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x86Bootstrap

x86 MBR bootloader wroten in GNU assembly syntax.

Some really good explaination...

Multiboot handled by a menu from which you can boot on the 4 MBR partitions.

Dependencies

You can compile the only source file with your hands if make is missing.

  • make (dev under V4.3)

  • as (dev under V2.34)

  • ld (dev under V2.34)

Build

make

Install

dd if=bootstrap.bin of=/dev/sdX bs=446 count=1

Master Boot Record Format

Offset Size (bytes) Description
0x000 440 MBR Bootstrap (flat binary executable code)
0x1B8 4 Optional "Unique Disk ID / Signature"2
0x1BC 2 Optional, reserved 0x00003
0x1BE 16 First partition table entry
0x1CE 16 Second partition table entry
0x1DE 16 Third partition table entry
0x1EE 16 Fourth partition table entry
0x1FE 2 (0x55, 0xAA) "Valid bootsector" signature bytes

Traditional MBR steps

  • relocate itself to 0x0000:0x0600
  • examine the byte at offset 0x1be, 0x1ce, 0x1de, and 0x1ee to determine the active partition
  • load only the first sector of the active partition (which is expected to contain a DOS bootsector) to 0x0000:0x7c00 (hence the previous relocation)
  • set SI
  • jump to 0x7c00 -- transferring control to the DOS bootsector.

Partition Table Entry

Offset1 Size (bytes) Description
0x00 1 Drive attributes (bit 7 set = active or bootable)
0x01 3 CHS Address of partition start
0x04 1 Partition type
0x05 3 CHS address of last partition sector
0x08 4 LBA of partition start
0x0C 4 Number of sectors in partition

Runtime

runtime

Epilogue

Just for the fun.

Feel free to fork, use, improve.

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