Swap between two variables by reference in assembly 8086-CodePudding

I need to make a function that swaps two variables by reference (Transferring the values should be on stack, not through registers), when i try to check if the values swapped I have an error (on emu8086): (11) wrong parameters: CMP [ax], [bx] (11) probably it's an undefined var: [ax]

org 100h 
include print.inc   ;printing library
start:
    x db 0x7Fh
    y db 0x80h
    lea ax, x   ;get address
    lea bx, y   ;get address 
    push ax
    push bx
    call swap
    cmp [ax], [bx]
    jl label1;check if values swapped
    jmp end2

label1:
    PRINTN "values swapped"   ;print if the values swapped
end2:
mov ah, 0
int 16h
ret


proc swap   ;swap function
    push bp
    mov bp, sp
    mov cx, [bp   4]
    xchg cx, [bp   6]
    mov [bp   4], cx  
    jmp end1

end1:
    pop bp
    ret
endp swap

CodePudding user response：

I'm guessing you're trying to implement the classic C swap:

void swap(int* a, int* b)
{ 
int temp = *a;
*a = *b;
*b = temp;
}

So I'll break down what needs to be done step-by-step. Firstly, you have your data defined after your start label and then code right after. Unless there's additional hardware abstraction going on here, the CPU will interpret the 0x7F and 0x80 as CPU instructions, doing who knows what. So move your declared data to a different section.

proc swap   ;swap function
    push bp
    mov bp, sp
;sp   0 = bp
;sp   2 = ret
;sp   4 = pushed bx
;sp   6 = pushed ax

I like to have comments like these to remind myself which variable is which, since things get confusing very quickly.

Now, you have to remember that since you used LEA to populate ax and bx, what you have in those registers are pointers to memory. So ax does not equal 0x007F, but rather the address where that 0x007F is stored. So you'll need to dereference the arguments after loading them off the stack.

proc swap   ;swap function
    push bp
    mov bp, sp
;sp   0 = bp
;sp   2 = ret
;sp   4 = pushed bx
;sp   6 = pushed ax
    mov di,[bp   4] ;loads &y into di
    mov si,[bp   6] ;loads &x into si
    mov ax,[di]     ;loads 0x0080 into ax
    mov cx,[si]     ;loads 0x007F into cx

;now swap the values at those addresses

    mov [si],ax     ;*a = 0x0080
    mov [di],cx     ;*b = 0x007F
    pop bp
    ret
endp swap