Boot image

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Grub starts execution with the boot image. The boot image is 512 bytes and can be found in file form at /boot/grub/boot.img. The boot image is 512 bytes because that is how big the master boot record is. On a BIOS-based computer, a disk is booted by reading the first 512 bytes of a disk into memory address 0x7c00 and jumping to that location to begin execution. On an x86 or x86-64 system, the source code for that boot image can be found in the grub source code in assembly as grub-core/boot/i386/pc/boot.S. The bootloader is written in 16-bit assembly so it may help to be familiar with the 16-bit instruction set as well as the register set used. It's also useful to know how interrupts work since they get enabled and disabled at various times. Intel makes their manuals specifying these details available at http://www.intel.com/products/processor/manuals.

/* -*-Asm-*- */

This is just a magic comment that tells certain text editors to treat this as assembly and edit in an appropriate mode.

/*
 *  GRUB  --  GRand Unified Bootloader
 *  Copyright (C) 1999,2000,2001,2002,2005,2006,2007,2008,2009  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/>.
 */

This is just the license for the code.

#include <grub/symbol.h>
#include <grub/boot.h>
#include <grub/machine/boot.h>

These includes include information about offsets and other symbols used in the assembly.

/*
 *  defines for the code go here
 */
 
	/* Print message string */
#define MSG(x)	movw $x, %si; call LOCAL(message)
#define ERR(x)	movw $x, %si; jmp LOCAL(error_message)

These are just some definitions for how to print messages.

	.file	"boot.S"

This tells the assembler that we are starting the boot.S logical file.

	.text

This tells the assembler that the text portion of the executable format starts here.

	/* Tell GAS to generate 16-bit instructions so that this code works
	   in real mode. */
	.code16

This tells the assembler to output 16-bit instructions. This seems to only apply to ARM though according to the documentation.

.globl _start, start;
_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	LOCAL(after_BPB)
	nop	/* do I care about this ??? */

This jumps past the BIOS Parameter Block that takes up the first portion of the sector.

	/*
	 * 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 + GRUB_BOOT_MACHINE_BPB_START
	. = _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 + GRUB_BOOT_MACHINE_BPB_END
 
	/*
	 * End of BIOS parameter block.
	 */

This reserves space for the BIOS parameter block. The BIOS parameter block is not necessary in the MBR, but Grub uses the same image for a volume boot records as well. A volume boot record may have a BIOS parameter block, so space is made for it.

kernel_address:
	.word	GRUB_BOOT_MACHINE_KERNEL_ADDR
 
	. = _start + GRUB_BOOT_MACHINE_KERNEL_SECTOR
kernel_sector:
	.long	1, 0
 
	. = _start + GRUB_BOOT_MACHINE_BOOT_DRIVE
boot_drive:
	.byte 0xff	/* the disk to load kernel from */
			/* 0xff means use the boot drive */

Some other magic values are set up.

LOCAL(after_BPB):
 
/* general setup */
	cli		/* we're not safe here! */

This is the first instruction after the jump. The cli instruction clears the interrupts flag so that the processor does not respond to external maskable interrupts.

        /*
         * 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).
         */
	. = _start + GRUB_BOOT_MACHINE_DRIVE_CHECK
boot_drive_check:
        jmp     3f	/* grub-setup may overwrite this jump */
        testb   $0x80, %dl
        jz      2f
3:
	/* Ignore %dl different from 0-0x0f and 0x80-0x8f.  */
	testb   $0x70, %dl
	jz      1f
2:	
        movb    $0x80, %dl
1:
	/*
	 * ljmp to the next instruction because some bogus BIOSes
	 * jump to 07C0:0000 instead of 0000:7C00.
	 */
	ljmp	$0, $real_start

Part of the BIOS interface is indicating which disk is being booted from. Some BIOSes get this wrong so this uses some heuristics to make a fallback guess if the BIOS did something dumb. The other thing done here is to deal with BIOSes that jump to 07C0:0000 instead of 0000:7C00. This is actually the same physical address, but 0000 is a different segment than 07C0. This can be problematic, so a long jump is made to allow execution to continue with a known segment.

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	$GRUB_BOOT_MACHINE_STACK_SEG, %sp
 
	sti		/* we're safe again */

The data segment and stack segment registers are zeroed out, and the stack pointer is set up. At this point, interrupts can be re-enabled.

	/*
	 *  Check if we have a forced disk reference here
	 */
	movb   boot_drive, %al
	cmpb	$0xff, %al
	je	1f
	movb	%al, %dl
1:
	/* save drive reference first thing! */
	pushw	%dx
 
	/* print a notification message on the screen */
	MSG(notification_string)
 
	/* set %si to the disk address packet */
	movw	$disk_address_packet, %si
 
	/* 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	LOCAL(chs_mode)
	cmpw	$0xaa55, %bx
	jne	LOCAL(chs_mode)
 
	andw	$1, %cx
	jz	LOCAL(chs_mode)

Grub tries to use LBA geometry, but if the BIOS does not support LBA, it falls back to using CHS geometry.

lba_mode:
	xorw	%ax, %ax
	movw	%ax, 4(%si)
 
	incw	%ax
	/* set the mode to non-zero */
	movb	%al, -1(%si)
 
	/* the blocks */
	movw	%ax, 2(%si)
 
	/* the size and the reserved byte */
	movw	$0x0010, (%si)
 
