Basic Function of Computer | Computer Architecture

Basic function of a computer in Architecture – The basic function performed by a computer is the execution of a program, which consists of a set of instructions stored in memory. The central processing unit (CPU) is a unit of a computer that performs a variety of functions given by the instructions.

In the simplest way, instruction processing consists of two steps:

1. The processor reads instructions from memory one at a time and
2. Executes each instruction. Reading and executing an instruction may involve several operations, and this depends on the meaning of that instruction. In general, program execution is therefore carried out as follows:
   1. The central processing unit (CPU) moves the instruction and input data (if necessary) from main memory to the internal registers of the CPU.
   2. Decode and execute the instructions in their stored sequence. However, the execution sequence may change in the presence of an interrupt or branching instruction.
   3. Transfers the output data, if any, from the CPU registers to the main memory.

The above three steps, for program execution, are shown in the figure below. The program starts with reading (fetching) and then executing the instructions. The program execution stops only if the machine is turned off, an error occurs, or a program instruction that stops the execution is encountered.

The processing required for a single instruction is called an instruction cycle. Each instruction cycle begins with the reading operation of an instruction from memory. This is called a fetch operation. The program counter (PC) register holds the address of the next instruction to be fetched.

The fetched instruction is then loaded into another register, known as the instruction register (IR). The instruction contains the number of bits that specify the action the processor is to take. The processor calculates the meaning of the instruction and performs the required action.

The actions of the CPU during an instruction cycle are defined by a sequence of micro operations, each of which typically involves a register transfer operation.

The flow chart shows the main function of a CPU.