View Full Version : GDT Protected Mode Programming

June 28th, 2012, 07:23 AM
I am trying to write a program that enter the protected mode and show a 'P', but it doesn't work. The code is shown below, anyone can solve this problem? Thanks

/* Boot file of nosdl;
* This file is loaded at 0x8000;
* Enter the protected mode and run init;
* The max size of this file is 8k;

#define BOOTSEG 0x8000 /* where I am in; */

/* Generate a gdt with 32bits base, 20bits limit and 12bits flag;
* We use a 32bits limit (low 20bits available) and
* 16bits flag (lower 4bits of higher byte are always 0) here;

.globl _start
movl $0xB800,%eax
movl %eax,%gs
mov $'0',%al
mov $0x0C,%ah
movl %eax,%gs:((80*0+5)*2)
/* Prepare to enter the protected mode; */
/* Clear interrupt flags; */
/* Load GDT; */
lgdt gdtptr
/* Reset registers; */
movl $sel_kdata,%eax
movl %eax,%ds
movl %eax,%es
movl %eax,%fs
movl %eax,%gs
/* Open A20 Line; */
inb $0x92,%al
orb $0b00000010,%al
outb %al,$0x92
/* Set cr0, enter protected mode; */
movl %cr0,%eax
orl $0x01,%eax
movl %eax,%cr0
ljmp $0x10,$(__enter32)
* Here we enter the 32bits world;
* The next step is init idt and other things;
/* Display a P; */
xorl %eax,%eax
movl $sel_video,%eax
movl %eax,%gs
movl $((80*0+10)*2),%edi
mov $0x0F,%ah
mov $'P',%al
movl %eax,%gs:(%edi)
jmp .
/* GDT table; */
gdt: .quad 0x0000000000000000 /* Not used; */
gdt_kstack: .quad 0x00CF92000000FFFF /* Reserved for kernel stack; 0x08; */
gdt_kcode: .quad 0x00CF9A000000FFFF /* Kernel code segment; 0x10; */
gdt_kdata: .quad 0x00CF92000000FFFF /* Kernel data segment; 0x18; */
gdt_ustack: .quad 0x00CFF2000000FFFF /* Reserved for user stack; 0x20; */
gdt_ucode: .quad 0x00CFFA000000FFFF /* User code segment; 0x28; */
gdt_udata: .quad 0x00CFF2000000FFFF /* User data segment; 0x30; */
gdt_video: .quad 0x00CFF200B800FFFF /* Video buffer; 0x38; */
.quad 0x0000000000000000 /* Not used; */
.set gdtlen, (.-gdt) /* GDT Length; */
gdtptr: .2byte (gdtlen-1) /* GDT Limit; */
gdtbase: .4byte gdt /* GDT Base address; */
/* Selectors; */
.set sel_kcode, gdt_kcode-gdt
.set sel_kdata, gdt_kdata-gdt
.set sel_ucode, gdt_ucode-gdt
.set sel_udata, gdt_udata-gdt
.set sel_video, gdt_video-gdt

June 28th, 2012, 07:28 AM
The development environment is consists of:
CentOS 6.2 (x86_64)
gcc 4.4.6
make 3.81
virtualbox 4.1.16 (Used to boot the floppy image)

June 28th, 2012, 03:30 PM
Need to know more than just the code - how are you building it and trying to run it?
Can you please post your Makefile?