A CDE Definition
(Basic Input Output System) A set of routines residing in firmware that boots the operating system and sets up the hardware in an x86-based PC. Prior to loading the operating system, the BIOS provides software drivers for the basic peripheral support that is part of the motherboard, including the keyboard, mouse, monitor and hard disk. The drivers enable the user to edit configuration settings and allow the hardware to boot from the hard disk or other storage device.
After the operating system is loaded, more elaborate drivers are typically loaded, which replace the BIOS routines. The BIOS also supports internal services such as the real-time clock (time and date).
The BIOS Sets Up the Computer
On startup, the BIOS tests the system and prepares the computer for operation based on the installed hardware and the configuration settings from the manufacturer and user. For example, it initializes memory and caches and assigns resources to all devices connected to the PCI bus. The BIOS searches for BIOS extensions (option ROMs) on plug-in cards and sets up pointers (interrupt vectors) in main memory to access those routines. It then loads the operating system and passes control to it.
From ROM BIOS to Flash Memory
The BIOS, which dates back to the first IBM PC in 1981, used to be permanently stored on a read-only memory (ROM) chip. In the early days, adding a larger disk or new type of peripheral sometimes required a BIOS change, and in order to update the BIOS, it had to be replaced. Later, the BIOS was stored in rewritable flash memory. See BIOS setup, BIOS upgrades, beep codes and UEFI.
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