The system is initialized by a software reset (Ctrl-Alt-Del), a hardware reset (reset button), or by turning the computer on. The Intel 80x8x series processors always look for their first instruction at the end of their address space (0FFFF0h) when powered up or reset. This address contains a jump to the first instruction for the ROM BIOS.
Built-in ROM programs (Power-On Self-Test, or POST, in the IBM) check machine status and run inspection programs of various sorts. Some machines set up a reserved RAM area with bytes indicating installed equipment (AT and PCjr).
The ROM routine looks for a disk drive at A: or an option ROM (usually a hard disk) at absolute address C:800h. BIOS\Dsk\ROM If no floppy drive or option ROM is found, the BIOS calls Int\19 (ROM BASIC if it is an IBM) or displays error message. If a bootable disk is found, the ROM BIOS loads the first sector of information from the disk and then jumps into the RAM location holding that code. This code normally is a routine to load the rest of the code off the disk, or to "boot" the system.
The following actions occur after a system initialization:
file: /Techref/bios/init.htm, 4KB, , updated: 1999/2/20 11:24, local time: 2024/10/6 00:02,
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