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Load function in assembly

W twh

If I have the address of particular function, is it possible load the function in assembly?

I understand that usually the function is loaded as

LCALL _function

but I was wondering whether it is possible to load the function with its address?

Appreciate anyone who can offer me advise on this. Thank you.

Parents
  • 0

    Can you clarify your question? Do you mean that you want to call a function in assembly language when you only have the address of the function to be called stored in a variable or registers?

    It is possible to use something like the (untested) code below. Basicly, the return address and the function pointer are loaded onto the stack and then a RET instruction causes the CPU to pop the address of the function pointer into the PC. When the function executes a RET, the return address will be popped into the PC and the CPU will be back at return_address.

        MOV     A,#LOW(return_address)
    PUSH    Acc
    MOV     A,#HIGH(return_address)
    PUSH    Acc
    MOV     A,function_pointer_low
    PUSH    Acc
    MOV     A,function_pointer_high
    PUSH    Acc
    RET
retern_address:
    ....

Reply
  • 0

    Can you clarify your question? Do you mean that you want to call a function in assembly language when you only have the address of the function to be called stored in a variable or registers?

    It is possible to use something like the (untested) code below. Basicly, the return address and the function pointer are loaded onto the stack and then a RET instruction causes the CPU to pop the address of the function pointer into the PC. When the function executes a RET, the return address will be popped into the PC and the CPU will be back at return_address.

        MOV     A,#LOW(return_address)
    PUSH    Acc
    MOV     A,#HIGH(return_address)
    PUSH    Acc
    MOV     A,function_pointer_low
    PUSH    Acc
    MOV     A,function_pointer_high
    PUSH    Acc
    RET
retern_address:
    ....

Children
  • 0 in reply to Graham Cole

    Basicly, the return address and the function pointer are loaded onto the stack and then a RET instruction causes the CPU to pop the address of the function pointer into the PC.

    That technique has no excuse being used on the 8051, which has a perfectly fine "computed goto" instruction:

           JMP @A+DPTR

    There are platforms where indeed you need such stack hacks to do a computed jump, but the 8051 is, fortunately, not one of them.

  • 0 in reply to HansBernhard Broeker

    Hi,
    Sometimes it needs to keep DPTR unmodified, for example, if you pass a parameter to called function via DPTR. So the usage of "call-via-stack" method is still usable with C51.

    Regards,
    Oleg

  • 0 in reply to Graham Cole

    Thank you all for your replies.

    To clarify my question, what I needed to do was to call a function whose address is stored in variables.

    Anyway, as suggested, I have tried by loading the return address and the function onto the stack and it is working fine.

    Thank you.