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assemblyemu8086

Passing offset difference or constants inside macro

It's a university assignment, so emu8086 must be used. With no emu8086.inc

Let's say I have

msg db "Hello"
msgend:
msglen1 equ $ - msg
msglen2 db $ - msg

Then:

mov ax, msgend - offset msg ; ax gets the correct length
mov ax, msglen1             ; correct length
mov ax, msglen2             ; correct length

; same names as offsets to make it clear, 
; how which parameters would be passed. 
; But not the same in real code.
TESTMACRO macro msg msgend msglen1 msglen2
    mov ax,    msg ; correct offset
    mov ax, msgend ; correct offset
    mov ax, msgend - offset msg ; zero length
    mov ax, msglen1             ; again zero length
    mov ax, msglen2             ; correct length, but it used up a word
endm

So. I can't pass the length of the string inside the macro in any way, except by passing the length allocated in a word. But I'm interested, if I can do it with a equ constant.

Solution

  • Use = to define your equate instead of equ.

    The value of an equ equate is evaulated at the point of usage, which in your case means that the $ in msglen1 is replaced by the address of the mov ax, msgLen1 instruction.

    The value of an = equate is evaulated at the point of definition, which should give you the value you want in this case.