stage2/start.S - platform/external/grub - Git at Google

 /*
  *  GRUB  --  GRand Unified Bootloader
  *  Copyright (C) 1999,2000,2001   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.
  */

 #define ASM_FILE
 #include <shared.h>

 #ifndef STAGE1_5
 #include <stage2_size.h>
 #endif

 /*
  *  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!) */
 #ifdef STAGE1_5
 # define ABS(x) (x-_start+0x2000)
 #else
 # define ABS(x) (x-_start+0x8000)
 #endif /* STAGE1_5 */

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

 	.file	"start.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 0x8000 and is jumped to with
 	 * CS:IP 0:0x8000 in stage2.
 	 */

 	/*
 	 * we continue to use the stack for stage1 and assume that
 	 * some registers are set to correct values. See stage1.S
 	 * for more information.
 	 */

 	/* save drive reference first thing! */
 	pushw	%dx

 	/* print a notification message on the screen */
 	pushw	%si
 	MSG(notification_string)
 	popw	%si

 	/* this sets up for the first run through "bootloop" */
 	movw	$ABS(firstlist - BOOTSEC_LISTSIZE), %di

 	/* save the sector number of the second sector in %ebp */
 	movl	(%di), %ebp

         /* this is the loop for reading the secondary boot-loader in */
 bootloop:

 	/* check the number of sectors to read */
 	cmpw	$0, 4(%di)

 	/* if zero, go to the start function */
 	je	bootit

 setup_sectors:
 	/* check if we use LBA or CHS */
 	cmpb	$0, -1(%si)

 	/* jump to chs_mode if zero */
 	je	chs_mode

 lba_mode:
 	/* load logical sector start */
 	movl	(%di), %ebx

 	/* the maximum is limited to 0x7f because of Phoenix EDD */
 	xorl	%eax, %eax
 	movb	$0x7f, %al

 	/* how many do we really want to read? */
 	cmpw	%ax, 4(%di)	/* compare against total number of sectors */

 	/* which is greater? */
 	jg	1f

 	/* if less than, set to total */
 	movw	4(%di), %ax

 1:
 	/* subtract from total */
 	subw	%ax, 4(%di)

 	/* add into logical sector start */
 	addl	%eax, (%di)

 	/* set up disk address packet */

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

 	/* the number of sectors */
 	movw	%ax, 2(%si)

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

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

 	/* save %ax from destruction! */
 	pushw	%ax

 	/* zero %eax */
 	xorl	%eax, %eax

 	/* the offset of buffer address */
 	movw	%ax, 4(%si)

 	/* the absolute address (high 32 bits) */
 	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

 	jc	read_error

 	movw	$BUFFERSEG, %bx
 	jmp	copy_buffer

 chs_mode:
 	/* load logical sector start (bottom half) */
 	movl	(%di), %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

 	/* determine the maximum sector length of this read */
 	movw	(%si), %ax	/* get number of sectors per track/head */

 	/* subtract sector start */
 	subb	10(%si), %al

 	/* how many do we really want to read? */
 	cmpw	%ax, 4(%di)	/* compare against total number of sectors */


 	/* which is greater? */
 	jg	2f

 	/* if less than, set to total */
 	movw	4(%di), %ax

 2:
 	/* subtract from total */
 	subw	%ax, 4(%di)

 	/* add into logical sector start */
 	addl	%eax, (%di)

 /*
  *  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
 	pushw	%dx

 	/* head number */
 	movb	11(%si), %dh

 	pushw	%ax	/* save %ax from destruction! */

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

 	xorw	%bx, %bx	/* %bx = 0, put it at 0 in the segment */
 	movb	$0x2, %ah	/* function 2 */
 	int	$0x13

 	jc	read_error

 	/* save source segment */
 	movw	%es, %bx

 copy_buffer:

 	/* load addresses for copy from disk buffer to destination */
 	movw	6(%di), %es	/* load destination segment */

 	/* restore %ax */
 	popw	%ax

 	/* determine the next possible destination address (presuming
 		512 byte sectors!) */
 	shlw	$5, %ax		/* shift %ax five bits to the left */
 	addw	%ax, 6(%di)	/* add the corrected value to the destination
 				   address for next time */

 	/* save addressing regs */
 	pusha
 	pushw	%ds

 	/* get the copy length */
 	shlw	$4, %ax
 	movw	%ax, %cx

 	xorw	%di, %di	/* zero offset of destination addresses */
 	xorw	%si, %si	/* zero offset of source addresses */
 	movw	%bx, %ds	/* restore the source segment */

 	cld		/* sets the copy direction to forward */

 	/* perform copy */
 	rep		/* sets a repeat */
 	movsb		/* this runs the actual copy */

