stage1/stage1.S - platform/external/grub - Git at Google

 /* -*-Asm-*- */
 /*
  *  GRUB  --  GRand Unified Bootloader
  *  Copyright (C) 1999,2000,2001,2002,2004   Free Software Foundation, Inc.
  *
  *  This program 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 2 of the License, or
  *  (at your option) any later version.
  *
  *  This program 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 this program; if not, write to the Free Software
  *  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */

 #include <stage1.h>

 /*
  *  defines for the code go here
  */

 	/* Absolute addresses
 	   This makes the assembler generate the address without support
 	   from the linker. (ELF can't relocate 16-bit addresses!) */
 #define ABS(x) (x-_start+0x7c00)

 	/* Print message string */
 #define MSG(x)	movw $ABS(x), %si; call message

 	/* XXX:	binutils-2.9.1.0.x doesn't produce a short opcode for this. */
 #define	MOV_MEM_TO_AL(x)	.byte 0xa0;  .word x

 	.file	"stage1.S"

 	.text

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

 .globl _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	after_BPB
 	nop	/* do I care about this ??? */

 	/*
 	 * This space is for the BIOS parameter block!!!!  Don't change
 	 * the first jump, nor start the code anywhere but right after
 	 * this area.
 	 */

 	. = _start + 4

 	/* 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... */

 	. = _start + STAGE1_BPBEND

 	/*
 	 * End of BIOS parameter block.
 	 */

 stage1_version:
 	.byte	COMPAT_VERSION_MAJOR, COMPAT_VERSION_MINOR
 boot_drive:
 	.byte	GRUB_INVALID_DRIVE	/* the disk to load stage2 from */
 force_lba:
 	.byte	0
 stage2_address:
 	.word	0x8000
 stage2_sector:
 	.long	1
 stage2_segment:
 	.word	0x800

 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).
 	 */
 boot_drive_check:
 	jmp	1f
 	testb	$0x80, %dl
 	jnz	1f
 	movb	$0x80, %dl
 1:

 	/*
 	 * ljmp to the next instruction because some bogus BIOSes
 	 * jump to 07C0:0000 instead of 0000:7C00.
 	 */
 	ljmp	$0, $ABS(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	$STAGE1_STACKSEG, %sp

 	sti		/* we're safe again */

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

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

 	/* do not probe LBA if the drive is a floppy */
 	testb	$STAGE1_BIOS_HD_FLAG, %dl
 	jz	chs_mode

 	/* 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	chs_mode
 	cmpw	$0xaa55, %bx
 	jne	chs_mode

 	/* check if AH=0x42 is supported if FORCE_LBA is zero */
 	MOV_MEM_TO_AL(ABS(force_lba))	/* movb	ABS(force_lba), %al */
 	testb	%al, %al
 	jnz	lba_mode
 	andw	$1, %cx
 	jz	chs_mode

 lba_mode:
 	/* save the total number of sectors */
 	movl	0x10(%si), %ecx

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

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

 	movl	ABS(stage2_sector), %ebx

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

 	/* the blocks */
 	movw	$1, 2(%si)

 	/* the absolute address (low 32 bits) */
 	movl	%ebx, 8(%si)

 	/* the segment of buffer address */
 	movw	$STAGE1_BUFFERSEG, 6(%si)

 	xorl	%eax, %eax
 	movw	%ax, 4(%si)
 	movl	%eax, 12(%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	chs_mode

 	movw	$STAGE1_BUFFERSEG, %bx
 	jmp	copy_buffer

 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	final_init

 	/*
 	 *  The call failed, so maybe use the floppy probe instead.
 	 */
 	testb	$STAGE1_BIOS_HD_FLAG, %dl
 	jz	floppy_probe

 	/* Nope, we definitely have a hard disk, and we're screwed. */
 	jmp	hd_probe_error

 final_init:

 	movw	$ABS(sectors), %si

 	/* set the mode to zero */
 	movb	$0, -1(%si)

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

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

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

 	xorw	%ax, %ax
 	movb	%dl, %al
 	shrb	$2, %al

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

 setup_sectors:
 	/* load logical sector start (bottom half) */
 	movl	ABS(stage2_sector), %eax

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

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

 	/* save sector start */
 	movb	%dl, 10(%si)

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

 	/* save head start */
 	movb	%dl, 11(%si)

 	/* save cylinder start */
 	movw	%ax, 12(%si)

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

 /*
  *  This is the loop for taking care of BIOS geometry translation (ugh!)
  */

 	/* get high bits of cylinder */
 	movb	13(%si), %dl

 	shlb	$6, %dl		/* shift left by 6 bits */
 	movb	10(%si), %cl	/* get sector */

 	incb	%cl		/* normalize sector (sectors go
 					from 1-N, not 0-(N-1) ) */
 	orb	%dl, %cl	/* composite together */
 	movb	12(%si), %ch	/* sector+hcyl in cl, cylinder in ch */

