CODEWITHSUNDEEP
assemblyx86division
tilikompowaa
tilikompowaa

Reputation: 69

Wrong result after dividing signed integer

Basic question:

I wrote the following block of code:

DATASEG

Result db ?
Modulo db ?

var3 db -9
var4 db 2


CODESEG

start:

    mov ah, 0
    mov al, [var3]
    mov bl, [var4]
    div bl
    mov [Result], al
    mov [Modulo], ah

I get wrong result for -9/2. The result I get is "7B", seems that it treats "F7" as 247.

How can I get this done correctly while still defining var3 as a databyte (db)?

Or, is there any other way?

Thanks for answers

Upvotes: 2

Views: 151

Answers (1)

Sep Roland
Sep Roland

Reputation: 39166

You must use idiv to perform a signed division.

div will not correctly process the negative number in var3

mov al, [var3]
cbw
idiv byte ptr [var4]

From your comments I see that you don't like the cbw instruction and that you don't want to use the shortest code for the division instruction. You can always code it like:

mov   al, [var3]
mov   ah, FFh      ;Only if you 'know' var3 has a negative value!
mov   bl, [var4]
idiv   bl
mov   [Result], al
mov   [Modulo], ah

Alternatively and still avoiding cbw:

movsx ax, byte ptr [var3]
mov   bl, [var4]
idiv   bl
mov   [Result], al
mov   [Modulo], ah

Upvotes: 3

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