Computer Architecture

Computer architecture deals with the functional behaviour of a computer system as viewed by a programmer. It can also be described as the logical structure of the system unit that house electronic components. The computer architecture forms the backbone for building successful computer system.

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Components of computer

A computer consists of following main components:

1. Input/output (I/O) unit

2. Central processing unit

3. Memory Unit

Input Unit

The computer accepts coded information through input unit by the user. It is a device that is used to give required information to the computer e.g. keyboard, mouse, etc.

An input unit performs the following functions

(a) It accepts the instructions and data from the user.

(b) It converts these instructions and data in computer in acceptable format.

(c) It supplies the converted instructions and data to the computer system for further processing.

Output Unit

This units sends the processed results to the user. It is mainly used to display the disired result to the user as per input instruction e.g. video monitor, printer and plotter, etc. 

The following functions are performed by an output unit

(a) It accepts the results produced by the computer which are in coded form and hence cannot be easily understood by us.

(b) It converts these coded results to human acceptable form.

(c) It supplies the converted results to the user.

Central processing unit (CPU)

It consists of set of registers, arithmetic logic unit and control unit,which together interpret and execute instructions in assembly language.

The primary functions of the CPU are as follows: 

(a) The CPU transfer instructions and input data from main memory to resisters, e.g. internal memory.

(b) The CPU executes the instructions in the stored sequence.

(c) When necessary, cpu transfer output data from registers to main memory.

Central processing unit is often called the brain of computer. The CPU is fabricated as a single Integrated Circuit (IC) and is also known as microprocessor.

Arithmetic Logic Unit

ALU contains the electronic circuitry that executes all arithmetic and logical operations on the available data.

ALU uses registers to hold the data that is being processed.

Most ALUs can perform the following operations 

(a) Arithmetic operations (addition, subtraction, multiplication and division).

(b) Logical operations (AND, NOT,OR, XOR)

(c) Comparisons operations (=,<,<=,>,>=)

Control Unit

CU coordinates with the input and output devices of a computer. It directs the computer to carry out stored program instructions by communicating with the ALU and the registers. It organises the processing of data and instructions.

Memory unit 

This unit is responsible to store programs or data on a temporary or permanent basis. It has primary memory ( main memory) and secondary memory (auxiliary memory). The input data which is to processed is brought into main memory before processing.

The needed instruction for processing and any kind of intermediate results are also stored in primary memory. Together with these, the final output is also stored in primary memory before transferring it to the output unit. Another kind of memory is referred as secondary memory of a computer system. 

Motherboard

The main circuit board contained in any computer called a motherboard. It is also known as the main board or logic board or system board or planer board. The biggest piece of silicon housed in the system unit of a computer is motherboard.

All the other electronic devices and circuits of computer system are attached to this board like, CPU, ROM, RAM, expansion slots, PCI slots and USB ports. It also includes controllers for devices like the hard drive, DVD drive, keyboard and mouse.

Interconnection of Units

CPU sends data, instructions and information to the components inside the computer as well as to the peripheral devices attached to it.

In other words, A bus is a set of wires used for interconnection, where each wire can carry one bit of data. In other words, bus is a set of electronic signal pathways that allows information and signals to travel between components inside or outside of a computer.

A computer bus can be divided into two types

1. Internal Bus  The internal Bus connects components inside the motherboard like CPU and system memory. It is also called the system bus.

2.  External Bus  

The external Bus connects the different external devices; peripherals, expansion slots, I/O ports and drive connections to the rest of computer. It is also referred to as the expansion bus.

Instruction Cycle

The instruction Cycle represents the sequence of events that takes place as an instruction is read from memory and executed.

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A simple instruction Cycle consists of the following steps

1. Fetching the instruction from the memory.

2. Decoding the instruction for operation.

3. Executing the instruction.

4. Storing in memory.

In above steps, step 1 and 2 instructions are same and known as fetch cycle and step 3 and 4 instructions are different and known as execute cycle.

Instructions format

Computer understands instructions only in terms of 0's and 1's which is called the machine language. A computer program is a set of instructions that describe the steps to be performed for carrying out a computation task.

The  processor must have two inputs; instructions and data. 

The instruction tell the processor what actions are needed to be performed on the data. An instruction is further divided into two parts; operation (op- code) and operand.