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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
• dseg.inc data segment locations, KB/DSK/VIDEO data areas
• keybd.asm keyboard BIOS INT16, INT9
• modref.inc BIOS I/O interface
• orgs.asm compatibility module
• postequ.inc equates used by POST and BIOS
• printer.asm printer adapter BIOS INT17
• rs232.asm communications BIOS RS232 INT14
• sysdata.inc 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 https://sites.google.com/site/pcdosretro/ibmpcbios

• 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.