Nov 9, 2012


Processing unit of a computer is also known as CPU(central processing unit). It is the brain of the computer system. CPU can do all mathematical and logical operations.

                     It directs, activates and controls the operation of different blocks of a computer system. CPU receives instructions, generates control signals and finally executes the instruction. CPU may take data from the memory and input device and send data to memory or output devices.

          The features of CPU......

1. CPU is the nerve center of the computer system.
2. It performs the central control functions.
3. All mathematical operations are controlled by CPU.
4. All logical and decision-making operations are controlled by CPU.

   The processing unit is divided into three parts -----

A>ALU(arithmetic logic unit)

CPU performs arithmetic and logical operations in ALU. It can perform four basic arithmetic operations. These are addition, subtraction, multiplication and division. But it follows binary arithmetic. It can perform logic operations. These are AND, NAND, OR, NOR, NOT etc. Accumulator is the main component of ALU. ALU uses accumulator for all arithmetic and logic operations. 

The function of accumulator are.....
1. Accumulator is used in all mathematical and logical operations.
2. During addition of two data, one must be kept in accumulator.
3. Results of mathematical and logical operations are kept in accumulator.
4. All input/output data pass through accumulator.

B>CU(control unit)
Control unit is a special type of electronic circuit. It controls all functioning of the computer. It coordinates input unit, output unit and memory unit. It controls execution of instructions. It helps the computer to understand the difference between data and instructions. It decodes program instructions stored in control memory. Each instruction is read from control memory into a register. These registers are known as instruction registers (IR).

         The instructions of a program are stored one after another in control memory. The instructions are written in binary notation. The binary instructions are read by CU into IR. This process is called fetch cycle. Then the instructions are followed. The binary patterns of the instructions are used to operate necessary electronic circuit designed for this purpose. The process of executing instructions stored in IR is called execute cycle.

           The function of CU are......

1. CU reads the binary instructions into IR.
2. It controls the fetch cycle and execute cycle.
3. During arithmetic operations, CU reads data from CM into ALU.
4. CU also writes data into CM in some operations.


Execution of a program by the CPU, requires movement of information between the various units of the computer system. In order to handle this process satisfactorily and to speed up the rate of data and instruction transfer, the computer uses a number of special memory units called registers. These registers are not a part of the main memory. These are used to retain information on a temporary basis. The internal registers, which expand the capabilities of the ALU, serve as the computer's internal memory. They may also perform some data processing operations.

             The number of registers varies among computers depending on the data-flow pattern. Most computers use several types of registers, each designed to perform a specific function. Each of these registers possess the ability to receive information, to hold it temporarily and to pass it on as directed by the control unit. The length of a register indicates the number of bits it can store. Thus, a register that can store 8 bits is normally referred to as 8-bit register. Although the number of registers varies from computer to computer , there are some registers which are common to all computers. The internal register section includes general purpose registers and special purpose registers such as an accumulator register, a memory address register, a memory data register, an instruction counter, stack registers and a status register.

General Purpose Register(GPR):
The general purpose registers, GPRs of which the registers may be from 4 to 64 determine to a certain extent the computational capabilities of the microprocessors. They are primarily used for the manipulation and temporary storage of data. Since they are used as a person would use a pad of paper to write down intermediate results of an arithmetic operation, they are called 'scratch pad memory' registers.

Accumulator(Ace. or A):
The accumulator register similarly serves as a scratch pad for arithmetic, logical and input or output operations. In most microprocessors the accumulator also stores the results of the mathematical operations which replace the operand that was originally stored there. Normally, data words fetched from memory and words to be written into memory are first loaded into the accumulator. When an operation involves two operands, the accumulator will hold only one and also the result of the operation. The size or width of the accumulator is the same as, the data word size. Some microprocessors may have two or more accumulators - this enhances their flexibility and efficiency in problem-solving.


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