grub-lakitu/grub-core/boot/i386/pc/boot.S - cos/cobble - Git at Google

 /* -*-Asm-*- */
 /*
  *  GRUB  --  GRand Unified Bootloader
  *  Copyright (C) 1999,2000,2001,2002,2005,2006,2007,2008,2009  Free Software Foundation, Inc.
  *
  *  GRUB is free software: you can redistribute it and/or modify
  *  it under the terms of the GNU General Public License as published by
  *  the Free Software Foundation, either version 3 of the License, or
  *  (at your option) any later version.
  *
  *  GRUB is distributed in the hope that it will be useful,
  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  *  GNU General Public License for more details.
  *
  *  You should have received a copy of the GNU General Public License
  *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
  */

 #include <grub/symbol.h>
 #include <grub/machine/boot.h>

 /*
  *  defines for the code go here
  */

 	/* Print message string */
 #define MSG(x)	movw $x, %si; call LOCAL(message)
 #define ERR(x)	movw $x, %si; jmp LOCAL(error_message)

 	.macro floppy
 part_start:

 LOCAL(probe_values):
 	.byte	36, 18, 15, 9, 0

 LOCAL(floppy_probe):
 	pushw	%dx
 /*
  *  Perform floppy probe.
  */
 #ifdef __APPLE__
 	LOCAL(probe_values_minus_one) = LOCAL(probe_values) - 1
 	movw	MACRO_DOLLAR(LOCAL(probe_values_minus_one)), %si
 #else
 	movw	MACRO_DOLLAR(LOCAL(probe_values)) - 1, %si
 #endif

 LOCAL(probe_loop):
 	/* reset floppy controller INT 13h AH=0 */
 	xorw	%ax, %ax
 	int	MACRO_DOLLAR(0x13)

 	incw	%si
 	movb	(%si), %cl

 	/* if number of sectors is 0, display error and die */
 	testb	%cl, %cl
 	jnz	1f

 /*
  * Floppy disk probe failure.
  */
 	MSG(fd_probe_error_string)
 	jmp	LOCAL(general_error)

 /* "Floppy" */
 fd_probe_error_string:	.asciz "Floppy"

 1:
 	/* perform read */
 	movw	MACRO_DOLLAR(GRUB_BOOT_MACHINE_BUFFER_SEG), %bx
 	movw	%bx, %es
 	xorw	%bx, %bx
 	movw	MACRO_DOLLAR(0x201), %ax
 	movb	MACRO_DOLLAR(0), %ch
 	movb	MACRO_DOLLAR(0), %dh
 	int	MACRO_DOLLAR(0x13)

 	/* if error, jump to "LOCAL(probe_loop)" */
 	jc	LOCAL(probe_loop)

 	/* %cl is already the correct value! */
 	movb	MACRO_DOLLAR(1), %dh
 	movb	MACRO_DOLLAR(79), %ch

 	jmp	LOCAL(final_init)
 	.endm

 	.macro scratch

 	/* scratch space */
 mode:
 	.byte	0
 disk_address_packet:
 sectors:
 	.long	0
 heads:
 	.long	0
 cylinders:
 	.word	0
 sector_start:
 	.byte	0
 head_start:
 	.byte	0
 cylinder_start:
 	.word	0
 	/* more space... */
 	.endm

 	.file	"boot.S"

 	.text

 	/* Tell GAS to generate 16-bit instructions so that this code works
 	   in real mode. */
 	.code16

 .globl _start, start;
 _start:
 start:
 	/*
 	 * _start is loaded at 0x7c00 and is jumped to with CS:IP 0:0x7c00
 	 */

 	/*
 	 * Beginning of the sector is compatible with the FAT/HPFS BIOS
 	 * parameter block.
 	 */

 	jmp	LOCAL(after_BPB)
 	nop	/* do I care about this ??? */

 #ifdef HYBRID_BOOT
 	nop
 	nop
 	nop
 	nop
 	nop
 	nop
 	nop
 	nop
 	nop
 	nop
 	nop
 	nop
 	nop

 	nop
 	nop
 	nop
 	nop
 	nop
 	nop
 	nop
 	nop

 	nop
 	nop
 	jmp	LOCAL(after_BPB)
 #else
 	/*
 	 * This space is for the BIOS parameter block!!!!  Don't change
 	 * the first jump, nor start the code anywhere but right after
 	 * this area.
 	 */

