487 lines
9.6 KiB
ArmAsm
487 lines
9.6 KiB
ArmAsm
/* -*-Asm-*- */
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/*
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* GRUB -- GRand Unified Bootloader
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* Copyright (C) 1999,2000,2001,2002,2005,2006,2007 Free Software Foundation, Inc.
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*
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* GRUB 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 3 of the License, or
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* (at your option) any later version.
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*
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* GRUB 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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*
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* You should have received a copy of the GNU General Public License
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* along with GRUB. If not, see <http://www.gnu.org/licenses/>.
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*/
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#include <grub/boot.h>
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#include <grub/machine/boot.h>
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/*
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* defines for the code go here
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*/
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/* Absolute addresses
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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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.file "boot.S"
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.text
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/* Tell GAS to generate 16-bit instructions so that this code works
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in real mode. */
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.code16
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.globl _start; _start:
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/*
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* _start is loaded at 0x7c00 and is jumped to with CS:IP 0:0x7c00
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*/
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/*
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* Beginning of the sector is compatible with the FAT/HPFS BIOS
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* parameter block.
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*/
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jmp after_BPB
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nop /* do I care about this ??? */
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/*
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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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* this area.
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*/
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. = _start + 4
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/* scratch space */
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mode:
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.byte 0
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disk_address_packet:
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sectors:
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.long 0
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heads:
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.long 0
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cylinders:
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.word 0
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sector_start:
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.byte 0
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head_start:
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.byte 0
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cylinder_start:
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.word 0
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/* more space... */
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. = _start + GRUB_BOOT_MACHINE_BPB_END
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/*
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* End of BIOS parameter block.
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*/
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boot_version:
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.byte GRUB_BOOT_VERSION_MAJOR, GRUB_BOOT_VERSION_MINOR
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kernel_address:
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.word GRUB_BOOT_MACHINE_KERNEL_ADDR
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kernel_segment:
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.word GRUB_BOOT_MACHINE_KERNEL_SEG
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kernel_sector:
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.long 1, 0
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boot_drive:
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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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after_BPB:
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/* general setup */
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cli /* we're not safe here! */
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/*
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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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*/
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boot_drive_check:
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jmp 1f
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testb $0x80, %dl
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jnz 1f
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movb $0x80, %dl
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1:
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/*
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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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*/
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ljmp $0, $ABS(real_start)
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real_start:
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/* set up %ds and %ss as offset from 0 */
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xorw %ax, %ax
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movw %ax, %ds
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movw %ax, %ss
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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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/*
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* Check if we have a forced disk reference here
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*/
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MOV_MEM_TO_AL(ABS(boot_drive)) /* movb ABS(boot_drive), %al */
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cmpb $0xff, %al
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je 1f
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movb %al, %dl
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1:
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/* save drive reference first thing! */
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pushw %dx
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/* print a notification message on the screen */
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MSG(notification_string)
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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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/* 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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jz chs_mode
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/* check if LBA is supported */
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movb $0x41, %ah
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movw $0x55aa, %bx
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int $0x13
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/*
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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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*/
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popw %dx
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pushw %dx
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/* use CHS if fails */
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jc chs_mode
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cmpw $0xaa55, %bx
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jne chs_mode
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andw $1, %cx
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jz chs_mode
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lba_mode:
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xorw %ax, %ax
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movw %ax, 4(%si)
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incw %ax
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/* set the mode to non-zero */
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movb %al, -1(%si)
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/* the blocks */
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movw %ax, 2(%si)
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/* the size and the reserved byte */
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movw $0x0010, (%si)
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/* the absolute address */
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movl ABS(kernel_sector), %ebx
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movl %ebx, 8(%si)
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movl ABS(kernel_sector + 4), %ebx
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movl %ebx, 12(%si)
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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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/*
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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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* %dl = drive number
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* %ds:%si = segment:offset of disk address packet
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* Return:
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* %al = 0x0 on success; err code on failure
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*/
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movb $0x42, %ah
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int $0x13
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/* LBA read is not supported, so fallback to CHS. */
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jc chs_mode
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movw $GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
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jmp copy_buffer
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chs_mode:
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/*
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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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*/
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movb $8, %ah
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int $0x13
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jnc final_init
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/*
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* The call failed, so maybe use the floppy probe instead.
