How it works XLAT?

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Andy

Joined: 17 Oct 2011
Posts: 55

Andy 02 Mar 2012, 12:37

I tried to understand how it works XLAT instruction and I wrote this short code for test:

Code:

mov bx,Table
mov al,4
xlatb
mov dl,al
mov ah,2
int 21h

mov ah,4Ch
int 21h

Table db '0123456789'

The compilations works fine but the result is not what I really expect. If I understood well, XLAT should put in al the ascii value of 4, this mean 34h, so I do expect to be write to STDOUT this char:

Where is my wrong?

02 Mar 2012, 12:37

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20513
Location: In your JS exploiting you and your system

revolution 02 Mar 2012, 12:38

Check your DS segment register.

Code:

org 0x100
mov ax,cs
mov ds,ax
mov bx,Table
;...

02 Mar 2012, 12:38

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8367
Location: Kraków, Poland

Tomasz Grysztar 02 Mar 2012, 12:47

revolution: .COM programs have CS=DS=ES=SS at startup, all with the value of PSP. It is enough to add "org 100h" in the beginning to get this working.

02 Mar 2012, 12:47

Andy

Joined: 17 Oct 2011
Posts: 55

Andy 02 Mar 2012, 12:49

I spent 2 hours to check what I'm doing wrong and all this because I didn't set the origin? I'm feel so dumb. Well this is the way that go all beginners. It's good to have on whom you ask.

Thanks revolution!

02 Mar 2012, 12:49

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20513
Location: In your JS exploiting you and your system

revolution 02 Mar 2012, 13:04

Tomasz Grysztar wrote:

revolution: .COM programs have CS=DS=ES=SS at startup, all with the value of PSP. It is enough to add "org 100h" in the beginning to get this working.

Thanks. It's been a while since I wrote a DOS program. I guess I was thinking of a boot sector where the segment registers need some coaxing to get them in the right position.

02 Mar 2012, 13:04

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8367
Location: Kraków, Poland

Tomasz Grysztar 02 Mar 2012, 13:13

Writing .COM programs was really like flat memory model programming, especially when combined with FRM. Wink

02 Mar 2012, 13:13

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20513
Location: In your JS exploiting you and your system

revolution 02 Mar 2012, 13:40

offtopic: That was your 5000th post! Hehe, not that it actually means anything of course.

02 Mar 2012, 13:40

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20513
Location: In your JS exploiting you and your system

revolution 02 Mar 2012, 13:42

Andy wrote:

I spent 2 hours to check what I'm doing wrong and all this because I didn't set the origin? I'm feel so dumb. Well this is the way that go all beginners. It's good to have on whom you ask.

We all gotta start somewhere. You came to the right place to solve the problem.

Andy wrote:

Thanks revolution!

Y'r welcome.

02 Mar 2012, 13:42

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8367
Location: Kraków, Poland

Tomasz Grysztar 02 Mar 2012, 13:48

revolution wrote:

offtopic: That was your 5000th post! Hehe, not that it actually means anything of course.

Well, I think that writing about flat model was a very good topic for a 5000th post. Smile

I did not notice that, however.

02 Mar 2012, 13:48

AsmGuru62

Joined: 28 Jan 2004
Posts: 1692
Location: Toronto, Canada

AsmGuru62 02 Mar 2012, 14:55

COM files were actually quite awesome.
There is about 64Kb of code and around 560Kb of memory!
I once written large stuff with these - in 1989!!

02 Mar 2012, 14:55

avcaballero

Joined: 02 Feb 2004
Posts: 212
Location: Madrid - Spain

avcaballero 07 Mar 2012, 17:09

Some time ago I did something similar with xlat:

http://www.abreojosensamblador.net/Productos/AOE/html/Pags_en/Chap07.html

Search for "7.1.1.6. XLAT"

You have to follow the program execution with a debugger:

Greetings.

It would also be good that you look in Ralph Brown Interrupt List for xlat Wink

. Well, maybe not here, sure, not an interrupt, where did I read it?, maybe here (only spanish text ha ha Wink

):

http://www.gui.uva.es/udigital/

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_________________
Siempre aprendiendo

07 Mar 2012, 17:09

16bitPM

Joined: 08 Jul 2011
Posts: 30

16bitPM 29 Mar 2012, 19:35

Tomasz Grysztar wrote:

Writing .COM programs was really like flat memory model programming, especially when combined with FRM.

OK my turn to feel dumb: what's FRM?

29 Mar 2012, 19:35

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8367
Location: Kraków, Poland

Tomasz Grysztar 29 Mar 2012, 20:42

16bitPM wrote:

OK my turn to feel dumb: what's FRM?

The Flat Real Mode.

29 Mar 2012, 20:42

16bitPM

Joined: 08 Jul 2011
Posts: 30

16bitPM 30 Mar 2012, 18:21

Ow.. that one... never been a fan.

30 Mar 2012, 18:21

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