Basic Components of Computer System and Instruction Execution Cycle

A computer system consists of various hardware components. Each component plays a specific role in computer system. These components are interconnected to each other in such a way so that the computer can perform its functions.

Processor:

Processor is the main component of a computer system. It controls the operations of the computer and co-ordinates the other components of the computer. It performs all the functions according to the program instructions. It decodes and executes the instructions. It is considered the brain of the computer. It contains a number of special-purpose registers and Arithmetic Logic Unit (ALU). A modem computer system consists of one or more processors. When there is only one processor in a computer system, it is often known as the Central Processing Unit (CPU).

Main Memory:

Programs and data must be loaded into the main -memory of the computer before their execution. The main part of operating system is also loaded into the main memory during booting process of the computer. The main memory, is a temporary storage of the computer. Main memory is also known as primary memory. If the computer is shutdown or power is suddenly cut-off, all data, programs and operating system are cleared from the memory. The data and programs are permanently stored on the secondary storage device (hard disk, CD-ROM, Tape etc.).

I/O Modules:

I/O module is an electronic component. It is also known as I/O controller or controller devices. The data or program instructions are moved between the computer and its exte
al devices through I/O module. Each I/O device (such as secondary storage communication equipments, and I/O devices etc.) has its own I/O module (or device controller), which is required for communication between device and processor. It also controls the I/O operations of the I/O devices. For example, disk controller exchanges data between disk and processor. The processor writes and reads data to and from disk through disk controller.

In some cases, the I/O module exchanges data directly between main memory and I/O device. In such case, the processor grants the authority to an I/O module to write and read data to and from the memory. For this purpose, the I/O module issues read or write commands to memory.

System Bus:

A system bus is an electrical path. The data and command signals are communicated between the computer components through bus. For example, data is communicated between main memory, processor and I/O module through the bus.

The CPU is connected to all other components of computer through the system bus The CPU controls and co-ordinates other components of the computer by sending command signals. The system bus is further divided into Data bus, Control bus and Address bus.

Instruction Execution Cycle:

For every instruction of the program, the control unit (a part of CPU) carries out three basic operations, known as the machine cycle. It is also called instruction cycle.

1.Fetch Instruction:

The process of transferring a program instruction from memory to CPU is called fetch instruction. The CPU gets a program instruction from main memory for taking action on it.

2. Decode Instruction:

The process of decoding the instruction so that the computer can understand is called Decode Instruction. Actually, in this step, the necessary circuits are activated that are required to execute the instruction.

3. Execute Instruction:

The process of taking action on the decoded instruction is called Execute Instruction. After decoding the instruction, the processor executes the instruction by using the activated circuits. The results of the execution are written back to main memory or CPU registers.