 	/* restore %dx */
 	popw	%dx

 	/* head number */
 	movb	11(%si), %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	$STAGE1_BUFFERSEG, %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	read_error

 	movw	%es, %bx

 copy_buffer:
 	movw	ABS(stage2_segment), %es

 	/*
 	 * We need to save %cx and %si because the startup code in
 	 * stage2 uses them without initializing them.
 	 */
 	pusha
 	pushw	%ds

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

 	cld

 	rep
 	movsw

 	popw	%ds
 	popa

 	/* boot stage2 */
 	jmp	*(stage2_address)

 /* END OF MAIN LOOP */

 /*
  * BIOS Geometry translation error (past the end of the disk geometry!).
  */
 geometry_error:
 	MSG(geometry_error_string)
 	jmp	general_error

 /*
  * Disk probe failure.
  */
 hd_probe_error:
 	MSG(hd_probe_error_string)
 	jmp	general_error

 /*
  * Read error on the disk.
  */
 read_error:
 	MSG(read_error_string)

 general_error:
 	MSG(general_error_string)

 /* go here when you need to stop the machine hard after an error condition */
 stop:	jmp	stop

 notification_string:	.string "GRUB "
 geometry_error_string:	.string "Geom"
 hd_probe_error_string:	.string "Hard Disk"
 read_error_string:	.string "Read"
 general_error_string:	.string " Error"

 /*
  * 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 */
 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.
 	 */

 	. = _start + STAGE1_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?
 	 */

 part_start:
 	. = _start + STAGE1_PARTSTART

 probe_values:
 	.byte	36, 18, 15, 9, 0

 floppy_probe:
 /*
  *  Perform floppy probe.
  */

 	movw	$ABS(probe_values-1), %si

 probe_loop:
 	/* reset floppy controller INT 13h AH=0 */
 	xorw	%ax, %ax
 	int	$0x13

 	incw	%si
 	movb	(%si), %cl

 	/* if number of sectors is 0, display error and die */
 	cmpb	$0, %cl
 	jne	1f

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

 fd_probe_error_string:	.string "Floppy"

 1:
 	/* perform read */
 	movw	$STAGE1_BUFFERSEG, %bx
 	movw	$0x201, %ax
 	movb	$0, %ch
 	movb	$0, %dh
 	int	$0x13

 	/* if error, jump to "probe_loop" */
 	jc	probe_loop

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

 	jmp	final_init

 	. = _start + STAGE1_PARTEND

 /* the last 2 bytes in the sector 0 contain the signature */
 	.word	STAGE1_SIGNATURE
	/* --Asm-- */
	/*
	* GRUB -- GRand Unified Bootloader
	* Copyright (C) 1999,2000,2001,2002,2004 Free Software Foundation, Inc.
	*
	* This program 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 2 of the License, or
	* (at your option) any later version.
	*
	* This program 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 this program; if not, write to the Free Software
	* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
	*/

	#include <stage1.h>

	/*
	* defines for the code go here
	*/

	/* Absolute addresses
	This makes the assembler generate the address without support
	from the linker. (ELF can't relocate 16-bit addresses!) */
	#define ABS(x) (x-_start+0x7c00)

	/* Print message string */
	#define MSG(x) movw $ABS(x), %si; call message

	/* XXX: binutils-2.9.1.0.x doesn't produce a short opcode for this. */
	#define MOV_MEM_TO_AL(x) .byte 0xa0; .word x

	.file "stage1.S"

	.text

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

	.globl _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 after_BPB
	nop /* do I care about this ??? */

	/*
	* This space is for the BIOS parameter block!!!! Don't change
	* the first jump, nor start the code anywhere but right after
	* this area.
	*/

	. = _start + 4

	/* 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... */

	. = _start + STAGE1_BPBEND

	/*
	* End of BIOS parameter block.
	*/

	stage1_version:
	.byte COMPAT_VERSION_MAJOR, COMPAT_VERSION_MINOR
	boot_drive:
	.byte GRUB_INVALID_DRIVE /* the disk to load stage2 from */
	force_lba:
	.byte 0
	stage2_address:
	.word 0x8000
	stage2_sector:
	.long 1
	stage2_segment:
	.word 0x800

	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).
	*/
	boot_drive_check:
	jmp 1f
	testb $0x80, %dl
	jnz 1f
	movb $0x80, %dl
	1:

	/*
	* ljmp to the next instruction because some bogus BIOSes
	* jump to 07C0:0000 instead of 0000:7C00.
	*/
	ljmp $0, $ABS(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 $STAGE1_STACKSEG, %sp

	sti /* we're safe again */

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

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

	/* do not probe LBA if the drive is a floppy */
	testb $STAGE1_BIOS_HD_FLAG, %dl
	jz chs_mode

