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Julio Guerra
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doc IBM PC AT v2 bios source code and docs Jan 10, 2015
src IBM PC AT v2 bios source code and docs Jan 10, 2015 IBM PC AT v2 bios source code and docs Jan 10, 2015

IBM PC AT version 2 (06/10/85) BIOS commented code from IBM which shows a correct implementation of POST and chipset initialization.


The code is divided in the following files:

  • ios.asm BIOS routines INT12, INT11, INT02
  • bios1.asm INT15 BIOS routines
  • bios2.asm BIOS routines INT1A, INT70, INT5, INT8
  • disk.asm fixed disk BIOS INT13
  • diskette.asm diskette BIOS INT13
  • data segment locations, KB/DSK/VIDEO data areas
  • keybd.asm keyboard BIOS INT16, INT9
  • BIOS I/O interface
  • orgs.asm compatibility module
  • equates used by POST and BIOS
  • printer.asm printer adapter BIOS INT17
  • rs232.asm communications BIOS RS232 INT14
  • protected mode EQU for post-test and BIOS routines
  • test1.asm POST TEST.1 through TEST.16
  • test2.asm POST TEST.17 through TEST.22
  • test3.asm POST exception interrupt tests
  • test4.asm POST and BIOS utility routines
  • test5.asm exception interrupt test handlers for POST
  • test6.asm POST and system bootstrap
  • video.asm video display BIOS INT10

The IBM PC AT version 2 BIOS was built using IBM MASM 2.0 on DOS.

Other legacy IBM PC BIOS sources can be found at


  • ibm-pc-techref.pdf complete IBM PC technical reference manual from the hardware design to the BIOS requirements (API, memory mapping, etc.).

  • biod-pnp-specification.pdf modern BIOS specification with Plug-and-Play specification.

  • bios-boot-specification.pdf focus on boot selection specification.

  • bios-advanced-power-management.pdf extension for advanced power management.

  • bios-pentium-pro.pdf example of BIOS writer's whose purpose is to provide to OEMs and BIOS writers recipes to initialize platforms based on a given processor like how to enable memory controllers, PCI bridges, processor's cores, etc.