Why won't pop work inside a function call? (NASM)

Question

I'm trying to pass arguments to a function call by pushing them onto the stack, then popping them into registers inside the function like this:

    push keystring+0x7c00
    call PrintString
    jmp $

;====functions====

PrintString:
    pop ecx
    mov ah, 0x0e
LoopHead:
    mov bl, 0
    mov al, [ecx]
    int 0x10
    inc ecx
    add bl, [ecx]
    jnz LoopHead
    ret

;====variables====

keystring:
    db "Press any key...", 0

;====padding====

    times 510-($-$$) db 0
    db 0x55, 0xAA

It works fine if I do it by storing the argument in ecx and using ecx in the function, but for some reason pop just won't work. Does anybody know how I can fix this?

Answer 1

I figured it out guys, I had to pop it into cx twice. I couldn't of figured it out without y'alls suggestions though, like trying 16 bit addresses, and knowing that the return address gets pushed to the stack automatically. Here's the fixed code if anyone's interested:

    org 0x7c00
    push keystring
    call PrintString
    jmp $

;====functions====

PrintString:
    pop di
    shl edi, 16
    pop di
    push eax
    push ebx
    push ecx
    xor ecx, ecx
    mov cx, di
    mov ah, 0x0e
LoopHead:
    mov al, [ecx]
    int 0x10
    inc ecx
    mov bl, [ecx]
    add bl, 0
    jnz LoopHead
    pop ecx
    pop ebx
    pop eax
    shr edi, 16
    push di
    ret

;====variables====

keystring:
    db "Press any key...", 0

;====padding====

    times 510-($-$$) db 0
    db 0x55, 0xAA