Computer Architecture – An Introduction

An instruction cycle, also known as fetch-decode-execute cycle is the basic operational process of a computer. This process is repeated continuously by CPU from boot up to shut down of computer. The way any operand is selected during the program execution is dependent on the addressing mode of the instruction. Different languages read their text in different orders. For example, English reads from left to right, while Arabic is read right to left. This is exactly what endianness is for computers. Endianness means that the bytes in computer memory are read in a certain order. Endianness is represented two ways Big-endian (BE) and Little-endian (LE). Based on the no. of addresses, we can classify the instructions into 4 types: Zero Address Instructions, One Address Instructions, Address Instructions, Three Address Instructions. The five hierarchies in the memory are registers, cache, main memory, magnetic discs, and magnetic tapes. The first three hierarchies are volatile memories which mean when there is no power, and then automatically they lose their stored data. Whereas the last two hierarchies are non-volatile which means they store the data permanently. To perform a given task an appropriate program consisting of a list of instructions is stored in the memory. Individual instructions are brought from the memory into the processor, which executes the specified operations. Data to be stored are also stored in the memory.