Printing a character

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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: 20335
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: 20335
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: 4037
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

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