Protected Mode Boot Sector Help
From: Ryan Sommers (ryans_at_gamersimpact.com)
Date: 04/25/04
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Date: Sat, 24 Apr 2004 23:20:44 +0000 (UTC)
I'm attempting to write a bootsector that enables the A20 line and
then puts the processor into protected mode. I've been stewing over
this code the past few days trying to find out why it's failing. I'm
using Bochs on FreeBSD to test. Any help would be greatly appreciated.
It seems to be failing after I enable the PE bit in the cr0 register.
Here is the ASM code:
.set SEL_CODE, 0x8
.set SEL_LINEAR, 0x10
.org 0x000
.code16
ljmp $0x7C0, $start # far jump to load the CS with known value
# some BIOS' start at 07c0:0000 and others
# at 0000:7c00
.globl start
start:
movw $0x7C0, %ax # load known address
movw %ax, %ds # into the segment
movw %ax, %es # registers.
movw %ax, %fs
movw %ax, %gs
xorw %ax, %ax # setup
movw %ax, %ss # ... the
movw $0xffff, %sp # ... stack
movw $bootmessage, %si # put something
call putstr # on the screen
call gateA20 # enable the A20 line
pushw $0x2
popf
lgdt (gdt_ptr+0x7C00) # Initialize GDTR, need a linear address
# not a segmented one.
movl %cr0, %eax
orb $0x1, %al
movl %eax, %cr0
ljmp $SEL_CODE, $pmode # long jump to reload %cs
gateA20:
push %si
push %ax
movw $A20Message, %si
call putstr
inb $0x92, %al
orb $0x02, %al
outb %al, $0x92
pop %ax
pop %si
ret
putstr:
push %ax
push %bx
movb $0x0e, %ah
movw $0x0007, %bx
putstr.0:
lodsb
testb %al, %al
jz putstr.1
int $0x10
jmp putstr.0
putstr.1:
pop %bx
pop %ax
ret
.code32
pmode:
jmp .
# Enter protected mode
bootmessage: .asciz "SOS Bootloader (v.01)\n"
A20Message: .asciz "Enabling A20 line...\n"
pmodemsg: .asciz "Jumping to PMode...\n"
# GDT
# Segment Descriptor Layout:
#
# 31 24 23 22 21 20 19 16 15 14 13 12 11 8 7
0 Byte
# _________________________________________________________________________
# | | |D | |A | Seg | | D | | |
|
# | Base 31:24 |G |/ |O |V | Limit |P | P |S | Type | Base
23:16| 4
# | | |B | |L | 19:16 | | L | | |
|
# |___________________|__|__|__|__|________|__|_____|__|________|___________|
#
# 31 16 15
0
# _________________________________________________________________________
# | |
|
# | Base Address 15:00 | Segment Limit 15:00
| 0
# |________________________________________|________________________________|
.org 0x0120
gdt_ptr: .word gdt_end-gdt-1
.word (gdt+0x7C00)
.word 0x0
.word 0x0 # align the GDT on an 8-byte boundary
.org 0x130
gdt:
gdt.0: # Null selector
.word 0 # [Off. 0] Segment Limit (15:00)
.word 0 # [Off. 2] Base Address (15:00)
.byte 0 # [Off. 4] Base Address (23:16)
.byte 0 # [Off. 5] Flags
.byte 0 # [Off. 6] Seg Limit & Flags
.byte 0 # [Off. 7] Base Address (31:24)
gdt.1: # Code (0x7C00 base)
.word 0xFFFF # [Off. 0] Segment Limit (15:00)
.word 0x7C00 # [Off. 2] Base Address (15:00)
.byte 0x00 # [Off. 4] Base Address (23:16)
.byte 0x9A # [Off. 5] Flags
.byte 0xCF # [Off. 6] Seg Limit & Flags
.byte 0x00 # [Off. 7] Base Address (31:24)
gdt.2: # Linear (0x0 base)
.word 0xFFFF # [Off. 0] Segment Limit (15:00)
.word 0x0 # [Off. 2] Base Address (15:00)
.byte 0x0 # [Off. 4] Base Address (23:16)
.byte 0x92 # [Off. 5] Flags
.byte 0xCF # [Off. 6] Seg Limit & Flags
.byte 0x0 # [Off. 7] Base Address (31:24)
gdt_end:
.org 0x01fe
.word 0xaa55
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