xref: /external/grub/stage2/start.S

/*
 *  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!!! */