The CPU

The CPU or Central Processing Unit is the computer. At the end of the twentieth century this means it is a postage stamp sized piece of silicon etched with thousands of circuits. All the trappings of today's personal computers are just interfaces between the microchip and the human wanting to accomplish a task. The potential of the computer is the chip, the limitations are the interfaces and the human's level of frustration with the interfaces. But this is about the chip.

There are couple different ways to look at the CPU.

The pieces and design of the CPU.
The history of CPU.

Pick one and continue. (Come back later and read the other one, too).

What is it, exactly?

The CPU is actually made of several parts.

ALU - The Arithmetic Logic Unit. This circuit performs binary addition. The computer works in 1s and 0s only, called binary numbers. Anything I tell a computer is translated into binary numbers for storage and manipulation. Binary numbers can also be thought of as YES and NO which makes it simple to design switches or gates to represent them. 1 for yes or open and 0 for no or closed.

Registers - This is where the ALU puts things while its working on them. Everything entering the ALU wipes out what was done before, but you can save the result of a previous calculation in the registers.
The Clock - The clock is the pace maker of the computer. At each tick the CPU knows that the information currently residing in the registers and the ALU is correct. Between ticks the CPU is making those calculations and the information in the curcuits may not be correct. The clock speed is chosen by the designers of the the CPU after they have calculated in nanoseconds the time it will take their chip to complete its most time consuming calculation.

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Where did it come from?

The history of theCPU can also be seen as the history of the computer. When Babbage conceived of the idea for a multipurpose programable calculating machine he worked with precision gears not switches and he worked with base ten (i.e. 0, 1, 2, 3, 4, 5, 6, 7, 8, 9) not base 2 (i.e. 0, 1). He was never able to finish his machine.

In the 1930's Konrad Zuse hit upon the idea of using binary numbers to make Babbage's machine. That required only 2 positions (not 10) and a simple phone relay switch could indicate these two positions.

In the 1940's ENIAC was built, using the same idea, binary numbers, but vacuum tubes instead of telephone relays, which increased the speed.

Both Zuse's machine and ENIAC took up enormous amounts of space. A circuit board for ENIAC was an entire panel which could be removed and reprogrammed by physically moving connecting wires around to change the circuitry. Everything was 'hardwired', there was no way to save programming to use over, it had to be re-wired.

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For SI 526, Winter 1997
19 March, 1997
Suky Morita