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The BIOS Program
In order for any computer to function, it must have software to run on it. All that a processor--or any hardware for that matter--knows how to do is to follow instructions. The software is that collection of instructions, as described in this part of the Introduction. All regular programs that you run on your PC are stored permanently on your hard disk, and are loaded into your system memory (RAM) when you need to use them. From the memory, the processor can access the instructions coded into the program and run them, which lets you do your work.
When you first turn on your PC, the processor is "raring to go", but it needs some instructions to execute. However, since you just turned on the machine, your system memory is empty; there are no programs to run. To make sure that the BIOS program is always available to the processor, even when it is first turned on, it is "hard-wired" into a read-only-memory (ROM) chip that is placed on your motherboard.
A uniform standard was created between the makers of processors and the makers of BIOS programs, so that the processor would always look in the same place in memory to find the start of the BIOS program. The processor gets its first instructions from this location, and the BIOS program begins executing. The BIOS program then begins the system boot sequence which calls other programs, gets your operating system loaded, and your PC up and running.
The BIOS program is always located in a special reserved memory area, the upper 64K of the first megabyte of system memory (addresses F000h to FFFFh). Some BIOSes use more than this 64K area.
Next: Other PC BIOSes