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.code16 # Tell GAS to generate 16 bit code
.global _start # Make the symbol visible to ld
.include "bios.inc"
# Define some constants for the GDT segment descriptor offsets
.set CODESEG, gdt_code - gdt_start
.set DATASEG, gdt_data - gdt_start
.set SYSSEG, 0x1000 # Historical load address
.set MAGIC, 0xAA55
.section .text.bootentry # Code that will start executing at
# special address (specified in linker)
_start:
jmp $0x0, $_start2 # Normalize the start address
# CS = 0 and IP = _start2
_start2:
mov %cs, %ax # AX = CS = 0 (see above)
mov %ax, %ds # Zero segment registers
mov %ax, %es
mov %ax, %ss
mov %ax, %sp
cld # Set direction flag for incrementing
mov %dl, boot_drive # BIOS stores our boot drive in DL,
# so we remember it
BIOS_PRINT $boot_real_mode_msg
load_kernel: # Load our kernel
BIOS_PRINT $boot_load_kern_msg
mov $0x02, %ah # Set BIOS read sector routine
mov boot_drive, %dl # Read drive number from $boot_drive
mov $0x00, %ch # Select cylinder 0
mov $0x00, %dh # Select head 0 [has a base of 0]
mov $0x02, %cl # Select sector 2 (next after the
# boot sector) [has a base of 1]
mov $0x01, %al # Read 1 sectors
mov $SYSSEG, %bx # Load sectors to ES:BX (0:$SYSSEG)
int $0x13 # Start reading from drive
jc disk_error # If carry flag set, bios failed to read
# FIXME: we must compare different register
cmp %al, %al # If AL(sect. read) != <>(sect. expected)
jne disk_error # then return disk error
switch_to_pm:
BIOS_PRINT $boot_prot_mode_msg
cli # Switch of interrupt until we have set
# up the protected mode interrupt vector
lgdt gdt_descriptor # Load our global descriptor table
mov %cr0, %eax # Set the first bit of CR0
or $0x01, %eax # to make the switch to protected mode
mov %eax, %cr0
# Make a far jump to our 32-bit code.
# This also forces the CPU to flush its cache of pre-fetched
# and real-mode decoded instructions, which can cause problems
jmp $CODESEG, $init_pm
disk_error:
BIOS_PRINT $disk_error_msg
jmp .
.code32
init_pm:
mov $DATASEG, %ax # Point segment registers to the
mov %ax, %ds # data selector we defined in our GDT
mov %ax, %es
mov %ax, %ss
mov %ax, %fs
mov %ax, %gs
mov $0x90000, %ebp # Update stack position so it is right
mov %ebp, %esp # at the top of the free space.
jmp . # infinite loop
# Global Descriptor Table (contains 8-byte entries)
gdt_start:
gdt_null: # The mandatory null descriptor
.quad 0x0
gdt_code: # The code segment descriptor
# Base = 0x0, limit = 0xfffff
# 1st flags: (present)1 (privilege)00 (descriptor type)1 -> b1001
# Type flags: (code)1 (conforming)0 (readable)1 (accessed)0 -> b1010
# 2nd flags: (granularity)1 (size)1 (64-bit seg)0 (AVL)0 -> b1100
.word 0xffff # Limit (bits 0-15)
.word 0x0 # Base (bits 0-15)
.byte 0x0 # Base (bits 16-23)
.byte 0b10011010 # 1st flags, type flags
.byte 0b11001111 # 2nd flags, limit (bits 16-19)
.byte 0x0 # Base (bits 24-31)
gdt_data: # the data segment descriptor
# Same as code segment except for the type flags:
# Type flags: (code)0 (direction)0 (writable)1, (accessed)0 -> b0010
# P.S: direction bit: 0 the segment grows up
.word 0xffff # Limit (bits 0-15)
.word 0x0 # Base (bits 0-15)
.byte 0x0 # Base (bits 16-23)
.byte 0b10010010 # 1st flags, type flags
.byte 0b11001111 # 2nd flags, limit (bits 16-19)
.byte 0x0 # Base (bits 24-31)
gdt_end:
# Global variables
boot_drive:
.byte 0
gdt_descriptor:
# The 6-byte GDT structure containing:
# - GDT size, 2 bytes (size always less one of the real size):
.word gdt_end - gdt_start - 1
# - GDT address, 4 bytes:
.long gdt_start
boot_real_mode_msg:
.asciz "Started mfsos in 16-bit real mode\r\n"
boot_prot_mode_msg:
.asciz "Entering 32-bit protected mode\r\n"
boot_load_kern_msg:
.asciz "Loading kernel into memory\r\n"
disk_error_msg:
.asciz "Disk read error!"
# Bootsector padding
.space 512 - 2 - (. - _start), 0
.word MAGIC
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