 	.org GRUB_BOOT_MACHINE_BPB_START
 	.org 4
 #endif
 #ifdef HYBRID_BOOT
 	floppy
 #else
 	scratch
 #endif

 	.org GRUB_BOOT_MACHINE_BPB_END
 	/*
 	 * End of BIOS parameter block.
 	 */

 LOCAL(kernel_address):
 	.word	GRUB_BOOT_MACHINE_KERNEL_ADDR

 #ifndef HYBRID_BOOT
 	.org GRUB_BOOT_MACHINE_KERNEL_SECTOR
 LOCAL(kernel_sector):
 	.long	1
 LOCAL(kernel_sector_high):
 	.long	0
 #endif

 	.org GRUB_BOOT_MACHINE_BOOT_DRIVE
 boot_drive:
 	.byte 0xff	/* the disk to load kernel from */
 			/* 0xff means use the boot drive */

 LOCAL(after_BPB):

 /* general setup */
 	cli		/* we're not safe here! */

         /*
          * This is a workaround for buggy BIOSes which don't pass boot
          * drive correctly. If GRUB is installed into a HDD, check if
          * DL is masked correctly. If not, assume that the BIOS passed
          * a bogus value and set DL to 0x80, since this is the only
          * possible boot drive. If GRUB is installed into a floppy,
          * this does nothing (only jump).
          */
 	.org GRUB_BOOT_MACHINE_DRIVE_CHECK
 boot_drive_check:
         jmp     3f	/* grub-setup may overwrite this jump */
         testb   $0x80, %dl
         jz      2f
 3:
 	/* Ignore %dl different from 0-0x0f and 0x80-0x8f.  */
 	testb   $0x70, %dl
 	jz      1f
 2:
         movb    $0x80, %dl
 1:
 	/*
 	 * ljmp to the next instruction because some bogus BIOSes
 	 * jump to 07C0:0000 instead of 0000:7C00.
 	 */
 	ljmp	$0, $real_start

 real_start:

 	/* set up %ds and %ss as offset from 0 */
 	xorw	%ax, %ax
 	movw	%ax, %ds
 	movw	%ax, %ss

 	/* set up the REAL stack */
 	movw	$GRUB_BOOT_MACHINE_STACK_SEG, %sp

 	sti		/* we're safe again */

 	/*
 	 *  Check if we have a forced disk reference here
 	 */
 	movb   boot_drive, %al
 	cmpb	$0xff, %al
 	je	1f
 	movb	%al, %dl
 1:
 	/* save drive reference first thing! */
 	pushw	%dx

 	/* print a notification message on the screen */
 	MSG(notification_string)

 	/* set %si to the disk address packet */
 	movw	$disk_address_packet, %si

 	/* check if LBA is supported */
 	movb	$0x41, %ah
 	movw	$0x55aa, %bx
 	int	$0x13

 	/*
 	 *  %dl may have been clobbered by INT 13, AH=41H.
 	 *  This happens, for example, with AST BIOS 1.04.
 	 */
 	popw	%dx
 	pushw	%dx

 	/* use CHS if fails */
 	jc	LOCAL(chs_mode)
 	cmpw	$0xaa55, %bx
 	jne	LOCAL(chs_mode)

 	andw	$1, %cx
 	jz	LOCAL(chs_mode)

 LOCAL(lba_mode):
 	xorw	%ax, %ax
 	movw	%ax, 4(%si)

 	incw	%ax
 	/* set the mode to non-zero */
 	movb	%al, -1(%si)

 	/* the blocks */
 	movw	%ax, 2(%si)

 	/* the size and the reserved byte */
 	movw	$0x0010, (%si)

 	/* the absolute address */
 	movl	LOCAL(kernel_sector), %ebx
 	movl	%ebx, 8(%si)
 	movl	LOCAL(kernel_sector_high), %ebx
 	movl	%ebx, 12(%si)

 	/* the segment of buffer address */
 	movw	$GRUB_BOOT_MACHINE_BUFFER_SEG, 6(%si)

