Printing a character

flat assembler
Message board for the users of flat assembler.

Index > OS Construction > Printing a character

Author

Thread

manfred

Joined: 28 Feb 2009
Posts: 43
Location: Racibórz, Poland

manfred 07 Mar 2009, 18:07

Hello!

I'm trying to print character in protected mode, but character doesn't print. I noticed that the value ecx is incorrect (for example 0xf000fea5, what isn't value of video_mem variable).

Code:

Code:

format BINARY

include './macros.inc'

use32
kmain:
  mov al, 'a'
  mov ah, 7
  call putc
  jmp $
  
; void putc(al = char, ah = color)  
putc:
  mov ecx, dword [ds:video_mem]
  mov [ecx], al
  mov [ecx + 1], ah
  add dword [ds:video_mem], 2
  ret

;data
  video_mem dd 0B8000h

_________________
Sorry for my English...

07 Mar 2009, 18:07

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20339
Location: In your JS exploiting you and your system

revolution 07 Mar 2009, 18:31

Are you using a 32bit OS? You need a 32bit OS to use native 32bit instructions.

07 Mar 2009, 18:31

manfred

Joined: 28 Feb 2009
Posts: 43
Location: Racibórz, Poland

manfred 07 Mar 2009, 20:59

32bit OS? I don't understand... I've entered into protected mode in bootloader and jumped to kernel... Other 32-bit instructions works. If i write ie. something like that:

Code:

mov ecx, 0xB8000
mov byte [ecx], 'a'
mov byte [ecx+1], 7

it works. Only that one value is incorrect. Or, if you talking about this, I'm compiling under 32bit OS...

_________________
Sorry for my English...

07 Mar 2009, 20:59

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20339
Location: In your JS exploiting you and your system

revolution 08 Mar 2009, 00:23

Okay, if you are in 32bit protected mode then that is fine.

You can check out that your ds segment is setup to map 1:1 with the cs segment and also that the memory you are accessing is in the paging tables.

08 Mar 2009, 00:23

egos

Joined: 10 Feb 2009
Posts: 144

egos 08 Mar 2009, 09:11

Code:

  mov ecx, dword [ds:video_mem]
  ...
  add dword [ds:video_mem], 2
  ...
  video_mem dd 0B8000h

What it means? Usually character output is made relative to cursor position. Additionally, you are using video_mem variable, but I don't see where you defined the starting displacement (using ORG directive).

08 Mar 2009, 09:11

bitRAKE

Joined: 21 Jul 2003
Posts: 4042
Location: vpcmpistri

bitRAKE 08 Mar 2009, 10:32

DS is the default segment.
What does your GDT look like?
What has been loaded into DS?

08 Mar 2009, 10:32

manfred

Joined: 28 Feb 2009
Posts: 43
Location: Racibórz, Poland

manfred 08 Mar 2009, 10:34

After setting origin it works. Thanks egos!

_________________
Sorry for my English...

08 Mar 2009, 10:34

< Last Thread | Next Thread >

Forum Rules:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum