BIOS is a collection of program stored in hardware ROM. BIOS contain device drivers for standard devices so that DOS could just call them to do I/O BIOS is located at the top of the address space. BIOS contain calls to read and write from absolute disk addresses and to read characters from keyboard and write to screen kernal access BIOS services by making procedure calls to io.sys instead of using interrupts to ROM. The file io.sys is loaded immediately after the computer is booted. BIOS also contain a rich collection of routines which can be accessed by user programs through software interrupts.
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