 /*
  * BIOS call "INT 0x13 Function 0x42" to read sectors from disk into memory
  *	Call with	%ah = 0x42
  *			%dl = drive number
  *			%ds:%si = segment:offset of disk address packet
  *	Return:
  *			%al = 0x0 on success; err code on failure
  */

 	movb	$0x42, %ah
 	int	$0x13

 	/* LBA read is not supported, so fallback to CHS.  */
 	jc	LOCAL(chs_mode)

 	movw	$GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
 	jmp	LOCAL(copy_buffer)

 LOCAL(chs_mode):
 	/*
 	 *  Determine the hard disk geometry from the BIOS!
 	 *  We do this first, so that LS-120 IDE floppies work correctly.
 	 */
 	movb	$8, %ah
 	int	$0x13
 	jnc	LOCAL(final_init)

 	popw	%dx
 	/*
 	 *  The call failed, so maybe use the floppy probe instead.
 	 */
 	testb	%dl, %dl
 	jnb	LOCAL(floppy_probe)

 	/* Nope, we definitely have a hard disk, and we're screwed. */
 	ERR(hd_probe_error_string)

 LOCAL(final_init):
 	/* set the mode to zero */
 	movzbl	%dh, %eax
 	movb	%ah, -1(%si)

 	/* save number of heads */
 	incw	%ax
 	movl	%eax, 4(%si)

 	movzbw	%cl, %dx
 	shlw	$2, %dx
 	movb	%ch, %al
 	movb	%dh, %ah

 	/* save number of cylinders */
 	incw	%ax
 	movw	%ax, 8(%si)

 	movzbw	%dl, %ax
 	shrb	$2, %al

 	/* save number of sectors */
 	movl	%eax, (%si)

 setup_sectors:
 	/* load logical sector start (top half) */
 	movl	LOCAL(kernel_sector_high), %eax

 	orl	%eax, %eax
 	jnz	LOCAL(geometry_error)

 	/* load logical sector start (bottom half) */
 	movl	LOCAL(kernel_sector), %eax

 	/* zero %edx */
 	xorl	%edx, %edx

 	/* divide by number of sectors */
 	divl	(%si)

 	/* save sector start */
 	movb	%dl, %cl

 	xorw	%dx, %dx	/* zero %edx */
 	divl	4(%si)		/* divide by number of heads */

 	/* do we need too many cylinders? */
 	cmpw	8(%si), %ax
 	jge	LOCAL(geometry_error)

 	/* normalize sector start (1-based) */
 	incb	%cl

 	/* low bits of cylinder start */
 	movb	%al, %ch

 	/* high bits of cylinder start */
 	xorb	%al, %al
 	shrw	$2, %ax
 	orb	%al, %cl

 	/* save head start */
 	movb	%dl, %al

 	/* restore %dl */
 	popw	%dx

 	/* head start */
 	movb	%al, %dh

 /*
  * BIOS call "INT 0x13 Function 0x2" to read sectors from disk into memory
  *	Call with	%ah = 0x2
  *			%al = number of sectors
  *			%ch = cylinder
  *			%cl = sector (bits 6-7 are high bits of "cylinder")
  *			%dh = head
  *			%dl = drive (0x80 for hard disk, 0x0 for floppy disk)
  *			%es:%bx = segment:offset of buffer
  *	Return:
  *			%al = 0x0 on success; err code on failure
  */

 	movw	$GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
 	movw	%bx, %es	/* load %es segment with disk buffer */

 	xorw	%bx, %bx	/* %bx = 0, put it at 0 in the segment */
 	movw	$0x0201, %ax	/* function 2 */
 	int	$0x13

 	jc	LOCAL(read_error)

 	movw	%es, %bx

 LOCAL(copy_buffer):
 	/*
 	 * We need to save %cx and %si because the startup code in
 	 * kernel uses them without initializing them.
 	 */
 	pusha
 	pushw	%ds

 	movw	$0x100, %cx
 	movw	%bx, %ds
 	xorw	%si, %si
 	movw	$GRUB_BOOT_MACHINE_KERNEL_ADDR, %di
 	movw	%si, %es

 	cld

 	rep
 	movsw

 	popw	%ds
 	popa

 	/* boot kernel */
 	jmp	*(LOCAL(kernel_address))

