/* -*-Asm-*- */ /* * GRUB -- GRand Unified Bootloader * Copyright (C) 1999,2000,2001,2002,2004 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. */ #include <stage1.h> /* * 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!) */ #define ABS(x) (x-_start+0x7c00) /* Print message string */ #define MSG(x) movw $ABS(x), %si; call message /* XXX: binutils-2.9.1.0.x doesn't produce a short opcode for this. */ #define MOV_MEM_TO_AL(x) .byte 0xa0; .word x .file "stage1.S" .text /* Tell GAS to generate 16-bit instructions so that this code works in real mode. */ .code16 .globl _start; _start: /* * _start is loaded at 0x7c00 and is jumped to with CS:IP 0:0x7c00 */ /* * Beginning of the sector is compatible with the FAT/HPFS BIOS * parameter block. */ jmp after_BPB nop /* do I care about this ??? */ /* * This space is for the BIOS parameter block!!!! Don't change * the first jump, nor start the code anywhere but right after * this area. */ . = _start + 4 /* scratch space */ mode: .byte 0 disk_address_packet: sectors: .long 0 heads: .long 0 cylinders: .word 0 sector_start: .byte 0 head_start: .byte 0 cylinder_start: .word 0 /* more space... */ . = _start + STAGE1_BPBEND /* * End of BIOS parameter block. */ stage1_version: .byte COMPAT_VERSION_MAJOR, COMPAT_VERSION_MINOR boot_drive: .byte GRUB_INVALID_DRIVE /* the disk to load stage2 from */ force_lba: .byte 0 stage2_address: .word 0x8000 stage2_sector: .long 1 stage2_segment: .word 0x800 after_BPB: /* general setup */ cli /* we're not safe here! */ /* * This is a workaround for buggy BIOSes which don't pass boot * drive correctly. If GRUB is installed into a HDD, check if * DL is masked correctly. If not, assume that the BIOS passed * a bogus value and set DL to 0x80, since this is the only * possible boot drive. If GRUB is installed into a floppy, * this does nothing (only jump). */ boot_drive_check: jmp 1f testb $0x80, %dl jnz 1f movb $0x80, %dl 1: /* * ljmp to the next instruction because some bogus BIOSes * jump to 07C0:0000 instead of 0000:7C00. */ ljmp $0, $ABS(real_start) real_start: /* set up %ds and %ss as offset from 0 */ xorw %ax, %ax movw %ax, %ds movw %ax, %ss /* set up the REAL stack */ movw $STAGE1_STACKSEG, %sp sti /* we're safe again */ /* * Check if we have a forced disk reference here */ MOV_MEM_TO_AL(ABS(boot_drive)) /* movb ABS(boot_drive), %al */ cmpb $GRUB_INVALID_DRIVE, %al je 1f movb %al, %dl 1: /* save drive reference first thing! */ pushw %dx /* print a notification message on the screen */ MSG(notification_string) /* do not probe LBA if the drive is a floppy */ testb $STAGE1_BIOS_HD_FLAG, %dl jz chs_mode /* check if LBA is supported */ movb $0x41, %ah movw $0x55aa, %bx int $0x13 /* * %dl may have been clobbered by INT 13, AH=41H. * This happens, for example, with AST BIOS 1.04. */ popw %dx pushw %dx /* use CHS if fails */ jc chs_mode cmpw $0xaa55, %bx jne chs_mode /* check if AH=0x42 is supported if FORCE_LBA is zero */ MOV_MEM_TO_AL(ABS(force_lba)) /* movb ABS(force_lba), %al */ testb %al, %al jnz lba_mode andw $1, %cx jz chs_mode lba_mode: /* save the total number of sectors */ movl 0x10(%si), %ecx /* set %si to the disk address packet */ movw $ABS(disk_address_packet), %si /* set the mode to non-zero */ movb $1, -1(%si) movl ABS(stage2_sector), %ebx /* the size and the reserved byte */ movw $0x0010, (%si) /* the blocks */ movw $1, 2(%si) /* the absolute address (low 32 bits) */ movl %ebx, 8(%si) /* the segment of buffer address */ movw $STAGE1_BUFFERSEG, 6(%si) xorl %eax, %eax movw %ax, 4(%si) 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 /* LBA read is not supported, so fallback to CHS. */ jc chs_mode movw $STAGE1_BUFFERSEG, %bx jmp copy_buffer chs_mode: /* * Determine the hard disk geometry from the BIOS! * We do this first, so that LS-120 IDE floppies work correctly. */ movb $8, %ah int $0x13 jnc final_init /* * The call failed, so maybe use the floppy probe instead. */ testb $STAGE1_BIOS_HD_FLAG, %dl jz floppy_probe /* Nope, we definitely have a hard disk, and we're screwed. */ jmp hd_probe_error final_init: movw $ABS(sectors), %si /* set the mode to zero */ movb $0, -1(%si) /* save number of heads */ xorl %eax, %eax movb %dh, %al incw %ax movl %eax, 4(%si) xorw %dx, %dx movb %cl, %dl shlw $2, %dx movb %ch, %al movb %dh, %ah /* save number of cylinders */ incw %ax movw %ax, 8(%si) xorw %ax, %ax movb %dl, %al shrb $2, %al /* save number of sectors */ movl %eax, (%si) setup_sectors: /* load logical sector start (bottom half) */ movl ABS(stage2_sector), %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 /* * 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 /* head number */ movb 11(%si), %dh /* * 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 $STAGE1_BUFFERSEG, %bx movw %bx, %es /* load %es segment with disk buffer */ xorw %bx, %bx /* %bx = 0, put it at 0 in the segment */ movw $0x0201, %ax /* function 2 */ int $0x13 jc read_error movw %es, %bx copy_buffer: movw ABS(stage2_segment), %es /* * We need to save %cx and %si because the startup code in * stage2 uses them without initializing them. */ pusha pushw %ds movw $0x100, %cx movw %bx, %ds xorw %si, %si xorw %di, %di cld rep movsw popw %ds popa /* boot stage2 */ jmp *(stage2_address) /* END OF MAIN LOOP */ /* * BIOS Geometry translation error (past the end of the disk geometry!). */ geometry_error: MSG(geometry_error_string) jmp general_error /* * Disk probe failure. */ hd_probe_error: MSG(hd_probe_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 notification_string: .string "GRUB " geometry_error_string: .string "Geom" hd_probe_error_string: .string "Hard Disk" 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 */ message: lodsb cmpb $0, %al jne 1b /* if not end of string, jmp to display */ ret /* * Windows NT breaks compatibility by embedding a magic * number here. */ . = _start + STAGE1_WINDOWS_NT_MAGIC nt_magic: .long 0 .word 0 /* * This is where an MBR would go if on a hard disk. The code * here isn't even referenced unless we're on a floppy. Kinda * sneaky, huh? */ part_start: . = _start + STAGE1_PARTSTART probe_values: .byte 36, 18, 15, 9, 0 floppy_probe: /* * Perform floppy probe. */ movw $ABS(probe_values-1), %si probe_loop: /* reset floppy controller INT 13h AH=0 */ xorw %ax, %ax int $0x13 incw %si movb (%si), %cl /* if number of sectors is 0, display error and die */ cmpb $0, %cl jne 1f /* * Floppy disk probe failure. */ MSG(fd_probe_error_string) jmp general_error fd_probe_error_string: .string "Floppy" 1: /* perform read */ movw $STAGE1_BUFFERSEG, %bx movw $0x201, %ax movb $0, %ch movb $0, %dh int $0x13 /* if error, jump to "probe_loop" */ jc probe_loop /* %cl is already the correct value! */ movb $1, %dh movb $79, %ch jmp final_init . = _start + STAGE1_PARTEND /* the last 2 bytes in the sector 0 contain the signature */ .word STAGE1_SIGNATURE
