Architecture

Contemporary computers put the ALU and control unit into a single integrated circuit known as the Central Processing Unit or CPU. Typically, the computer's memory is located on a few small integrated circuits near the CPU. The overwhelming majority of the computer's mass is either ancillary systems (for instance, to supply electrical power) or I/O devices.
Some larger computers differ from the above model in one major respect - they have multiple CPUs and control units working simultaneously. Additionally, a few computers, used mainly for research purposes and scientific computing, have differed significantly from the above model, but they have found little commercial application, because their programming model has not yet standardized.
The functioning of a computer is therefore in principle quite straightforward. Typically, on each clock cycle, the computer fetches instructions and data from its memory. The instructions are executed, the results are stored, and the next instruction is fetched. This procedure repeats until a halt instruction is encountered.