How The Computer Works
Part 5, Boot-up and Operating System

Last time we looked at power-up of your system and started to look at what BIOS does when you flip the on switch. We pick-up now where we left off with more on BIOS and boot-up!

The next thing your BIOS does is search for software instructions to make the computer run. This software is called an operating system. All computers originally got their operating system off floppy disk so that is one of the places BIOS will look, but in more modern computers made after 1985 the first place BIOS will looks is for a hard drive with a boot sector. If it does not find this it will move to the floppy drive.

At the point just before this search for boot up occurs if you generally tap on the key rapidly most PC compatibles will start a visual front end on your screen (a front end is some type of interface the computer operator can use to do functions) that is stored in BIOS instructions. This is called the CMOS set up. It allows you to configure BIOS and some hardware. In this mode you can select what drives to boot from in what order. Some people use this to run multiple operating systems, where they might have Windows XP on C drive and Linux on D drive. By changing the order of boot search you can switch from starting in Windows to starting in Linux. You can’t, however, run both operating systems at the same time.

This also lets you set up your primary and secondary hard drives if your BIOS is so old it can’t do this automatically. There are also other features you can activate or defeat that may help the system out but may also make it run slower, such as cache memory for the hard drives or monitor.

The only time you have to run this CMOS set up is when you add a new device such as a DVD, CD-R or extra hard drive that may need to be confirmed or given a space in BIOS memory.

Once BIOS is ready to start your operating system it looks for the designated drive (they are given letters, A and B for floppy disks, C through Z for hard disks and CD players), tells the drive to activate (you often hear a buzz from the floppy drive when this happens) makes the arm with the magnetic read-write head move to a designated area and then the disk is told to turn. BIOS then scans the disk looking for an area on the disk that matches a protocol in permanent memory. This is called a boot sector. The boot sector then passes some information to BIOS via the CPU.

The BIOS and CPU now start reading other sectors (areas of data) of the hard drive that include a table of contents for the drive (in the PC this is called a File Allocation Table or FAT, in the original IBM of 1980 this was called a FCB or File Control Block, both of these protocols are still used through Windows ME but more than likely not found in Windows XP which has down away with the original concepts of 1980).

The FAT information is placed into active memory by BIOS instructions to the CPU which receives the data into a register, then “pushes” that data into what is called a memory stack.

Every computer has memory. This is a special passive chipset often called a SIMM or DIMM. Originally these were rated in KB (kilo bytes or thousands of bytes) then around mid 1980 we started rating them in MB (mega bytes or millions of bytes). Today most computers come with 64 or 128 MB of RAM memory and this is where data is stored for use in the form of a stack.
The CPU places data into the memory chip and stacks it one by one. The memory area is 32 bits in size. If the data is only 8 bits in size the rest of the 24 bits of memory may remain blank or the BIOS, Operating System and CPU may actually allocate memory on a bit by bit basis, but generally they are allocated in 32 bit sizes for direct transfer from the 32 bit registers of the CPU.

Each area in memory has an address, just like your house. The CPU places that address into another register and then stacks that address into another area of memory.

After the operating system is placed into memory BIOS sits silently, polling the 15 interrupts on the PC (a computer like a Macintosh may do this somewhat differently and may have a larger physical number of polling points because they used a 24 bit addressing system when the Mac was first created instead of the 18 bit system used by the PC). If a keyboard button is pressed, some data enters via a modem, you move the mouse or turn on a printer the BIOS instantly senses this and uses either the operating system or the internal pre-written instructions to handle this incoming information, routing it via the CPU to memory or other pathways on the computer, such as to the monitor.

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