 /* END OF MAIN LOOP */

 /*
  * BIOS Geometry translation error (past the end of the disk geometry!).
  */
 LOCAL(geometry_error):
 	ERR(geometry_error_string)

 /*
  * Read error on the disk.
  */
 LOCAL(read_error):
 	movw	$read_error_string, %si
 LOCAL(error_message):
 	call	LOCAL(message)
 LOCAL(general_error):
 	MSG(general_error_string)

 /* go here when you need to stop the machine hard after an error condition */
         /* tell the BIOS a boot failure, which may result in no effect */
         int	$0x18
 LOCAL(stop):
 	jmp	LOCAL(stop)

 notification_string:	.asciz "GRUB "
 geometry_error_string:	.asciz "Geom"
 hd_probe_error_string:	.asciz "Hard Disk"
 read_error_string:	.asciz "Read"
 general_error_string:	.asciz " Error\r\n"

 /*
  * message: write the string pointed to by %si
  *
  *   WARNING: trashes %si, %ax, and %bx
  */

 	/*
 	 * Use BIOS "int 10H Function 0Eh" to write character in teletype mode
 	 *	%ah = 0xe	%al = character
 	 *	%bh = page	%bl = foreground color (graphics modes)
 	 */
 1:
 	movw	$0x0001, %bx
 	movb	$0xe, %ah
 	int	$0x10		/* display a byte */
 LOCAL(message):
 	lodsb
 	cmpb	$0, %al
 	jne	1b	/* if not end of string, jmp to display */
 	ret

 	/*
 	 *  Windows NT breaks compatibility by embedding a magic
 	 *  number here.
 	 */

 #ifdef HYBRID_BOOT
 	.org 0x1b0
 LOCAL(kernel_sector):
 	.long	1
 LOCAL(kernel_sector_high):
 	.long	0
 #endif
 	.org GRUB_BOOT_MACHINE_WINDOWS_NT_MAGIC
 nt_magic:
 	.long 0
 	.word 0

 	/*
 	 *  This is where an MBR would go if on a hard disk.  The code
 	 *  here isn't even referenced unless we're on a floppy.  Kinda
 	 *  sneaky, huh?
 	*/

 	.org GRUB_BOOT_MACHINE_PART_START

 #ifndef HYBRID_BOOT
 	floppy
 #else
 	scratch
 #endif

 	.org GRUB_BOOT_MACHINE_PART_END

 /* the last 2 bytes in the sector 0 contain the signature */
 	.word	GRUB_BOOT_MACHINE_SIGNATURE
	/* --Asm-- */
	/*
	* GRUB -- GRand Unified Bootloader
	* Copyright (C) 1999,2000,2001,2002,2005,2006,2007,2008,2009 Free Software Foundation, Inc.
	*
	* GRUB is free software: you can redistribute it and/or modify
	* it under the terms of the GNU General Public License as published by
	* the Free Software Foundation, either version 3 of the License, or
	* (at your option) any later version.
	*
	* GRUB is distributed in the hope that it will be useful,
	* but WITHOUT ANY WARRANTY; without even the implied warranty of
	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	* GNU General Public License for more details.
	*
	* You should have received a copy of the GNU General Public License
	* along with GRUB. If not, see <http://www.gnu.org/licenses/>.
	*/

	#include <grub/symbol.h>
	#include <grub/machine/boot.h>

	/*
	* defines for the code go here
	*/

	/* Print message string */
	#define MSG(x) movw $x, %si; call LOCAL(message)
	#define ERR(x) movw $x, %si; jmp LOCAL(error_message)

	.macro floppy
	part_start:

	LOCAL(probe_values):
	.byte 36, 18, 15, 9, 0

	LOCAL(floppy_probe):
	pushw %dx
	/*
	* Perform floppy probe.
	*/
	#ifdef __APPLE__
	LOCAL(probe_values_minus_one) = LOCAL(probe_values) - 1
	movw MACRO_DOLLAR(LOCAL(probe_values_minus_one)), %si
	#else
	movw MACRO_DOLLAR(LOCAL(probe_values)) - 1, %si
	#endif

