I hate to prevent a delightful presentation, but it is very difficult (if not impossible) to write a boot block for MBR on a PC for several reasons, mainly related to the dodgy loading of a PC / outdated BIOS:
- The BIOS will load your block, but then write it in the exact place to provide information. This is the "boot parameter block" [BPB]. Among other things, it has a disk number to download.
- BPB starts at offset 0x03. So, the first 3 bytes must have a
jmp asm instruction to skip this. - The last two bytes of the sector must have a BIOS boot signature of 0x55AA
- There must be space in the exact location of the partition table [which will be overwritten by partition editors such as
fdisk
There is a bit of free space left and the code is complex and strongly tied to the BIOS. You must use int asm instructions to query the BIOS and issue disk read calls. They have an exact sequence, and for some you should be able to change the registers of x86 segments, as well as manage general registers with byte loads (for example, you should be able to set values to %ah regardless of %al ).
Most downloads [almost all] write the boot sector to the assembly. To save space, some boot blocks execute some code and then overwrite this code with data, because the code is executed once and the space is reused. This is hard to do in C.
By the time you have adjusted your C program, you may find out that it simply does not correspond to 512 bytes.
In the case of grub its boot block loads a sector or two from the area after the boot block and transfers control. This, using parameters configured by the boot block, is read more by grub . grub "co-ops" part of the "no man's land" from the second sector to the first section [which starts at 1 MB]. Then it transfers control, which then loads the remainder and decides how to load the OS. Later stages are recorded in C.
For Windows, the MBR will load only the first sector of the partition, marked as boot, and then transfer control. This second step is the NT bootloader. He knows that the first N blocks after it in the NTFS file system contain the rest of the download, so he just loads them sequentially and transfers control.
Please note that the second stage bots have a bit of assembly, but after downloading and checking the bits, they can switch to C-code. I recommend this approach.
As an example, here is the source for the boot block for grub2 :
/* -*-Asm-*- */ /* * 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/>. */ #include <grub/symbol.h> #include <grub/machine/boot.h> /* * 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) .file "boot.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 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 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. */ 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 */ LOCAL(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). */ . = _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 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 */ /* * 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) 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) /* * 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" /* * 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 %bx, %es xorw %bx, %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