Question

I am trying to learn some Assembler code; specifically how to use the stack and how it works, etc. However, I run into an error when I use the code below:

entry:
    push 0x42
    call teststack
    jmp hang

hang:
    jmp hang

teststack:
    mov ah, 0x0e
    pop al
    mov bh, 0x00
    mov bl, 0x04
    mov cx, 0x01
    int 10h
    ret

What I am trying to do is pass the number 42 (life, universe, everything :D) into my teststack procedure, where it will print Ascii 0x42 (I think capital B).
My issue is a compiler error, for the line that reads pop al:

**error: invalid combination of opcode and operands**

I am using NASM inside a Windows 98 virtual machine.
Any help with my error would be greatly appreciated.

Answer 1

Didn’t ever use NASM but I presume that you are in 16 bit mode and pop is by default using 16 bit registers, so you can’t use ah. When you pushed 0x42 to stack that was a 16 bit value (0x0042). It’s safe to use pop ax and then set ah :

entry:
    push 0x42
    call teststack
    jmp hang

hang:
    jmp hang

teststack:
    pop ax           ; ax = 0x0042
    mov ah, 0x0e     ; ax = 0x0e42
    mov bh, 0x00
    mov bl, 0x04
    mov cx, 0x01
    int 10h
ret