CENTRAL PROCESSING UNIT

CPU or central processing unit in computer is an electronic circuitry that carries out the instruction given by a computer program.

The central processing unit (CPU) is the unit which performs most of the processing inside a computer. To control instructions and data flow to and from other parts of the computer, the CPU relies heavily on a chipset, which is a group of microchips located on the motherboard.

To function properly, the CPU relies on the system clock, memory, secondary storage, and data and address buses. This term is also known as a central processor, microprocessor or chip.

Functions of CPU in computer: 

1. Controlling all other parts of the machine and sending timing signals.
2. Transferring data between memory and I/O devices
3. Fetching data and instructions from memory
4. Decoding instruction
5. Performing arithmetical and logical operations
6. Executing programs stored in memory
7. Performing communication among the I/O devices etc.

Control Unit (CU)

A control unit (CU) handles all processor control signals. It directs all input and output flow, fetches code for instructions from micro programs and directs other units and models by providing control and timing signals. A CU component is considered the processor brain because it issues orders to just about everything and ensures correct instruction execution.

CU functions are as follows:

1. Controls sequential instruction execution
2. Interprets instructions
3. Guides data flow through different computer areas
4. Regulates and controls processor timing
5. Sends and receives control signals from other computer devices
6. Handles multiple tasks, such as fetching, decoding, execution handling and storing results

CUs are designed in two ways

Hardwired control

Design is based on a fixed architecture. The CU is made up of flip-flops, logic gates, digital circuits and encoder and decoder circuits that are wired in a specific and fixed way. When instruction set changes are required, wiring and circuit changes must be made. This is preferred in a reduced instruction set computing (RISC) architecture, which only has a small number of instructions.

Micro program control

Micro programs are stored in a special control memory and are based on flowcharts. They are replaceable and ideal because of their simplicity.

Arithmetic and Logic Unit (ALU)

An arithmetic logic unit (ALU) is a major component of the central processing unit of a computer system. It does all processes related to arithmetic and logic operations that need to be done on instruction words.

In some microprocessor architectures, the ALU is divided into the arithmetic unit (AU) and the logic unit (LU). An arithmetic logic unit is also known as an integer unit (IU).

ALUs routinely perform the following operations:

Logical Operations

These include AND, OR, NOT, XOR, NOR, NAND, etc.

Bit-Shifting Operations

This pertains to shifting the positions of the bits by a certain number of places to the right or left, which is considered a multiplication operation.

Arithmetic Operation

This refers to bit addition and subtraction. Although multiplication and division are sometimes used, these operations are more expensive to make. Addition can be used to substitute for multiplication and subtraction for division.

Register Set

A processor register (CPU register) is one of a small set of data holding places that are part of the computer processor.

A register may hold an instruction, a storage address, or any kind of data (such as a bit sequence or individual characters). Some instructions specify registers as part of the instruction. For example, an instruction may specify that the contents of two defined registers be added together and then placed in a specified register.

There are various types of computer register.  Such as

• Data Register.
• Accumulator Register.
• Memory Address Register.
• Memory Buffer Register.
• Program Counter.
• Instruction Register.
• Stack Control Register.
• Flag Register.
• I/O Address Register.
• I/O Buffer Register.

Introduction to Bus

The system bus is a pathway composed of cables and connectors used to carry data between a computer microprocessor and the main memory. The bus provides a communication path for the data and control signals moving between the major components of the computer system.

Data Bus: Carries the data that needs processing

Address Bus: Determines where data should be sent

Control Bus: Determines data processing

Data bus

1. Bi-directional
2. The data lines provide a path for moving data between system modules.
3. The data bus may consist of from 32 to hundreds of separate lines, the number of lines being referred to as the width of the data bus.
4. The width determines the overall system performance.
5. The bandwidth of a data bus is the number of bits it can transfer in a single operation, called a bus cycle.

Address Bus

1. unidirectional

The computer must be able to access every character of memory rapidly, so every character has its own address number. The central processor specifies which addresses it wants to read or write and the address bus carries this information to a memory controller circuit, which locates and fetches the information.

Control Bus

A control bus is a computer bus that is used by the CPU to communicate with devices that are contained within the computer. This occurs through physical connections such as cables or printed circuits. The CPU transmits a variety of control signals to components and devices to transmit control signals to the CPU using the control bus. One of the main objectives of a bus is to minimize the lines that are needed for communication. An individual bus permits communication between devices using one data channel. The control bus is bidirectional and assists the CPU in synchronizing control signals to internal devices and external components. It is comprised of interrupt lines, byte enable lines, read/write signals and status lines.