How A Computer Works
Part 4: The CPU and BIOS.

Last time we looked at how chips or integrated circuits are created and how they evolved from the transistor technologies that replaced vacuum tubes in electronic devices. This time we look at two specific chips: The Central Processing Unit (CPU) and Basic In and Out Systems (BIOS) which work together to boot the computer up so you can use it.

The primary differences between PC compatible, Apple Macintosh or older computers like the Commodore C-64 and Kaypro II is their internal chip designs and specific start up instructions. On a general level every computer from your lap top to the most powerful super computer used by government essential starts up using the same principles.

The CPU is the heart of the computer. It pumps instructions through special flip-flop devices called registers. Registers, if youíll remember from last time, are like the fingers on your hands. No finger is 0 and a finger up is 1. The number of fingers on a hand determines how high the register can count. An 8 bit (or finger) register can count to 255. A 16 bit register can count to 65,535. A 20 bit register can count to 1,048,575. A 24 bit register can count to almost 17 million. A 32 bit register can count to just over 4 billion.

The first computers like the Altair, Commodore C-64, TRS-80 and even the IBM were based on 8 and 16 bit technologies. IBM quickly switch to 16 and 20 bit technologies, while Apple Macintosh, Commodore Amiga and Atari ST all embraced 16 and 24 bit technology. In the 1990ís everyone switch to 32 bit technology with some 64 or even 128 bit functionality. But all computers basically start off in a similar way.

If the CPU is the heart, then the BIOS is like the central nervous system of the body. It regulates and runs everything else using pre-programmed instructions. These instructions are written in CPU native code.

Each CPU is made up of three sections, registers (see above), simple math processing and what is called an instruction set. This instruction set makes the various registers and math processors do their flip flops and directing information from register to register and out to various devices. The BIOS determines the flow path in and out of the CPU (data interchange) as well as within the CPU (data intrachange).

The BIOS also provide a blue print for putting raw data onto the monitor screen, getting it from the keyboard and sending it to the printer. The BIOS also controls the time-share slices each input or output gets on the computer. In the PC compatible computer this is called the interrupt system and there were originally 7 and now 15 physical interrupt paths, but with all the new devices that were not around when the first PC came on the scene around 1980 a system was created to allow several devices to share these physical interrupts and this is one of the reason a PC can crash or boot improperly.

Some devices must get priority, for these interrupts are permanently allocated and no other device can use them. These would be for monitor, drives and dedicated ports for communication.

Some devices like hard drives were not planned for and got added several years after the first computers were marketed. Some manufacturers made add-in gismo you could plug into to accommodate a hard drive, but quickly the makers of the mother board added a separate controller for these drives and assigned an interrupt in the BIOS for their operation.

When you turn on the computer by pressing the power switch (see our first installment) low voltage power is sent to the motherboard (see our second installment) and this provides power to the chips (see our third installment).

The BIOS chip immediate sends instructions to the CPU in native language (so you canít use a PC BIOS on a Macintosh as they donít speak the same language) and a set of diagnostics are done. The CPU processes instructions from the BIOS and sends it down a pathway selected by the BIOS to see if a monitor and keyboard is present. On the PC the keyboard then generates an A20 line code which starts up high memory. If the keyboard is not connected or defective your boot-up will stop here.

The BIOS will look for devices based on the instructions. If you have a very old BIOS from 1993 and you add a card for a USB port it will see the card but it has no idea was a USB is, because that wasnít around in 1993. An interrupt, however, will be assigned to this card but BIOS remains ignorant of the device or how it works.

In our article last September on ATA devices we talked about protocols. If you have a computer with 1993 BIOS it is behind the time an may only work to ATA-4 protocol, so it will treat modern ATA100 hard drives as if they were an old fashioned ATA33 drive. The makers of some hard drives often include software BIOS to help compensate for this old age problem. The primary factor in this 1993 concept is that your computer will not automatically detect the drives or their configuration using the new cord protocols. Not even the software upgrade may compensate for this. You would need to upgrade your mother board for the newest connector that has more grounding and can sense the new color coded cord, plus a new BIOS that is designed to look for and configure your primary and secondary hard drives automatically.


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