	LOCAL(probe_loop):
	/* reset floppy controller INT 13h AH=0 */
	xorw %ax, %ax
	int MACRO_DOLLAR(0x13)

	incw %si
	movb (%si), %cl

	/* if number of sectors is 0, display error and die */
	testb %cl, %cl
	jnz 1f

	/*
	* Floppy disk probe failure.
	*/
	MSG(fd_probe_error_string)
	jmp LOCAL(general_error)

	/* "Floppy" */
	fd_probe_error_string: .asciz "Floppy"

	1:
	/* perform read */
	movw MACRO_DOLLAR(GRUB_BOOT_MACHINE_BUFFER_SEG), %bx
	movw %bx, %es
	xorw %bx, %bx
	movw MACRO_DOLLAR(0x201), %ax
	movb MACRO_DOLLAR(0), %ch
	movb MACRO_DOLLAR(0), %dh
	int MACRO_DOLLAR(0x13)

	/* if error, jump to "LOCAL(probe_loop)" */
	jc LOCAL(probe_loop)

	/* %cl is already the correct value! */
	movb MACRO_DOLLAR(1), %dh
	movb MACRO_DOLLAR(79), %ch

	jmp LOCAL(final_init)
	.endm

	.macro scratch

	/* scratch space */
	mode:
	.byte 0
	disk_address_packet:
	sectors:
	.long 0
	heads:
	.long 0
	cylinders:
	.word 0
	sector_start:
	.byte 0
	head_start:
	.byte 0
	cylinder_start:
	.word 0
	/* more space... */
	.endm

	.file "boot.S"

	.text

	/* Tell GAS to generate 16-bit instructions so that this code works
	in real mode. */
	.code16

	.globl _start, start;
	_start:
	start:
	/*
	* _start is loaded at 0x7c00 and is jumped to with CS:IP 0:0x7c00
	*/

	/*
	* Beginning of the sector is compatible with the FAT/HPFS BIOS
	* parameter block.
	*/

	jmp LOCAL(after_BPB)
	nop /* do I care about this ??? */

	#ifdef HYBRID_BOOT
	nop
	nop
	nop
	nop
	nop
	nop
	nop
	nop
	nop
	nop
	nop
	nop
	nop

	nop
	nop
	nop
	nop
	nop
	nop
	nop
	nop

	nop
	nop
	jmp LOCAL(after_BPB)
	#else
	/*
	* This space is for the BIOS parameter block!!!! Don't change
	* the first jump, nor start the code anywhere but right after
	* this area.
	*/

	.org GRUB_BOOT_MACHINE_BPB_START
	.org 4
	#endif
	#ifdef HYBRID_BOOT
	floppy
	#else
	scratch
	#endif

	.org GRUB_BOOT_MACHINE_BPB_END
	/*
	* End of BIOS parameter block.
	*/

	LOCAL(kernel_address):
	.word GRUB_BOOT_MACHINE_KERNEL_ADDR

	#ifndef HYBRID_BOOT
	.org GRUB_BOOT_MACHINE_KERNEL_SECTOR
	LOCAL(kernel_sector):
	.long 1
	LOCAL(kernel_sector_high):
	.long 0
	#endif

	.org GRUB_BOOT_MACHINE_BOOT_DRIVE
	boot_drive:
	.byte 0xff /* the disk to load kernel from */
	/* 0xff means use the boot drive */

	LOCAL(after_BPB):

	/* general setup */
	cli /* we're not safe here! */

	/*
	* This is a workaround for buggy BIOSes which don't pass boot
	* drive correctly. If GRUB is installed into a HDD, check if
	* DL is masked correctly. If not, assume that the BIOS passed
	* a bogus value and set DL to 0x80, since this is the only
	* possible boot drive. If GRUB is installed into a floppy,
	* this does nothing (only jump).
	*/
	.org GRUB_BOOT_MACHINE_DRIVE_CHECK
	boot_drive_check:
	jmp 3f /* grub-setup may overwrite this jump */
	testb $0x80, %dl
	jz 2f
	3:
	/* Ignore %dl different from 0-0x0f and 0x80-0x8f. */
	testb $0x70, %dl
	jz 1f
	2:
	movb $0x80, %dl
	1:
	/*
	* ljmp to the next instruction because some bogus BIOSes
	* jump to 07C0:0000 instead of 0000:7C00.
	*/
	ljmp $0, $real_start

	real_start:

