Basic operational concepts

 Basic operational concepts

• The main function of a computer system is to execute a program(sequence of instructions). These instructions are stored in system memory.

• To process data which is already loaded in the computer memory through some input devices,the instructions are executed.

• After processing the data, the result is either stored in the memory, or it is sent to the outside world through some output port.

• To execute the instructions stored in the computer memory it requires ALU, control unit, some registers to store the temporary data, and some special function registers.

• The special function registers include program counters (PC), instruction registers (IR), memory address registers (MAR) and memory data registers (MDR).

• The Program counter is one of the most important registers in CPU.

• The Program Counter is used to hold the address of the instruction which is to be executed next.

• The instruction register IR is used to hold the instruction that is currently being executed.

• The control Unit is able to get the contents of IR register to generate the timing signals that control, the various processing elements involved in executing the instruction.

• To transfer the data between main memory and the processor MAR and MDR registers are used.

• The MDR register contains the data to be read from or written into the main memory.

• The MAR register holds the address of the main memory to or from which data is to be transferred.

• Whenever the processor is asked to communicate with devices, we say that the processor is servicing the devices. The processor can service these devices in one of the two ways.

• One way is to use the polling routine, and the other way is to use an interrupt.

• Polling enables the processor software to check each of the input and output devices frequently. During this check, the processor tests to see if any devices need servicing or not.

• Interrupt method provides an external asynchronous input that informs the processor that it should complete whatever instruction that is currently being executed and fetch a new routine that will service the requesting device.