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Posted: **Mon Sep 19, 2011 6:46 pm**

Hello. After some work, I managed to get into 32-bit protected mode. I can access some memory correctly, but not all.

[bits 16]
[org 0x8000] ; stage1 loads me at 0x0000:0x8000
...
set video mode, disable interrupts, enable A20, load gdt, set cr0 LSB
jmp 0x8:pmode_start
...
pmode_start:
[bits 32]
mov  al,[tmpvar]
mov  ebx,[tmpvar2]
mov  [0xb8000+ebx],al
mov  byte [0xb8000+ebx+1],0x7

jmp hang

tmpvar db 'A'
tmpvar2 dw 0

I would expect this to write 'A' at the top left corner. It does not. However, if I replace "[tmpvar2]" with "0", it does write 'A'. So it seems like I can get to tmpvar, but not tmpvar2. Can anyone shed some light?

Posted: **Mon Sep 19, 2011 7:02 pm**

I think you need

Code: Select all

tmpvar2 dd 0

or else you will be loading garbage into the upper word of ebx

Posted: **Tue Sep 20, 2011 3:30 am**

Usually following logic is used:

Code: Select all

  movzx ebx,[superpos]
  mov [videobuf+ebx*2],al
  mov byte [videobuf+ebx*2+1],7
  ...
  superpos dw 0

Posted: **Tue Sep 20, 2011 6:49 am**

gerryg400 wrote:I think you need
Code: Select all
tmpvar2 dd 0
or else you will be loading garbage into the upper word of ebx

Facepalm

It works now.... Thanks.

OSDev.org

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