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.