	/* set up %ds and %ss as offset from 0 */
	xorw %ax, %ax
	movw %ax, %ds
	movw %ax, %ss

	/* set up the REAL stack */
	movw $GRUB_BOOT_MACHINE_STACK_SEG, %sp

	sti /* we're safe again */

	/*
	* Check if we have a forced disk reference here
	*/
	movb boot_drive, %al
	cmpb $0xff, %al
	je 1f
	movb %al, %dl
	1:
	/* save drive reference first thing! */
	pushw %dx

	/* print a notification message on the screen */
	MSG(notification_string)

	/* set %si to the disk address packet */
	movw $disk_address_packet, %si

	/* check if LBA is supported */
	movb $0x41, %ah
	movw $0x55aa, %bx
	int $0x13

	/*
	* %dl may have been clobbered by INT 13, AH=41H.
	* This happens, for example, with AST BIOS 1.04.
	*/
	popw %dx
	pushw %dx

	/* use CHS if fails */
	jc LOCAL(chs_mode)
	cmpw $0xaa55, %bx
	jne LOCAL(chs_mode)

	andw $1, %cx
	jz LOCAL(chs_mode)

	LOCAL(lba_mode):
	xorw %ax, %ax
	movw %ax, 4(%si)

	incw %ax
	/* set the mode to non-zero */
	movb %al, -1(%si)

	/* the blocks */
	movw %ax, 2(%si)

	/* the size and the reserved byte */
	movw $0x0010, (%si)

	/* the absolute address */
	movl LOCAL(kernel_sector), %ebx
	movl %ebx, 8(%si)
	movl LOCAL(kernel_sector_high), %ebx
	movl %ebx, 12(%si)

	/* the segment of buffer address */
	movw $GRUB_BOOT_MACHINE_BUFFER_SEG, 6(%si)

	/*
	* BIOS call "INT 0x13 Function 0x42" to read sectors from disk into memory
	* Call with %ah = 0x42
	* %dl = drive number
	* %ds:%si = segment:offset of disk address packet
	* Return:
	* %al = 0x0 on success; err code on failure
	*/

	movb $0x42, %ah
	int $0x13

	/* LBA read is not supported, so fallback to CHS. */
	jc LOCAL(chs_mode)

	movw $GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
	jmp LOCAL(copy_buffer)

	LOCAL(chs_mode):
	/*
	* Determine the hard disk geometry from the BIOS!
	* We do this first, so that LS-120 IDE floppies work correctly.
	*/
	movb $8, %ah
	int $0x13
	jnc LOCAL(final_init)

	popw %dx
	/*
	* The call failed, so maybe use the floppy probe instead.
	*/
	testb %dl, %dl
	jnb LOCAL(floppy_probe)

	/* Nope, we definitely have a hard disk, and we're screwed. */
	ERR(hd_probe_error_string)

	LOCAL(final_init):
	/* set the mode to zero */
	movzbl %dh, %eax
	movb %ah, -1(%si)

	/* save number of heads */
	incw %ax
	movl %eax, 4(%si)

	movzbw %cl, %dx
	shlw $2, %dx
	movb %ch, %al
	movb %dh, %ah

	/* save number of cylinders */
	incw %ax
	movw %ax, 8(%si)

	movzbw %dl, %ax
	shrb $2, %al

	/* save number of sectors */
	movl %eax, (%si)

	setup_sectors:
	/* load logical sector start (top half) */
	movl LOCAL(kernel_sector_high), %eax

	orl %eax, %eax
	jnz LOCAL(geometry_error)

	/* load logical sector start (bottom half) */
	movl LOCAL(kernel_sector), %eax

	/* zero %edx */
	xorl %edx, %edx

	/* divide by number of sectors */
	divl (%si)

