The BIOS, or Basic Input/Output System, is the low-level firmware stored in a PC’s read-only memory. When the machine is switched on, the processor begins executing the BIOS, which tests the hardware, finds a bootable disk, and loads the operating system. Beyond booting, the BIOS provides a set of routines, reached through software interrupts, that read and write the keyboard, screen, and disks. These routines let the operating system and application programs talk to hardware through a stable interface instead of addressing each device directly.
The IBM Personal Computer made the BIOS famous by publishing it. The IBM Personal Computer Technical Reference of August 1981 contains the complete assembly-language source listing of the system BIOS, alongside the hardware documentation. This served two purposes: it taught programmers exactly how the machine worked, and it defined the interface that all PC software would target. Because applications and operating systems called the BIOS rather than the bare hardware, the BIOS became the contract that defined what it meant to be a PC.
That same published listing created the central legal problem of the early PC industry. The printed BIOS source was IBM’s copyrighted work, so a competitor could not legally copy it into a compatible machine. Yet to be truly compatible, a rival’s firmware had to behave identically. The resolution was to treat the BIOS as a specification to be reimplemented from its observable behavior, written by people who had never read IBM’s code, rather than as text to be copied. Companies such as Compaq and Phoenix Technologies produced such independent BIOS implementations, and later IBM reference manuals like the 1987 PS/2 and PC BIOS Interface Technical Reference continued to document the interface itself.
This division between a copyrightable implementation and a freely reimplementable interface is the BIOS’s lasting contribution to computing history. It made the BIOS the thin, well-defined seam along which the PC architecture could be cleanly separated from IBM, and it is the reason a single layer of firmware could both boot a machine and unlock an entire industry of compatible hardware.