BIOS interrupt call

BIOS interrupt calls are a facility that operating systems and application programs use to invoke the facilities of the Basic Input/Output System on IBM PC compatible computers. Traditionally, BIOS calls are mainly used by MS-DOS programs and some other software such as boot loaders (including, mostly historically, relatively simple application software that boots directly and runs without an operating system—especially game software.) BIOS only runs in the real address mode (Real Mode) of the x86 CPU, so programs that call BIOS either must also run in real mode or must switch from protected mode to real mode before calling BIOS and then switch back again. For this reason, modern operating systems that use the CPU in Protected Mode generally do not use the BIOS to support system functions, although some of them use the BIOS to probe and initialize hardware resources during their early stages of booting.

In all computers, software instructions control the physical hardware (screen, disk, keyboard, etc.) from the moment the power is switched on. In a PC, the BIOS, pre-loaded in ROM on the main board, takes control immediately after the processor is reset, including during power-up or when a hardware reset button is pressed. The BIOS initializes the hardware, finds, loads and runs the boot program (usually, but not necessarily, an OS loader), and provides basic hardware control to the operating system running on the machine, which is usually an operating system but may be a directly booting single software application.

For IBM's part, they provided all the information needed to use their BIOS fully or to directly utilize the hardware and avoid BIOS completely, when programming the early IBM PC models (prior to the PS/2). From the beginning, programmers had the choice of using BIOS or not, on a per-hardware-peripheral basis. Today, the BIOS in a new PC still supports most, if not all, of the BIOS interrupt function calls defined by IBM for the IBM AT (introduced in 1984), along with many more newer ones, plus extensions to some of the originals (e.g. expanded parameter ranges). This, combined with a similar degree of hardware compatibility, means that most programs written for an IBM AT can still run correctly on a new PC today, assuming that the faster speed of execution is acceptable (which it typically is for all but games that use CPU-based timing). Despite the considerable limitations of the services accessed through the BIOS interrupts, they have proven extremely useful and durable to technological change.

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