	/* save sector start */
	movb %dl, %cl

	xorw %dx, %dx /* zero %edx */
	divl 4(%si) /* divide by number of heads */

	/* do we need too many cylinders? */
	cmpw 8(%si), %ax
	jge LOCAL(geometry_error)

	/* normalize sector start (1-based) */
	incb %cl

	/* low bits of cylinder start */
	movb %al, %ch

	/* high bits of cylinder start */
	xorb %al, %al
	shrw $2, %ax
	orb %al, %cl

	/* save head start */
	movb %dl, %al

	/* restore %dl */
	popw %dx

	/* head start */
	movb %al, %dh

	/*
	* BIOS call "INT 0x13 Function 0x2" to read sectors from disk into memory
	* Call with %ah = 0x2
	* %al = number of sectors
	* %ch = cylinder
	* %cl = sector (bits 6-7 are high bits of "cylinder")
	* %dh = head
	* %dl = drive (0x80 for hard disk, 0x0 for floppy disk)
	* %es:%bx = segment:offset of buffer
	* Return:
	* %al = 0x0 on success; err code on failure
	*/

	movw $GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
	movw %bx, %es /* load %es segment with disk buffer */

	xorw %bx, %bx /* %bx = 0, put it at 0 in the segment */
	movw $0x0201, %ax /* function 2 */
	int $0x13

	jc LOCAL(read_error)

	movw %es, %bx

	LOCAL(copy_buffer):
	/*
	* We need to save %cx and %si because the startup code in
	* kernel uses them without initializing them.
	*/
	pusha
	pushw %ds

	movw $0x100, %cx
	movw %bx, %ds
	xorw %si, %si
	movw $GRUB_BOOT_MACHINE_KERNEL_ADDR, %di
	movw %si, %es

	cld

	rep
	movsw

	popw %ds
	popa

	/* boot kernel */
	jmp *(LOCAL(kernel_address))

	/* END OF MAIN LOOP */

	/*
	* BIOS Geometry translation error (past the end of the disk geometry!).
	*/
	LOCAL(geometry_error):
	ERR(geometry_error_string)

	/*
	* Read error on the disk.
	*/
	LOCAL(read_error):
	movw $read_error_string, %si
	LOCAL(error_message):
	call LOCAL(message)
	LOCAL(general_error):
	MSG(general_error_string)

	/* go here when you need to stop the machine hard after an error condition */
	/* tell the BIOS a boot failure, which may result in no effect */
	int $0x18
	LOCAL(stop):
	jmp LOCAL(stop)

	notification_string: .asciz "GRUB "
	geometry_error_string: .asciz "Geom"
	hd_probe_error_string: .asciz "Hard Disk"
	read_error_string: .asciz "Read"
	general_error_string: .asciz " Error\r\n"

	/*
	* message: write the string pointed to by %si
	*
	* WARNING: trashes %si, %ax, and %bx
	*/

	/*
	* Use BIOS "int 10H Function 0Eh" to write character in teletype mode
	* %ah = 0xe %al = character
	* %bh = page %bl = foreground color (graphics modes)
	*/
	1:
	movw $0x0001, %bx
	movb $0xe, %ah
	int $0x10 /* display a byte */
	LOCAL(message):
	lodsb
	cmpb $0, %al
	jne 1b /* if not end of string, jmp to display */
	ret

	/*
	* Windows NT breaks compatibility by embedding a magic
	* number here.
	*/

	#ifdef HYBRID_BOOT
	.org 0x1b0
	LOCAL(kernel_sector):
	.long 1
	LOCAL(kernel_sector_high):
	.long 0
	#endif
	.org GRUB_BOOT_MACHINE_WINDOWS_NT_MAGIC
	nt_magic:
	.long 0
	.word 0

	/*
	* This is where an MBR would go if on a hard disk. The code
	* here isn't even referenced unless we're on a floppy. Kinda
	* sneaky, huh?
	*/

	.org GRUB_BOOT_MACHINE_PART_START

	#ifndef HYBRID_BOOT
	floppy
	#else
	scratch
	#endif

	.org GRUB_BOOT_MACHINE_PART_END

	/* the last 2 bytes in the sector 0 contain the signature */
	.word GRUB_BOOT_MACHINE_SIGNATURE