Newbie

[Alexander]

hello all,

i have declared a global descriptor as below:


gdt:

null_descriptor:

dw 0 ; seg_length1_15
dw 0 ; base_addr0_15
db 0 ; base_addr16_23
db 0 ; dflags
db 0 ; access
db 0 ; base_addr24_31

code_descriptor:

dw 0xFFFF ; seg_length1_15
dw 0 ; base_addr0_15
db 0 ; base_addr16_23
db 0x9A ; dflags
db 0xCF ; access
db 0 ; base_addr24_31

data_descriptor:

dw 0xFFFF
dw 0
db 0
db 0x92
db 0xCF
db 0

video_descriptor:

dw 0x00EF
dw 0xb800
db 0
db 0x92
db 0xCF
db 0

gdt_end:

gdt_descriptor:
dw gdt_end - gdt - 1
dd gdt



But, when i try to move the character 'a' into the video segment, it fails. The coding for moving is also given.


mov ax, 10h
mov ds, ax
mov byte [ds:0], 'A'
mov byte [ds:1], 1BH


Please, help me out of this problem.


Thanks in advance.

[Adek336]

Is your code located at segment 0x0000? Rememeber, GDTR.base and GDT.base are absolute. You've got to add CS<<4 to them to work.

it is the gdt register, not the gdt descriptor.

Cheers,
Adrian

[DaveHK]

also, from the code you give, your video segment if 0x18 not 0x10

[nicky_girl]

also, from the code you give, your video segment if 0x18 not 0x100x10

[nicky_girl]

also, from the code you give, your video segment if 0x18 not 0x100x10

[pepito]

You need properly switch to protected mode, I recommend you get some step-by-step tutorial. There are some well protected mode tutorials at:

http://osdev.neopages.net/tutorials.php?cat=4&sort=1

The author Chris Giese write the best protected mode examples I have found, but I don´t remeber where I get these examples. Ask him!

There is also an example at:

Intel Architecture Software Developer’s Manual Volume 3: System Programming
CHAPTER 8 PROCESSOR MANAGEMENT AND INITIALIZATION
8.9. INITIALIZATION AND MODE SWITCHING EXAMPLE

You can request a copy at the INTEL web site!

pepito

[pepito]

OK, you need at least this steps to switch to protected mode:

1) Create a GDT (Your code have one!)

2) Load the GDTR with the base and limit of your GDT:

   lgdt [gdt_descriptor]      

   (but be careful about what Adrian say!)

3) Set the PE bit at CR0 register:

   mov eax, cr0            
   or al, 1
   mov cr0, eax

4) Do a far jump to enable protected mode, you need a label before your PM code:

   jmp 8:some_label   ; Code descriptor

   [BITS 32]
   some_label:

   ; Update data segment registers

   mov ax, 16 ; Data descriptor
   mov ds, ax
   mov es, ax
   mov fs, ax
   mov gs, ax

   ; HERE YOU CAN TRY YOUR VIDEO CODE !!!

   mov ax, 24 ; Video descriptor
   mov ds, ax
   mov byte [ds:0], 'A'
   mov byte [ds:1], 1BH

pepito