Introduction to Computer Organisation
Computer organisation studies the structural and functional units of a computer and how they interact. It bridges the gap between hardware engineering and software programming, explaining how instructions are executed at the hardware level.
Von Neumann Architecture
The Von Neumann architecture stores both data and instructions in the same memory. It consists of: CPU (ALU + Control Unit), Memory, Input/Output devices, and a Bus system. The stored-program concept allows programs to be modified as data. The bottleneck is the single bus between CPU and memory.
CPU Components
The CPU has two main parts. The Arithmetic Logic Unit (ALU) performs arithmetic and logical operations. The Control Unit (CU) fetches instructions, decodes them, and generates control signals. Registers provide fast temporary storage: program counter (PC), instruction register (IR), accumulator, and general-purpose registers.
Instruction Cycle
The instruction cycle (fetch-decode-execute) repeats for each instruction. Fetch: read instruction from memory at address in PC. Decode: interpret the opcode and operands. Execute: perform the operation. Store: write results. The cycle time determines processor speed.
Bus System
A bus is a set of parallel wires transferring data between components. The data bus carries data (width determines word size). The address bus carries memory addresses (width determines addressable memory). The control bus carries timing and control signals.
Memory Hierarchy
Memory is organised in a hierarchy by speed and cost: registers (fastest, smallest), cache (L1, L2, L3), main memory (RAM), secondary storage (SSD, HDD), and tertiary storage (tape). The principle of locality (temporal and spatial) makes caching effective.
I/O Organisation
I/O devices connect via controllers/interfaces. I/O methods: programmed I/O (CPU polls device), interrupt-driven I/O (device signals CPU), and DMA (Direct Memory Access — device transfers directly to memory). DMA frees the CPU for other tasks during large transfers.
Summary
Computer organisation covers the fundamental structure of a computer system: the Von Neumann model, CPU components, instruction execution, bus architecture, memory hierarchy, and I/O methods.