大会中的单词大小mul

Question

I'm making a subroutine for a factorial operation. All of my local variables are in word size since I'm using stack for pass-by-value.

[ebp+4] is the counter/decrementing multiplier
[ebp-2] is the multiplicand

Currently, I have this code block like this:

while1:

    cmp byte[ebp+4], 1
    je exit1
    dec word[ebp+4]
    mov ax, [ebp-2]
    mov dx, 0
    mul byte[ebp+4]
    mov word[ebp-2], ax

    jmp while1

And it has these following input and output results:

1 -> 02561
2 -> 2
3 -> 6
4 -> 24
5 -> 120
6 -> 208
7 -> 176
8 -> 128

It's only correct from 2 to 5. I realized that I'm making a byte-sized multiplication because of mul byte[ebp+4] , thus, hindering the result of 6! = 720. So I changed mul byte[ebp+4] to mul word[ebp+4] :

while1:

    cmp byte[ebp+4], 1
    je exit1
    dec word[ebp+4]
    mov ax, [ebp-2]
    mov dx, 0
    mul word[ebp+4]
    mov word[ebp-2], ax

    jmp while1

It then has these following input and output results:

1 -> 02561
2 -> 07682
3 -> 28166
4 -> 62488
5 -> 46200
6 -> 60112
7 -> 35760
8 -> 15744

What am I doing wrong? I already cleared in the dx before the mul operation, but why is it not functioning properly?

Note: I have the correct printing routine of a word-size variable to 5 characters in ASCII for stdout. Also, I have a special case of 0! done in cmp before the iteration code block.

Im using Ubuntu 13 x86 and NASM.

---

update. here's the code block right on top of the while1 :

fact: 

    mov ebp, esp
    sub esp, 2
    mov ax, [ebp+4]
    mov word[ebp-2], ax

    ; checks if num is zero
    cmp byte[ebp+4], 0
    je one

Answer 1

If cmp word[ebp+4], 1 isn't working, that indicates that what's at word[ebp+4] may not be what you expect or what it should be. The upper byte of the word is evidently not 0 . Using the byte compare instead is covering up an issue somewhere. So you'll need to examine what's happening with the word at that location.