CPU

From ArticleWorld

The central processing unit or CPU is a component of a digital computer that interprets instructions and processes data. The CPU coordinates the entire activity of the computer, driven by computer programs.

Since the 1970s, the CPU has been concentrated on a single chip (initially it could go many square meters in surface). Today, few CPUs are still not concentrated on a single chip. Such a chip is called a microprocessor and is a fundamental component of today's computers.

Functions

The fundamental function of any CPU is that of executing a number of stored instructions. This is done in a three-step cycle:

• the Fetch cycle, in which the instruction is retrieved from a certain memory location. This is usually done using a special CPU register (a location that holds a value) called Program Counter (PC). The PC stores a value which identifies the position of the current instruction in the computer's memory. The instruction is retrieved and usually placed in a special register.

• the Decode cycle, in which the value retrieved from memory is broken down into its components. A value may well store a single instruction, but it may also store an instruction and its argument, depending on the CPU.

• the Execute cycle. In this cycle, the CPU executes the operations associated with the instruction that has been decoded. Some claim that this is followed by a fourth step, in which the results of the execution are written into the memory, the writeback step. However, other specialists simply consider this to be a part of the Execute cycle.

What defines a CPU?

There are currently many types of CPUs, and, in order to distinguish between them, various characteristics are taken into consideration.

The first one is the integer range. The integer range refers to the number of bits used by the CPU to make a word. A CPU deals with its information in chunks, much like humans communicate using words. The number of bits that make up a word denote the CPU's word size or integer size. For an 8-bit CPU, a word is 8-bit long, meaning that the largest integer that can be stored in a word is 256 (or 2 raised at the power of 8).

The other characteristic is the clock rate. In order for a CPU to work, its activity is coordinated by a "clock". Every time the clock ticks, something happens in the CPU: an instruction is fetched, part of its decoding is done, and so on. Therefore, a high clock rate (a high number of ticks in a certain time interval) means a high number of operations done in that interval. Most clock rates are measured in MHz or even GHz today, since the internal clock is really represented as a square wave.