division bug?

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kty11

Joined: 30 Dec 2012
Posts: 21
Location: south korea

kty11 11 Jan 2013, 06:41

is there anyone suffer division bug in dos programming?
following code with dx not 0, it shutdown

Code:

org 100h
mov dx, 0
mov ax, 10
mov bx, 10
div bx
add ax, '0'
int 29h   
mov ax, 00h
int 16h
mov ax, 4c00h
int 21h

and following code with dx greater than 0xf, it shut down

Code:

org 100h
mov dx, 0xf
mov ax, 10
div word [divby]
add ax, '0'
int 29h
mov ax, 00h
int 16h
mov ax, 4c00h
int 21h
divby:
        dw 0x10

I have no idea why I can't divide.
any solution?

_________________
since 1990 November 4th

11 Jan 2013, 06:41

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20761
Location: In your JS exploiting you and your system

revolution 11 Jan 2013, 06:55

DX:AX / divby == 0x000f0010 / 0x0010.

It will overflow with the high portion in DX greater than the divisor.

11 Jan 2013, 06:55

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