what is central processing unit

what is central processing unit

The center of the microcomputer is microprocessor. Another name of the microprocessor is CPU (Central Processing Unit). CPU controls the whole computer. For this purpose it gets help from the motherboard. CPU is connected with the motherboard and all other cards and components are directly or indirectly connected with the motherboard.

Though the CPU controls the computer, but the CPU is controlled by the programs. Programmers write and store their programs in disks (hard, floppy or compact). CPU retrieves the programs form the disks and stores them in the RAM. Then it starts executing the orders (i.e., commands or statements) given in the programs.

So the top-down structure is the programmer writes the program, program controls the CPU, CPU controls the motherboard and motherboard controls the rest of the devices. Though CPU is only a part of a computer, CPU performance determines, computer performance. CPUs have several properties, which affect their performance.

The processor is a semiconductor device. The main semiconductor portion is secured in plastic or ceramic casing. Pins connected with the semiconductor circuit in the processor, come out from the processor casing. Using these pins, the processor is connected with the motherboard. The processor’s semiconductor circuit is normally a 0.5 to 2 cm square. The plastic or ceramic casing is 4 cm to 10-cm length rectangle or square. A processor has 40 to 300 pins.

Many semiconductor industries today are producing processors. Of them Intel, AMD, Cyrix, Motorola etc. are popular. The processors used for microcomputers are 8085, 8086, 80286, Z80, Pentium, etc.

CPU Speed
There are many components inside the CPU. The CPU clock keeps the working of the components synchronized. By striking each clock a certain amount of work is done. Heart bit in our body is an example of clock tick in computer. The unit used for clock speed is megahertz. Hertz means 1 clock tick per second. So ‘Mega’ ‘Hertz’ means 1 million (106) clock tick per second.

Megahertz is the measure of speed of the processor. A microcomputer's speed mostly depends on the processor. So a faster clock means a faster computer. But this is not hundred percent true. A computer's speed depends on various other things such as memory size, motherboard, bus speed etc. So increasing only CPU speed by 50% does not make an overall increase of computer’s speed by 50%, it may increase by 20%.

Nowadays, the slowest computer you may find will be 25 or 33 MHz 386DX. The fastest speed you’ll hear may get up around 400 MHz Pentium II.

Word Size
Word size determines how much big a number the computer can operate in one operation. The larger the word size, the bigger the number. Unit of word size is bit. Today most of the processors support 32-bit word size. The largest integer number that a processor with 32-bit word size can manipulate is 232-1. A processor with larger word size is faster than a processor with smaller word size.

 Internal Cache Memory
While working the CPU stores its program and data in the RAM. There are 2 main types of RAM – static and dynamic.
# Static RAM or SRAM: Faster and 10-20 times more expensive than DRAM.
# Dynamic RAM or DRAM: Slower and 10-20 times less expensive than SRAM.

To save money, the PC uses a lot of DRAM, but it decreases the PC’s speed. To get some speed, small amount of SRAM is used in to the CPU. This SRAM is called the cache memory. The size of the cache memory is much smaller than the main memory.

The often-used data is stored in this cache memory. Thus the CPU need not access the relatively slow DRAM and the overall speed increases. There are 2 types of cache memory – internal and external. A PC can have any one or both or none of these two.

# Internal cache memory: This cache memory is built in to the CPU. It is also called level-1 or L1 cache. Size of the internal cache memory is smaller than the external cache memory.

# External cache memory: This cache memory is built on the motherboard. It is also called level-2 or L2 cache. The Pentium processors normally have 16K internal L1 cache and 256-512K external L2 cache. The Pentium-Pro has all the 2 level cache built in to the processor.

MMX Technology
For multimedia operations (video, audio etc.) MMX processors can work faster and with improved quality of color and sound. The MMX versions of Intel processors use 57 special purpose instructions. These instructions can work in parallel. These instructions are used to speed up the core algorithms, which are often used for multimedia operations.

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