The CPU

So how does the computer function? Obviously, simply storing data doesn't do much help - you need to be able to access, manipulate, and move it. That's where the CPU comes in.

The CPU reads in instructions from memory one at a time and executes them. This is known as the fetch-execute cycle. The CPU contains the following elements to accomplish this:

• Program Counter

• Instruction Decoder

• Data bus

• General-purpose registers

• Arithmetic and logic unit

The program counter is used to tell the computer where to fetch the next instruction from. We mentioned earlier that there is no difference between the way data and programs are stored, they are just interpreted differently by the CPU. The program counter holds the memory address of the next instruction to be executed. The CPU begins by looking at the program counter, and fetching whatever number is stored in memory at the location specified. It is then passed on to the instruction decoder which figures out what the instruction means. This includes what process needs to take place (addition, subtraction, multiplication, data movement, etc.) and what memory locations are going to be involved in this process. Computer instructions usually consist of both the actual instruction and the list of memory locations that are used to carry it out.

Now the computer uses the data bus to fetch the memory locations to be used in the calculation. The data bus is the connection between the CPU and memory. It is the actual wire that connects them. If you look at the motherboard of the computer, the wires that go out from the memory are your data bus.

In addition to the memory on the outside of the processor, the processor itself has some special, high-speed memory locations called registers. There are two kinds of registers - general registers and special-purpose registers. General-purpose registers are where the main action happens. Addition, subtraction, multiplication, comparisions, and other operations generally use general-purpose registers for processing. However, computers have very few general-purpose registers. Most information is stored in main memory, brought in to the registers for processing, and then put back into memory when the processing is completed. special-purpose registers are registers which have very specific purposes. We will discuss these as we come to them.

Now that the CPU has retrieved all of the data it needs, it passes on the data and the decoded instruction to the arithmetic and logic unit for further processing. Here the instruction is actually executed. After the results of the computation have been calculated, the results are then placed on the data bus and sent to the appropriate location in memory or in a register, as specified by the instruction.

This is a very simplified explanation. Processors have advanced quite a bit in recent years, and are now much more complex. Although the basic operation is still the same, it is complicated by the use of cache hierarchies, superscalar processors, pipelining, branch prediction, out-of-order execution, microcode translation, coprocessors, and other optimizations. Don't worry if you don't know what those words mean, you can just use them as Internet search terms if you want to learn more about the CPU.