BIOS Loads from 0x7c00

Posted by in Software, Tutorial

Today I was investigating a bug where a simple program wasn’t executing properly. It emerged that BIOS loads the code from the MBR at 0x7c00.

Relevant info reproduced here:

The (legacy) BIOS checks bootable devices for a boot signature, a so called magic number. The boot signature is in a boot sector (sector number 0) and it contains the byte sequence 0x55, 0xAA at byte offsets 510 and 511 respectively. When the BIOS finds such a boot sector, it is loaded into memory at 0x0000:0x7c00 (segment 0, address 0x7c00). (However, some BIOS’ load to 0x7c0:0x0000 (segment 0x07c0, offset 0), which resolves to the same physical address, but can be surprising. A good practice is to enforce CS:IP at the very start of your boot sector.)

Execution is then transferred to the freshly loaded boot record. On a floppy disk, all 512 bytes of the boot record may contain executable code. On a hard drive, the Master Boot Record (MBR) holds executable code at offset 0x0000 – 0x01bd, followed by table entries for the four primary partitions, using sixteen bytes per entry (0x01be – 0x01fd), and the two-byte signature (0x01fe – 0x01ff).


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