BIOS

Latest revision as of 08:19, 2 August 2025

BIOS (Basic Input Output System), is a ROM chip containing code (firmware) to control peripherals attached to a PC.

The x86 BIOS entry point is located at physical address 0xfffffff0 (usually corresponding to 16 bytes from the end of the BIOS image) and is executed in 16-bit real mode; it usually contains a jump instruction (either far or near). There are no other guarantees about BIOS image layout, although many vendors have developed in-house formats.

UEFI firmware images usually contain an embedded Firmware File System.

See also Firmware.

[edit] Identification

ROM images start with bytes 0xaa 0x55. At offset 3 the initialization vector is stored. In many cases this is a 3 byte jump instruction starting with hexadecimal value 0xE9. Images for cards with PCI interface contain a 4 byte signature PCIR. Images for cards which support plug and Play feature contain a 4 byte signature $PnP.

[edit] Sample files

• http://minuszerodegrees.net/rom/rom.htm - many older ROM images without Plug and Play.
• https://www.xtideuniversalbios.org/ - BIOS for IDE ATA hard disks.
• http://rayer.g6.cz/romos/romose.htm - romos.bin for loading FreeDOS.
• https://www.qemu.org/ - Download QEMU and extract or install → *.bin - ROM examples for cards with PCI interface.

[edit] Links

• Wikipedia article
• bios_extract
• BIOS Disassembly Ninjutsu Uncovered