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*/
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testb $GRUB_BOOT_MACHINE_BIOS_HD_FLAG, %dl
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jz floppy_probe
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/* Nope, we definitely have a hard disk, and we're screwed. */
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jmp hd_probe_error
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final_init:
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/* set the mode to zero */
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movzbl %dh, %eax
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movb %ah, -1(%si)
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/* save number of heads */
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incw %ax
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movl %eax, 4(%si)
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movzbw %cl, %dx
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shlw $2, %dx
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movb %ch, %al
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movb %dh, %ah
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/* save number of cylinders */
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incw %ax
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movw %ax, 8(%si)
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movzbw %dl, %ax
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shrb $2, %al
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/* save number of sectors */
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movl %eax, (%si)
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setup_sectors:
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/* load logical sector start (top half) */
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movl ABS(kernel_sector + 4), %eax
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orl %eax, %eax
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jnz geometry_error
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/* load logical sector start (bottom half) */
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movl ABS(kernel_sector), %eax
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/* zero %edx */
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xorl %edx, %edx
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/* divide by number of sectors */
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divl (%si)
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/* save sector start */
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movb %dl, %cl
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xorw %dx, %dx /* zero %edx */
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divl 4(%si) /* divide by number of heads */
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/* do we need too many cylinders? */
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cmpw 8(%si), %ax
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jge geometry_error
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/* normalize sector start (1-based) */
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incb %cl
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/* low bits of cylinder start */
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movb %al, %ch
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/* high bits of cylinder start */
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xorb %al, %al
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shrw $2, %ax
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orb %al, %cl
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/* save head start */
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movb %dl, %al
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/* restore %dl */
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popw %dx
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/* head start */
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movb %al, %dh
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/*
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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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* %ch = cylinder
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* %cl = sector (bits 6-7 are high bits of "cylinder")
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* %dh = head
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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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* Return:
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* %al = 0x0 on success; err code on failure
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*/
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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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int $0x13
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jc read_error
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movw %es, %bx
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copy_buffer:
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movw ABS(kernel_segment), %es
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/*
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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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*/
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pusha
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pushw %ds
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movw $0x100, %cx
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movw %bx, %ds
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xorw %si, %si
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xorw %di, %di
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cld
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rep
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movsw
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popw %ds
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popa
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/* boot kernel */
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jmp *(kernel_address)
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/* END OF MAIN LOOP */
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/*
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* BIOS Geometry translation error (past the end of the disk geometry!).
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*/
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geometry_error:
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MSG(geometry_error_string)
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jmp general_error
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/*
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* Disk probe failure.
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*/
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hd_probe_error:
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MSG(hd_probe_error_string)
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jmp general_error
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/*
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* Read error on the disk.
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*/
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read_error:
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MSG(read_error_string)
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general_error:
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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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/* tell the BIOS a boot failure, which may result in no effect */
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int $0x18
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stop: jmp stop
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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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/*
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* message: write the string pointed to by %si
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*
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* WARNING: trashes %si, %ax, and %bx
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*/
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/*
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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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*/
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1:
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movw $0x0001, %bx
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movb $0xe, %ah
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int $0x10 /* display a byte */
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message:
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lodsb
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cmpb $0, %al
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jne 1b /* if not end of string, jmp to display */
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ret
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/*
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* Windows NT breaks compatibility by embedding a magic
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* number here.
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*/
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. = _start + GRUB_BOOT_MACHINE_WINDOWS_NT_MAGIC
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nt_magic:
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.long 0
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.word 0
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/*
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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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* sneaky, huh?
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*/
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part_start:
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. = _start + GRUB_BOOT_MACHINE_PART_START
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probe_values:
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.byte 36, 18, 15, 9, 0
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floppy_probe:
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/*
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* Perform floppy probe.
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*/
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movw $ABS(probe_values-1), %si
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probe_loop:
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/* reset floppy controller INT 13h AH=0 */
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xorw %ax, %ax
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int $0x13
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incw %si
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movb (%si), %cl
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/* if number of sectors is 0, display error and die */
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cmpb $0, %cl
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jne 1f
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/*
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* Floppy disk probe failure.
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*/
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MSG(fd_probe_error_string)
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jmp general_error
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fd_probe_error_string: .string "Floppy"
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1:
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/* perform read */
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movw $GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
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movw $0x201, %ax
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movb $0, %ch
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movb $0, %dh
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int $0x13
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/* if error, jump to "probe_loop" */
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jc probe_loop
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/* %cl is already the correct value! */
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movb $1, %dh
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movb $79, %ch
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jmp final_init
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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
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