	/* 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 chs_mode
	cmpw $0xaa55, %bx
	jne chs_mode

	/* check if AH=0x42 is supported if FORCE_LBA is zero */
	MOV_MEM_TO_AL(ABS(force_lba)) /* movb ABS(force_lba), %al */
	testb %al, %al
	jnz lba_mode
	andw $1, %cx
	jz chs_mode

	lba_mode:
	/* save the total number of sectors */
	movl 0x10(%si), %ecx

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

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

	movl ABS(stage2_sector), %ebx

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

	/* the blocks */
	movw $1, 2(%si)

	/* the absolute address (low 32 bits) */
	movl %ebx, 8(%si)

	/* the segment of buffer address */
	movw $STAGE1_BUFFERSEG, 6(%si)

	xorl %eax, %eax
	movw %ax, 4(%si)
	movl %eax, 12(%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 chs_mode

	movw $STAGE1_BUFFERSEG, %bx
	jmp copy_buffer

	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 final_init

	/*
	* The call failed, so maybe use the floppy probe instead.
	*/
	testb $STAGE1_BIOS_HD_FLAG, %dl
	jz floppy_probe

	/* Nope, we definitely have a hard disk, and we're screwed. */
	jmp hd_probe_error

	final_init:

	movw $ABS(sectors), %si

	/* set the mode to zero */
	movb $0, -1(%si)

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

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

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

	xorw %ax, %ax
	movb %dl, %al
	shrb $2, %al

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

	setup_sectors:
	/* load logical sector start (bottom half) */
	movl ABS(stage2_sector), %eax

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

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

	/* save sector start */
	movb %dl, 10(%si)

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

	/* save head start */
	movb %dl, 11(%si)

	/* save cylinder start */
	movw %ax, 12(%si)

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

	/*
	* This is the loop for taking care of BIOS geometry translation (ugh!)
	*/

	/* get high bits of cylinder */
	movb 13(%si), %dl

	shlb $6, %dl /* shift left by 6 bits */
	movb 10(%si), %cl /* get sector */

	incb %cl /* normalize sector (sectors go
	from 1-N, not 0-(N-1) ) */
	orb %dl, %cl /* composite together */
	movb 12(%si), %ch /* sector+hcyl in cl, cylinder in ch */

	/* restore %dx */
	popw %dx

	/* head number */
	movb 11(%si), %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 $STAGE1_BUFFERSEG, %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 read_error

	movw %es, %bx

	copy_buffer:
	movw ABS(stage2_segment), %es

	/*
	* We need to save %cx and %si because the startup code in
	* stage2 uses them without initializing them.
	*/
	pusha
	pushw %ds

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

	cld

	rep
	movsw

	popw %ds
	popa

	/* boot stage2 */
	jmp *(stage2_address)

	/* END OF MAIN LOOP */

	/*
	* BIOS Geometry translation error (past the end of the disk geometry!).
	*/
	geometry_error:
	MSG(geometry_error_string)
	jmp general_error

	/*
	* Disk probe failure.
	*/
	hd_probe_error:
	MSG(hd_probe_error_string)
	jmp general_error

	/*
	* Read error on the disk.
	*/
	read_error:
	MSG(read_error_string)

	general_error:
	MSG(general_error_string)

	/* go here when you need to stop the machine hard after an error condition */
	stop: jmp stop

	notification_string: .string "GRUB "
	geometry_error_string: .string "Geom"
	hd_probe_error_string: .string "Hard Disk"
	read_error_string: .string "Read"
	general_error_string: .string " Error"

	/*
	* 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 */
	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.
	*/

	. = _start + STAGE1_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?
	*/

	part_start:
	. = _start + STAGE1_PARTSTART

	probe_values:
	.byte 36, 18, 15, 9, 0

	floppy_probe:
	/*
	* Perform floppy probe.
	*/

	movw $ABS(probe_values-1), %si

	probe_loop:
	/* reset floppy controller INT 13h AH=0 */
	xorw %ax, %ax
	int $0x13

	incw %si
	movb (%si), %cl

	/* if number of sectors is 0, display error and die */
	cmpb $0, %cl
	jne 1f

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

	fd_probe_error_string: .string "Floppy"

	1:
	/* perform read */
	movw $STAGE1_BUFFERSEG, %bx
	movw $0x201, %ax
	movb $0, %ch
	movb $0, %dh
	int $0x13

	/* if error, jump to "probe_loop" */
	jc probe_loop

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

	jmp final_init

	. = _start + STAGE1_PARTEND

	/* the last 2 bytes in the sector 0 contain the signature */
	.word STAGE1_SIGNATURE