 	/* restore addressing regs and print a dot with correct DS
 	   (MSG modifies SI, which is saved, and unused AX and BX) */
 	popw	%ds
 	MSG(notification_step)
 	popa

 	/* check if finished with this dataset */
 	cmpw	$0, 4(%di)
 	jne	setup_sectors

 	/* update position to load from */
 	subw	$BOOTSEC_LISTSIZE, %di

 	/* jump to bootloop */
 	jmp	bootloop

 /* END OF MAIN LOOP */

 bootit:
 	/* print a newline */
 	MSG(notification_done)
 	popw	%dx	/* this makes sure %dl is our "boot" drive */
 #ifdef STAGE1_5
 	ljmp	$0, $0x2200
 #else /* ! STAGE1_5 */
 	ljmp	$0, $0x8200
 #endif /* ! STAGE1_5 */


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

 #ifdef STAGE1_5
 notification_string:	.string "Loading stage1.5"
 #else
 notification_string:	.string "Loading stage2"
 #endif

 notification_step:	.string "."
 notification_done:	.string "\r\n"

 geometry_error_string:	.string "Geom"
 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 */

 	incw	%si
 message:
 	movb	(%si), %al
 	cmpb	$0, %al
 	jne	1b	/* if not end of string, jmp to display */
 	ret
 lastlist:

 /*
  *  This area is an empty space between the main body of code below which
  *  grows up (fixed after compilation, but between releases it may change
  *  in size easily), and the lists of sectors to read, which grows down
  *  from a fixed top location.
  */

 	.word 0
 	.word 0

 	. = _start + 0x200 - BOOTSEC_LISTSIZE

         /* fill the first data listing with the default */
 blocklist_default_start:
 	.long 2		/* this is the sector start parameter, in logical
 			   sectors from the start of the disk, sector 0 */
 blocklist_default_len:
 			/* this is the number of sectors to read */
 #ifdef STAGE1_5
 	.word 0		/* the command "install" will fill this up */
 #else
 	.word (STAGE2_SIZE + 511) >> 9
 #endif
 blocklist_default_seg:
 #ifdef STAGE1_5
 	.word 0x220
 #else
 	.word 0x820	/* this is the segment of the starting address
 			   to load the data into */
 #endif

 firstlist:	/* this label has to be after the list data!!! */
	/*
	* GRUB -- GRand Unified Bootloader
	* Copyright (C) 1999,2000,2001 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.
	*/

	#define ASM_FILE
	#include <shared.h>

	#ifndef STAGE1_5
	#include <stage2_size.h>
	#endif

	/*
	* 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!) */
	#ifdef STAGE1_5
	# define ABS(x) (x-_start+0x2000)
	#else
	# define ABS(x) (x-_start+0x8000)
	#endif /* STAGE1_5 */

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

	.file "start.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 0x8000 and is jumped to with
	* CS:IP 0:0x8000 in stage2.
	*/

	/*
	* we continue to use the stack for stage1 and assume that
	* some registers are set to correct values. See stage1.S
	* for more information.
	*/

	/* save drive reference first thing! */
	pushw %dx

	/* print a notification message on the screen */
	pushw %si
	MSG(notification_string)
	popw %si

	/* this sets up for the first run through "bootloop" */
	movw $ABS(firstlist - BOOTSEC_LISTSIZE), %di

	/* save the sector number of the second sector in %ebp */
	movl (%di), %ebp

	/* this is the loop for reading the secondary boot-loader in */
	bootloop:

	/* check the number of sectors to read */
	cmpw $0, 4(%di)

	/* if zero, go to the start function */
	je bootit

	setup_sectors:
	/* check if we use LBA or CHS */
	cmpb $0, -1(%si)

	/* jump to chs_mode if zero */
	je chs_mode

	lba_mode:
	/* load logical sector start */
	movl (%di), %ebx

	/* the maximum is limited to 0x7f because of Phoenix EDD */
	xorl %eax, %eax
	movb $0x7f, %al

	/* how many do we really want to read? */
	cmpw %ax, 4(%di) /* compare against total number of sectors */

	/* which is greater? */
	jg 1f

	/* if less than, set to total */
	movw 4(%di), %ax

	1:
	/* subtract from total */
	subw %ax, 4(%di)

