What is Central Processing Unit ?

Central Processing Unit

The processing unit or the Central Processing Unit is responsible for carrying out The processing job. The CPU is the 'brain' of the computer. It is where all the searching, sorting, calculating and decision making take place. The CPU is the control center for a computer. It guides, directs and controls a computer's performance.

The CPU is a single large scale semiconductor circuit chip and is also known as the microprocessor. It can be represented by a simple diagram as shown below

The processor is placed and secured into a compatible CPU socket found on the motherboard. Processors produce heat, so they are covered with a heat sink to keep them cool and running smoothly.

As you can see in the above picture, the chip are hundreds of connector pins that plug into each of the corresponding holes in the socket. Today, most CPU's resemble the picture shown above. However, Intel and AMD have also experimented with slot processors that were much larger and slid into a slot on the motherboard. Also, over the years, there have been dozens of different types of sockets on motherboards. Each socket only supports specific types of processors and each has its own pin layout.

CPU history

The CPU was first developed at Intel with the help of Ted Hoff and others in the early 1970's. The first processor released by Intel was the 4004 processor, shown in the picture to the right.

HOW a CPU works

The computer does its primary work in a part of the machine we cannot see, a control center that converts data input to information output. This control center, called the central processing unit (CPU), is a highly complex, extensive set of electronic circuitry that executes stored program instructions.

Broadly, a CPU has following three sub-units :

• Arithmetic Logic Unit (ALU)
• Register Unit (RU)
• COntrol Unit(CU)

i. Arithmetic Logic Unit(ALU):

Arithmetic Logic Unit (ALU): A subunit within a computer's CENTRAL PROCESSING UNIT that performs mathematical operations such as addition, and logical shifts on the values held in the processors REGISTERS or its ACCUMULATOR.

The Arithmetic Logic Unitperforms arithmetic and logic operations. The arithmetic operation are addition(+),subtraction(-,) multiplication(x;* in computer),and division(/).The logic operations are comparision e.g. greater then (<),less then(>),greater then or equal(>=),less then or equal to (<=),equal(=), not equal<> etc.

So whenever an instruction contains an arithmetic or logic operator, the ALU will carry out that job for you.

For arithmetic operations the reasult of the arithmetic expression (e.g.5*4 reaults into 20) is stored in the memory. For logic operations, the logical result, which is either TRUE or FALSE, is stored in the memory.

The ALU has circuits to perform the basic arithmetic operation (add, subtract),logical operations. (and ,or, complement,exclusive-OR), register operations.(clear, shift,load ,move etc.), memory operations (store, read),program sequencing control operations (such as jump, conditional jump, jump to sumrouting) and input/output operations.

In addition,it usally has two registers Known as accumulator and temporary register. It also has a status register which typically has bits which are to 1 to indicate the reaults of ALU operations. The status bit are.

1. Negative result
2. Zero result
3. Overflow of result
4. Carry
5. Parity

Interrupt mask bits to enable interrupt from I/O devices. Fig. 2.6 givens the structure of microprocessor and its connection.

register Unit

This unit content several register meant for housing data during executing of any operations. Register act like a temporary data store. The data may either be stored in a register or may be called whenever required, by executing different commands. For the purpose of identification separate names are given to the registers. Some of the registers can be used by user with help of appropriate commonds and instructions.

The time required to retrieve data from registers and to store results in them is at east four times faster than that required to retrive and store data in the RAM. Thus some working registers are provided in the CPU store temporarily intermediate result obtained during computation. besides these, are two other registers Known as PC (program counter) and SP is a register which holds the address of the first location (also known as the top location) of the stack.

The number of working registers in the CPU has to be kept small as they should be addressable. As was pointed uot, the main memory in CPU,i.e. cache memory is used in the CPU to alleviate the speed mismach. The normal size of instruction and data caches are around 32KB each.

Control Unit:

As CPU referes to central processing unit of a system of which, The ALU is responsible for calculations and comparisions. And the control unit is the one which controls the entire processing. Actually, the necessory timing and control signals required for execution of any operation is also decided by this unit.

In other words, the entire processing is monitored and controlled by the control unit. Whenever an input reaches into memory, the control unit determines what type of instruction it is. And accordingly it (the control unit) instructs the ALU, or input/output devices. To understand the functioning of control unit, consider the following set of Instructions:

1. Read value A
2. Read value B
3. If A > B then
4. Set C = A * B
5. Print C
6. Otherwise
7. Set C = A - B
8. Print C
9. End