You've seen the Central Processing Unit (CPU) in its role of control center for the computer. It carries out all instructions sent to it by the operating system or applications software. Now let's look at the device itself to see what makes it do its job.

The CPU contains two circuit elements that perform tasks and several memory locations where data/instructions are held temporarily while actions are performed.

Here they are.

• Control Unit. Directs and coordinates all elements of the computer. The control unit does not do, it only directs.
• Arithmetic/Logic Unit. This is the unit where the seven basic mathematical and logical operations you learned about earlier take place.
• Registers. Temporary storage locations.

Registers in Detail

As pointed out, registers are temporary storage areas within the CPU that are used when data must be manipulated or instructions carried out. You will usually find at least four registers, and often more. The most common are:

• Accumulator. This register is used to "accumulate" the results of calculations.
• Storage. A holding area for information taken from or to be sent to memory.
• Address. Holds the location of information or instructions the computer needs for processing.
• General Purpose. Can be used for multiple functions to include arithmetic or addressing.

Registers vary in length (number of bits) with the computer. Early personal computers had 8- or 16-bit registers, larger PC's are 32-bits, and new ones will have 64-bit registers.

This length is commonly known as a word and the longer it is, the more powerful the computer. Sometimes the registers have different lengths, depending on their use.

CPU Block Diagram

The basic operation of the CPU can be illustrated with the following diagram. (Assumed computer word size is 8-bits with 16-bit address line, like a 64K personal computer. Modern computers are much more complex.)

Commands and instructions flow to the CPU and data is moved back and forth.

All information needed is addressed by the CPU as is all transfer to and from memory. In modern designs some of these tasks are carried out by auxiliary processors.

Today's CPUs

CPUs in personal computers have moved from 8-bit to 32-bit and show every sign of not slowing down. The amount of memory each can address has also gone up with each generation, from 1 megabyte for early micros to four gigabytes (remember, that's billions), and growing.

For microcomputers, the most common CPUs are built by Intel. Early models were the 8088 and 8086, an intermediate model is the 80286, and current models are the 80386 and i486 with the Pentium just being released [1994]. New generations typically are introduced about every 18 months so. (The most popular alternatives to Intel CPUs are built by Motorola and are the 68000, 68020, 68030, and 68040; used in Apple Macintosh and other computers.) The PowerPC is another generation CPU that will bring even more computing power to the desktop.

Expect new CPUs to continue to be smaller and faster. Also, as chip-making technology advances you can expect to see other parts of the computer integrated onto a single chip with the CPU. The functions are all there, just in a smaller package.

 Floppy Disks