	/* add into logical sector start */
	addl %eax, (%di)

	/* set up disk address packet */

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

	/* the number of sectors */
	movw %ax, 2(%si)

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

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

	/* save %ax from destruction! */
	pushw %ax

	/* zero %eax */
	xorl %eax, %eax

	/* the offset of buffer address */
	movw %ax, 4(%si)

	/* the absolute address (high 32 bits) */
	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

	jc read_error

	movw $BUFFERSEG, %bx
	jmp copy_buffer

	chs_mode:
	/* load logical sector start (bottom half) */
	movl (%di), %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

	/* determine the maximum sector length of this read */
	movw (%si), %ax /* get number of sectors per track/head */

	/* subtract sector start */
	subb 10(%si), %al

	/* how many do we really want to read? */
	cmpw %ax, 4(%di) /* compare against total number of sectors */


	/* which is greater? */
	jg 2f

	/* if less than, set to total */
	movw 4(%di), %ax

	2:
	/* subtract from total */
	subw %ax, 4(%di)

	/* add into logical sector start */
	addl %eax, (%di)

	/*
	* 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
	pushw %dx

	/* head number */
	movb 11(%si), %dh

	pushw %ax /* save %ax from destruction! */

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

	xorw %bx, %bx /* %bx = 0, put it at 0 in the segment */
	movb $0x2, %ah /* function 2 */
	int $0x13

	jc read_error

	/* save source segment */
	movw %es, %bx

	copy_buffer:

	/* load addresses for copy from disk buffer to destination */
	movw 6(%di), %es /* load destination segment */

	/* restore %ax */
	popw %ax

	/* determine the next possible destination address (presuming
	512 byte sectors!) */
	shlw $5, %ax /* shift %ax five bits to the left */
	addw %ax, 6(%di) /* add the corrected value to the destination
	address for next time */

	/* save addressing regs */
	pusha
	pushw %ds

	/* get the copy length */
	shlw $4, %ax
	movw %ax, %cx

	xorw %di, %di /* zero offset of destination addresses */
	xorw %si, %si /* zero offset of source addresses */
	movw %bx, %ds /* restore the source segment */

	cld /* sets the copy direction to forward */

	/* perform copy */
	rep /* sets a repeat */
	movsb /* this runs the actual copy */

	/* restore addressing regs and print a dot with correct DS
	(MSG modifies SI, which is saved, and unused AX and BX) */
	popw %ds
	MSG(notification_step)
	popa

	/* check if finished with this dataset */
	cmpw $0, 4(%di)
	jne setup_sectors

	/* update position to load from */
	subw $BOOTSEC_LISTSIZE, %di

	/* jump to bootloop */
	jmp bootloop

	/* END OF MAIN LOOP */

	bootit:
	/* print a newline */
	MSG(notification_done)
	popw %dx /* this makes sure %dl is our "boot" drive */
	#ifdef STAGE1_5
	ljmp $0, $0x2200
	#else /* ! STAGE1_5 */
	ljmp $0, $0x8200
	#endif /* ! STAGE1_5 */


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

	#ifdef STAGE1_5
	notification_string: .string "Loading stage1.5"
	#else
	notification_string: .string "Loading stage2"
	#endif

	notification_step: .string "."
	notification_done: .string "\r\n"

	geometry_error_string: .string "Geom"
	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 */

	incw %si
	message:
	movb (%si), %al
	cmpb $0, %al
	jne 1b /* if not end of string, jmp to display */
	ret
	lastlist:

	/*
	* This area is an empty space between the main body of code below which
	* grows up (fixed after compilation, but between releases it may change
	* in size easily), and the lists of sectors to read, which grows down
	* from a fixed top location.
	*/

	.word 0
	.word 0

	. = _start + 0x200 - BOOTSEC_LISTSIZE

	/* fill the first data listing with the default */
	blocklist_default_start:
	.long 2 /* this is the sector start parameter, in logical
	sectors from the start of the disk, sector 0 */
	blocklist_default_len:
	/* this is the number of sectors to read */
	#ifdef STAGE1_5
	.word 0 /* the command "install" will fill this up */
	#else
	.word (STAGE2_SIZE + 511) >> 9
	#endif
	blocklist_default_seg:
	#ifdef STAGE1_5
	.word 0x220
	#else
	.word 0x820 /* this is the segment of the starting address
	to load the data into */
	#endif

	firstlist: /* this label has to be after the list data!!! */