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.code16 # Tell GAS to generate 16 bit code
.global _start # Make the symbol visible to ld
.include "bios.inc"
.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_os_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 $0x9000, %bx # Load sectors to ES:BS (0x9000)
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
jmp . # TODO: jump into the darkness
disk_error:
BIOS_PRINT $disk_error_msg
jmp .
# Global variables
boot_os_msg:
.asciz "Booting mfsos...\r\n"
disk_error_msg:
.asciz "Disk read error!"
boot_drive:
.byte 0
# Bootsector padding
.space 512 - 2 - (. - _start), 0
.word MAGIC
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