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/*
* GRUB -- GRand Unified Bootloader
* Copyright (C) 1999,2000,2001,2002,2006,2007,2009,2010 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
*/
#define MSG(x) movw $x, %si; call LOCAL(message)
.file "diskboot.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 0x2000 and is jumped to with
* CS:IP 0:0x2000 in kernel.
*/
/*
* we continue to use the stack for boot.img and assume that
* some registers are set to correct values. See boot.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 $LOCAL(firstlist), %di
/* save the sector number of the second sector in %ebp */
movl (%di), %ebp
/* this is the loop for reading the rest of the kernel in */
LOCAL(bootloop):
/* check the number of sectors to read */
cmpw $0, 8(%di)
/* if zero, go to the start function */
je LOCAL(bootit)
LOCAL(setup_sectors):
/* check if we use LBA or CHS */
cmpb $0, -1(%si)
/* use CHS if zero, LBA otherwise */
je LOCAL(chs_mode)
/* load logical sector start */
movl (%di), %ebx
movl 4(%di), %ecx
/* 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, 8(%di) /* compare against total number of sectors */
/* which is greater? */
jg 1f
/* if less than, set to total */
movw 8(%di), %ax
1:
/* subtract from total */
subw %ax, 8(%di)
/* add into logical sector start */
addl %eax, (%di)
adcl $0, 4(%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 */
movl %ebx, 8(%si)
movl %ecx, 12(%si)
/* the segment of buffer address */
movw $GRUB_BOOT_MACHINE_BUFFER_SEG, 6(%si)
/* save %ax from destruction! */
pushw %ax
/* the offset of buffer address */
movw $0, 4(%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 LOCAL(read_error)
movw $GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
jmp LOCAL(copy_buffer)
LOCAL(chs_mode):
/* load logical sector start (top half) */
movl 4(%di), %eax
orl %eax, %eax
jnz LOCAL(geometry_error)
/* 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 LOCAL(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, 8(%di) /* compare against total number of sectors */
/* which is greater? */
jg 2f
/* if less than, set to total */
movw 8(%di), %ax
2:
/* subtract from total */
subw %ax, 8(%di)
/* add into logical sector start */
addl %eax, (%di)
adcl $0, 4(%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 $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 */
movb $0x2, %ah /* function 2 */
int $0x13
jc LOCAL(read_error)
/* save source segment */
movw %es, %bx
LOCAL(copy_buffer):
/* load addresses for copy from disk buffer to destination */
movw 10(%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, 10(%di) /* add the corrected value to the destination
address for next time */
/* save addressing regs */
pusha
pushw %ds
/* get the copy length */
shlw $3, %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 */
movsw /* 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, 8(%di)
jne LOCAL(setup_sectors)
/* update position to load from */
subw $GRUB_BOOT_MACHINE_LIST_SIZE, %di
/* jump to bootloop */
jmp LOCAL(bootloop)
/* END OF MAIN LOOP */
LOCAL(bootit):
/* print a newline */
MSG(notification_done)
popw %dx /* this makes sure %dl is our "boot" drive */
ljmp $0, $(GRUB_BOOT_MACHINE_KERNEL_ADDR + 0x200)
/*
* BIOS Geometry translation error (past the end of the disk geometry!).
*/
LOCAL(geometry_error):
MSG(geometry_error_string)
jmp LOCAL(general_error)
/*
* Read error on the disk.
*/
LOCAL(read_error):
MSG(read_error_string)
LOCAL(general_error):
MSG(general_error_string)
/* go here when you need to stop the machine hard after an error condition */
LOCAL(stop): jmp LOCAL(stop)
notification_string: .asciz "loading"
notification_step: .asciz "."
notification_done: .asciz "\r\n"
geometry_error_string: .asciz "Geom"
read_error_string: .asciz "Read"
general_error_string: .asciz " 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
LOCAL(message):
movb (%si), %al
cmpb $0, %al
jne 1b /* if not end of string, jmp to display */
ret
/*
* 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 - GRUB_BOOT_MACHINE_LIST_SIZE
LOCAL(firstlist): /* this label has to be before the first list entry!!! */
/* fill the first data listing with the default */
blocklist_default_start:
/* this is the sector start parameter, in logical sectors from
the start of the disk, sector 0 */
.long 2, 0
blocklist_default_len:
/* this is the number of sectors to read. grub-mkimage
will fill this up */
.word 0
blocklist_default_seg:
/* this is the segment of the starting address to load the data into */
.word (GRUB_BOOT_MACHINE_KERNEL_SEG + 0x20)