WHAT IS PROCESSOR?

CENTRAL PROCESSING UNIT(CPU)

CPU is the brain of the computer system. It performs all major calculations and comparisons and also activates and controls the operations of other units of the computer system. Hence, no other single component of a computer determines its overall performance as much as its CPU. In order, to evaluate a computer's capabilities quickly, it is important to know how CPUs are internally structured, how different CPUs differ from each other, and how to evaluate CPU speed. These and other components are described below.

CONTROL UNIT(CU)

The Control Unit of a CPU selects and interprets program instructions and then coordinates their execution. It has some special purpose resisters and a decoder to perform these activities. The special purpose registers named Instruction register and Program control register, respectively, hold the current instruction and the next instruction for execution, and in this way help the control unit in instruction selection. On the other hand, the decoder has the necessary circuitry to decode and interpret the meaning of every instruction supported by the CPU. Each instruction is accompanied by a microcode - a very basic direction that tells the CPU how to execute the instruction.

Although the Control Unit does not perform any actual processing of data, it acts as the central nervous system for all other components of the computer. It manages and coordinates the entire computer system including its input and output units. It obtains instructions from a program stored in the main memory, interprets the instruction, and issues signals that cause other units of the system to execute them.

ARITHMETIC LOGIC UNIT(ALU)

ALU of the CPU is the place where the actual execution of instructions takes place during the data processing. When the Control Unit encounters an instruction that involves an arithmetic operation (such as add, subtract, multiply, divide) or a logic operation (such as less than, equal to, or greater than) it passes control to ALU. The ALU has some special purpose registers and necessary circuitry to carry out all arithmetic and logic operations included in the set of instructions supported by the CPU.

When the entire CPU (both CU and ALU) is contained on a single tiny silicon chip, it is called a microprocessor.

INSTRUCTION SET

Every CPU has a built-in ability to execute a set of machine instructions called its instruction set. Most CPUs have 200 or more instructions (such as add, subtract, and compare) in their instruction set. The list of instructions supported by a CPU in its instruction set forms the basis for designing the machine language for the CPU.

REGISTERS

As a computer's CPU interprets and executes instructions, there is a movement of information between various units of the computer. To handle this process satisfactorily and to speed up the rate of information transfer, the CPU uses a number of special memory units called registers. These registers hold information temporarily and are part of the CPU (not main memory).

The length of a register is equal to the number of bits it can store. Hence, we refer to a register that can store 8 bits as an 8-bit register. Most CPUs today have 32-bit or 64-bit registers.

Although the number of registers varies from computer to computer, there are some registers common to all computers. The functions of such registers are described below.

1. Memory Address Register (MAR): Holds the address of the active memory location.
2. Memory Buffer Register (MBR): Holds information on its way to and from memory.
3. Program Control Register (PCR): Holds the address of the next instruction to be executed.
4. Accumulator Register (AR): Accumulates results and data to be operated upon.
5. Instruction Register (IR): Holds as instruction while it is being executed.
6. Input/Output Register (I/O R): Communicates with I/O devices.