Computer architecture and organization

Computer architecture means the programmer-visible design of a computer: instruction set, registers, data types, addressing modes, memory model and input-output behaviour. Computer organization means the internal implementation of that architecture: datapath width, control signals, cache design, bus structure, clocking and hardware units. Two computers can have the same architecture but different organization; for example, two processors may run the same machine code while using different cache sizes or pipeline depths.

The von Neumann model is the standard starting point. It contains a central processing unit, memory, input-output devices and interconnections. Its key idea is the stored-program concept: instructions and data are both stored in memory as binary patterns. The CPU repeatedly fetches an instruction, decodes it, executes it and stores the result if required. The main limitation is the von Neumann bottleneck, where the single path between CPU and memory restricts the rate at which instructions and data can be supplied.

Harvard architecture separates instruction memory from data memory, allowing simultaneous instruction fetch and data access. Many modern processors use a modified Harvard style inside the chip, with separate instruction and data caches but a unified main memory view for programmers.

Core point: architecture tells what the machine appears to do; organization tells how the hardware makes it happen.