Write short notes on (Any two):

  1. Program Counter
  2. Von-Neumann Architecture
  3. Interrupt Masking


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Program Counter (PC)

This 16-bit register deals with sequencing the execution of instructions. This register is a memory pointer. The microprocessor uses this register to sequence the execution of the instructions. The function of the program counter is to point to the memory address from which the next byte is to be fetched. When a byte is being fetched, the program counter is automatically incremented by one to point to the next memory location. For example, when your computer is turned on, a signal places the decimal number F000 into the CPU. This action tells the computer to look at the first instruction on the motherboards flash memory chip.

Von Neumann Architecture

Von Neumann architecture was first published by John von Neumann in 1945.

His computer architecture design consists of a Control Unit, Arithmetic and Logic Unit (ALU), Memory Unit, Registers and Inputs/Outputs.

Von Neumann architecture is based on the stored-program computer concept, where instruction data and program data are stored in the same memory.  This design is still used in most computers produced today.

Von Neumann Architecture Diagram - Computer Science GCSE

Central Processing Unit (CPU)

The Central Processing Unit (CPU) is the electronic circuit responsible for executing the instructions of a computer program.

It is sometimes referred to as the microprocessor or processor.

The CPU contains the ALU, CU and a variety of registers.


Registers are high speed storage areas in the CPU.  All data must be stored in a register before it can be processed.

MAR Memory Address Register Holds the memory location of data that needs to be accessed
MDR Memory Data Register Holds data that is being transferred to or from memory
AC Accumulator Where intermediate arithmetic and logic results are stored
PC Program Counter Contains the address of the next instruction to be executed
CIR Current Instruction Register Contains the current instruction during processing

Arithmetic and Logic Unit (ALU)

The ALU allows arithmetic (add, subtract etc) and logic (AND, OR, NOT etc) operations to be carried out.

Control Unit (CU)

The control unit controls the operation of the computer’s ALU, memory and input/output devices, telling them how to respond to the program instructions it has just read and interpreted from the memory unit.

The control unit also provides the timing and control signals required by other computer components.


Buses are the means by which data is transmitted from one part of a computer to another, connecting all major internal components to the CPU and memory.

A standard CPU system bus is comprised of a control busdata bus and address bus.

Address Bus Carries the addresses of data (but not the data) between the processor and memory
Data Bus Carries data between the processor, the memory unit and the input/output devices
Control Bus Carries control signals/commands from the CPU (and status signals from other devices) in order to control and coordinate all the activities within the computer

Memory Unit

The memory unit consists of RAM, sometimes referred to as primary or main memory.  Unlike a hard drive (secondary memory), this memory is fast and also directly accessible by the CPU.

RAM is split into partitions.  Each partition consists of an address and its contents (both in binary form).

The address will uniquely identify every location in the memory.

Loading data from permanent memory (hard drive), into the faster and directly accessible temporary memory (RAM), allows the CPU to operate much quicker.



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