	/* the absolute address */
	movl	kernel_sector, %ebx
	movl	%ebx, 8(%si)
	movl	kernel_sector + 4, %ebx
	movl	%ebx, 12(%si)
 
	/* the segment of buffer address */
	movw	$GRUB_BOOT_MACHINE_BUFFER_SEG, 6(%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	LOCAL(chs_mode)
 
	movw	$GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
	jmp	LOCAL(copy_buffer)
 
LOCAL(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	LOCAL(final_init)
 
	/*
	 *  The call failed, so maybe use the floppy probe instead.
	 */
	testb	$GRUB_BOOT_MACHINE_BIOS_HD_FLAG, %dl
	jz	LOCAL(floppy_probe)
 
	/* Nope, we definitely have a hard disk, and we're screwed. */
	ERR(hd_probe_error_string)
 
LOCAL(final_init):
	/* set the mode to zero */
	movzbl	%dh, %eax
	movb	%ah, -1(%si)
 
	/* save number of heads */
	incw	%ax
	movl	%eax, 4(%si)
 
	movzbw	%cl, %dx
	shlw	$2, %dx
	movb	%ch, %al
	movb	%dh, %ah
 
	/* save number of cylinders */
	incw	%ax
	movw	%ax, 8(%si)
 
	movzbw	%dl, %ax
	shrb	$2, %al
 
	/* save number of sectors */
	movl	%eax, (%si)
 
setup_sectors:
	/* load logical sector start (top half) */
	movl	kernel_sector + 4, %eax
 
	orl	%eax, %eax
	jnz	LOCAL(geometry_error)
 
	/* load logical sector start (bottom half) */
	movl	kernel_sector, %eax
 
	/* zero %edx */
	xorl	%edx, %edx
 
	/* divide by number of sectors */
	divl	(%si)
 
	/* save sector start */
	movb	%dl, %cl
 
	xorw	%dx, %dx	/* zero %edx */
	divl	4(%si)		/* divide by number of heads */
 
	/* do we need too many cylinders? */
	cmpw	8(%si), %ax
	jge	LOCAL(geometry_error)
 
	/* normalize sector start (1-based) */
	incb	%cl
 
	/* low bits of cylinder start */
	movb	%al, %ch
 
	/* high bits of cylinder start */
	xorb	%al, %al
	shrw	$2, %ax
	orb	%al, %cl
 
	/* save head start */
	movb	%dl, %al
 
	/* restore %dl */
	popw	%dx
 
	/* head start */
	movb	%al, %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	$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 */
	movw	$0x0201, %ax	/* function 2 */
	int	$0x13
 
	jc	LOCAL(read_error)
 
	movw	%es, %bx
 
LOCAL(copy_buffer):
	/*
	 * We need to save %cx and %si because the startup code in
	 * kernel uses them without initializing them.
	 */
	pusha
	pushw	%ds
 
	movw	$0x100, %cx
	movw	%bx, %ds
	xorw	%si, %si
	movw	$GRUB_BOOT_MACHINE_KERNEL_ADDR, %di
	movw	%si, %es
 
	cld
 
	rep
	movsw
 
	popw	%ds
	popa
 
	/* boot kernel */
	jmp	*(kernel_address)
 
/* END OF MAIN LOOP */
/*
 * BIOS Geometry translation error (past the end of the disk geometry!).
 */
LOCAL(geometry_error):
	ERR(geometry_error_string)
</syntaxhighlight lang="asm">
This is where Grub prints the error message when there is an issue with the disk geometry.
<syntaxhighlight lang="asm">
 
/*
 * Read error on the disk.
 */
LOCAL(read_error):
	movw	$read_error_string, %si
LOCAL(error_message):
	call	LOCAL(message)
LOCAL(general_error):
	MSG(general_error_string)
 
/* go here when you need to stop the machine hard after an error condition */
        /* tell the BIOS a boot failure, which may result in no effect */
        int	$0x18
LOCAL(stop):
	jmp	LOCAL(stop)
notification_string:	.asciz "GRUB "
geometry_error_string:	.asciz "Geom"
hd_probe_error_string:	.asciz "Hard Disk"
read_error_string:	.asciz "Read"
general_error_string:	.asciz " Error\r\n"

Several null-terminated ASCII strings are set up.

/*
 * 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 */
LOCAL(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 + GRUB_BOOT_MACHINE_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?
	 */
 
	. = _start + GRUB_BOOT_MACHINE_PART_START
part_start:
 
probe_values:
	.byte	36, 18, 15, 9, 0
 
LOCAL(floppy_probe):
/*
 *  Perform floppy probe.
 */
 
	movw	$probe_values - 1, %si
 
LOCAL(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	LOCAL(general_error)
 
/* "Floppy" */
fd_probe_error_string:	.asciz "Floppy"
 
1:
	/* perform read */
	movw	$GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
	movw	$0x201, %ax
	movb	$0, %ch
	movb	$0, %dh
	int	$0x13
 
	/* if error, jump to "LOCAL(probe_loop)" */
	jc	LOCAL(probe_loop)
 
	/* %cl is already the correct value! */
	movb	$1, %dh
	movb	$79, %ch
 
	jmp	LOCAL(final_init)
 
	. = _start + GRUB_BOOT_MACHINE_PART_END
/* the last 2 bytes in the sector 0 contain the signature */
	.word	GRUB_BOOT_MACHINE_SIGNATURE

The MBR has some magic bytes that let the BIOS recognize it instead of being random data.