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* GRUB -- GRand Unified Bootloader
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* Copyright (C) 1999,2000,2001,2002,2005 Free Software Foundation, Inc.
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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#include <grub/boot.h>
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#include <grub/machine/boot.h>
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* defines for the code go here
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This makes the assembler generate the address without support
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from the linker. (ELF can't relocate 16-bit addresses!) */
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#define ABS(x) (x-_start+0x7c00)
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/* Print message string */
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#define MSG(x) movw $ABS(x), %si; call message
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/* XXX: binutils-2.9.1.0.x doesn't produce a short opcode for this. */
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#define MOV_MEM_TO_AL(x) .byte 0xa0; .word x
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/* Tell GAS to generate 16-bit instructions so that this code works
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.globl _start; _start:
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* _start is loaded at 0x7c00 and is jumped to with CS:IP 0:0x7c00
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* Beginning of the sector is compatible with the FAT/HPFS BIOS
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nop /* do I care about this ??? */
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* This space is for the BIOS parameter block!!!! Don't change
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* the first jump, nor start the code anywhere but right after
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. = _start + GRUB_BOOT_MACHINE_BPB_END
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* End of BIOS parameter block.
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.byte GRUB_BOOT_VERSION_MAJOR, GRUB_BOOT_VERSION_MINOR
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.byte 0xff /* the disk to load kernel from */
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/* 0xff means use the boot drive */
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.word GRUB_BOOT_MACHINE_KERNEL_ADDR
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.word GRUB_BOOT_MACHINE_KERNEL_SEG
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cli /* we're not safe here! */
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* This is a workaround for buggy BIOSes which don't pass boot
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* drive correctly. If GRUB is installed into a HDD, check if
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* DL is masked correctly. If not, assume that the BIOS passed
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* a bogus value and set DL to 0x80, since this is the only
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* possible boot drive. If GRUB is installed into a floppy,
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* this does nothing (only jump).
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* ljmp to the next instruction because some bogus BIOSes
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* jump to 07C0:0000 instead of 0000:7C00.
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ljmp $0, $ABS(real_start)
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/* set up %ds and %ss as offset from 0 */
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/* set up the REAL stack */
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movw $GRUB_BOOT_MACHINE_STACK_SEG, %sp
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sti /* we're safe again */
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* Check if we have a forced disk reference here
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MOV_MEM_TO_AL(ABS(boot_drive)) /* movb ABS(boot_drive), %al */
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/* save drive reference first thing! */
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/* print a notification message on the screen */
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MSG(notification_string)
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/* do not probe LBA if the drive is a floppy */
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testb $GRUB_BOOT_MACHINE_BIOS_HD_FLAG, %dl
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/* check if LBA is supported */
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* %dl may have been clobbered by INT 13, AH=41H.
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* This happens, for example, with AST BIOS 1.04.
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/* use CHS if fails */
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/* check if AH=0x42 is supported if FORCE_LBA is zero */
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MOV_MEM_TO_AL(ABS(force_lba)) /* movb ABS(force_lba), %al */
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/* set %si to the disk address packet */
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movw $ABS(disk_address_packet), %si
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/* set the mode to non-zero */
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movl ABS(kernel_sector), %ebx
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/* the size and the reserved byte */
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/* the absolute address (low 32 bits) */
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/* the segment of buffer address */
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movw $GRUB_BOOT_MACHINE_BUFFER_SEG, 6(%si)
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* BIOS call "INT 0x13 Function 0x42" to read sectors from disk into memory
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* Call with %ah = 0x42
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* %ds:%si = segment:offset of disk address packet
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* %al = 0x0 on success; err code on failure
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/* LBA read is not supported, so fallback to CHS. */
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movw $GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
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* Determine the hard disk geometry from the BIOS!
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* We do this first, so that LS-120 IDE floppies work correctly.
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* The call failed, so maybe use the floppy probe instead.
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testb $GRUB_BOOT_MACHINE_BIOS_HD_FLAG, %dl
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/* Nope, we definitely have a hard disk, and we're screwed. */
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movw $ABS(sectors), %si
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/* set the mode to zero */
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/* save number of heads */
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/* save number of cylinders */
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/* save number of sectors */
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/* load logical sector start (bottom half) */
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movl ABS(kernel_sector), %eax
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/* divide by number of sectors */
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/* save sector start */
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xorl %edx, %edx /* zero %edx */
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divl 4(%si) /* divide by number of heads */
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/* save head start */
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/* save cylinder start */
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/* do we need too many cylinders? */
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* This is the loop for taking care of BIOS geometry translation (ugh!)
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/* get high bits of cylinder */
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shlb $6, %dl /* shift left by 6 bits */
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movb 10(%si), %cl /* get sector */
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incb %cl /* normalize sector (sectors go
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from 1-N, not 0-(N-1) ) */
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orb %dl, %cl /* composite together */
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movb 12(%si), %ch /* sector+hcyl in cl, cylinder in ch */
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* BIOS call "INT 0x13 Function 0x2" to read sectors from disk into memory
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* Call with %ah = 0x2
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* %al = number of sectors
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* %cl = sector (bits 6-7 are high bits of "cylinder")
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* %dl = drive (0x80 for hard disk, 0x0 for floppy disk)
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* %es:%bx = segment:offset of buffer
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* %al = 0x0 on success; err code on failure
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movw $GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
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movw %bx, %es /* load %es segment with disk buffer */
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xorw %bx, %bx /* %bx = 0, put it at 0 in the segment */
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movw $0x0201, %ax /* function 2 */
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movw ABS(kernel_segment), %es
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* We need to save %cx and %si because the startup code in
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* kernel uses them without initializing them.
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jmp *(kernel_address)
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/* END OF MAIN LOOP */
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* BIOS Geometry translation error (past the end of the disk geometry!).
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MSG(geometry_error_string)
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* Disk probe failure.
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MSG(hd_probe_error_string)
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* Read error on the disk.
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MSG(read_error_string)
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MSG(general_error_string)
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/* go here when you need to stop the machine hard after an error condition */
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notification_string: .string "GRUB "
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geometry_error_string: .string "Geom"
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hd_probe_error_string: .string "Hard Disk"
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read_error_string: .string "Read"
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general_error_string: .string " Error"
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* message: write the string pointed to by %si
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* WARNING: trashes %si, %ax, and %bx
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* Use BIOS "int 10H Function 0Eh" to write character in teletype mode
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* %ah = 0xe %al = character
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* %bh = page %bl = foreground color (graphics modes)
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int $0x10 /* display a byte */
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jne 1b /* if not end of string, jmp to display */
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* Windows NT breaks compatibility by embedding a magic
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. = _start + GRUB_BOOT_MACHINE_WINDOWS_NT_MAGIC
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* This is where an MBR would go if on a hard disk. The code
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* here isn't even referenced unless we're on a floppy. Kinda
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. = _start + GRUB_BOOT_MACHINE_PART_START
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.byte 36, 18, 15, 9, 0
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* Perform floppy probe.
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movw $ABS(probe_values-1), %si
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/* reset floppy controller INT 13h AH=0 */
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/* if number of sectors is 0, display error and die */
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* Floppy disk probe failure.
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MSG(fd_probe_error_string)
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fd_probe_error_string: .string "Floppy"
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movw $GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
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/* if error, jump to "probe_loop" */
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/* %cl is already the correct value! */
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. = _start + GRUB_BOOT_MACHINE_PART_END
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/* the last 2 bytes in the sector 0 contain the signature */
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.word GRUB_BOOT_MACHINE_SIGNATURE
added
removed
boot/i386/pc/boot.S
