The BIOS and booting up a computer
What happens when you boot-up a computer?
Lots of things happen to a computer from the moment you turn the power on to the moment you start using it.
Description of the boot-up process and the role of the BIOS
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- When you power up a computer, a little program of instructions (known as the BIOS or Basic Input Output System and which is held in the ROM chip) starts running.
- The job of the BIOS is to check to see if the computer has the basic, error-free hardware needed to get the PC up and running. It runs what is known as a POST routine, or ‘Power On Self Test’. Specifically, this small program checks to see if there is a keyboard, a hard disk, a floppy disk and some memory (RAM). If it finds any of these absent or if it finds an error with any of them, then it displays an error message (and beeps)!
- Some of the BIOS is held in ROM. Some of it, however, is held in battery-backed RAM (known as CMOS) and you the user can configure it! Don't play with the BIOS settings on your computer unless you know what you are doing! You can create a lot of serious problems!
- Once the basic hardware check is complete, the BIOS then looks in some pre-set places for a special program called the ‘bootstrap program’. Often, the first place it looks in is in the first sector (known as the 'boot sector') of the hard drive and then if it can't find the bootstrap there, it looks in the first sector of any floppy disk in the floppy disk drive or possibly the CD drive. We said above, however, that you could configure certain settings in the battery-backed part of the BIOS. One thing the user can configure is the order that the BIOS looks at devices when it is looking for the bootstrap. Once again, I don't recommend you play with the BIOS, but your teacher may show you the BIOS if you ask nicely! So one thing the user might change is to get the computer to always look for the bootstrap program on the floppy disk first, or last, depending upon what the user wants and needs!
- When the BIOS has located the bootstrap program, wherever it is, it loads it into RAM and runs.
- The bootstrap program then takes over control of loading whatever operating system you are using. It first of all locates the operating system on the hard drive and then transfers it from the hard disk into RAM and runs it.
- Once your operating system has loaded, the computer automatically looks for ‘boot files’. These are special files that the user can play with to set up the computer in exactly the way that the user wants.
More about boot files
Once the operating system has been loaded, users will often want to customise their system. For example, they might want a virus-checking program to run automatically and for the computer to open Word. To customise a system, users can add their own commands to certain files known as ‘boot files’ (e.g. command.com and autoexec.bat in Windows). These files are looked at automatically after the operating system has been loaded. If present, the instructions in them are carried out. The user then uses the computer. Boot files allow a user to set up (or 'configure') their PC in exactly the way they want.