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Basic Operational Concepts

• The primary function of a computer system is to execute a program, sequence of instructions. These instructions are stored in computer memory.
• These instructions are executed to process data which are already loaded in the computer memory through some input devices.
• After processing the data, the result is either stored in the memory for further reference, or it is sent to the outside world through some output port.
• To perform the execution of an instruction, in addition to the arithmetic logic unit, and control unit, the processor contains a number of registers used for temporary storage of data and some special function registers.
• The special function registers include program counters (PC), instruction registers (IR), memory address registers (MAR) and memory and memory data registers (MDR).
• The Program counter is one of the most critical registers in CPU.
• The Program counter monitors the execution of instructions. It keeps track on which instruction is being executed and what the next instruction will be.
• The instruction register IR is used to hold the instruction that is currently being executed.
• The contents of IR are available to the control unit, which generate the timing signals that control, the various processing elements involved in executing the instruction.
• The two registers MAR and MDR are used to handle the data transfer between the main memory and the processor.
• The MAR holds the address of the main memory to or from which data is to be transferred.
• The MDR contains the data to be written into or read from the addressed word of the main memory.
• 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.