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/* Segment constants */
#define BOOT_SEG 0x7C0 /* Segment we reside on startup */
#define BOOT_RELOC_SEG 0x7A0 /* Relocation segment */
.code16
_start:
/* Normalize CS + other selectors */
ljmp $BOOT_SEG,$1f
1: xor %ax, %ax
mov %ax, %ss
mov %ax, %gs
mov %ax, %fs
/*
* The second stage bootloader expects to be loaded to 0x7C00 but since we are
* already executing at this address we'll need to relocate ourselves first before
* we clobber it.
*/
mov $BOOT_SEG, %ax
mov %ax, %ds
mov $BOOT_RELOC_SEG, %ax
mov %ax, %es
xor %si, %si
xor %di, %di
mov $0x200, %cx
cld
rep movsb
/* Jump to our relocated self */
ljmp $BOOT_RELOC_SEG,$reloc
reloc:
/* Save the drive number */
mov %dl, drive_num
/* Set our stack */
mov $0x7C00, %sp
/* Write out our boot string */
mov $boot_msg, %si
call putstr
cli
hlt
jmp reloc
/*
* Write a string to the console
*
* %si: String to write
*
* XXX: Clobbers %AL
*/
putstr:
lodsb
or %al, %al
jz 1f
call putchr
jmp putstr
1: ret
/*
* Write a single character to the console
*
* %al: Character to write
*/
putchr:
push %dx
mov $0x3F8, %dx
out %al, %dx
pop %dx
ret
drive_num: .byte 0x00
boot_msg: .ascii "booting\0"
.org 510
.byte 0x55
.byte 0xAA
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