The CPU of a computer controls all the other parts. Most standard CPUs consists of a control unit, arithmetic and logic unit (ALU) and memory (in the form of registers, RAM, ROM etc...). L1 cache or "level one cache" is a small fast cache memory usually contained within the CPU chip or at least on the same integrated circuit. Second level cache or L2 cache is larger and slower located between the L1 cache and main memory.

The control unit fetches and decodes instructions from memory and produces signals which control the other parts of the computer. The fetch-execute cycle involves a sequence of actions that the CPU carries out in order to execute a machine code instruction in a program.

First the CPU puts the value of the Program Counter (PC) onto the address bus. The CPU then fetches the instruction from main memory through the data bus into the Instruction Register (IR).

From the Instruction Register, the data is decoded and passed to the control unit which then sends various control signals to the relevant parts of the CPU so that it can carry out the necessary operations and calculations. The Program Counter is increased in value by 1 (incremented) for each cycle so that the next instruction can be processed.

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