Central Processing Unit (CPU): Also known as the computer processor, the CPU is an electronic circuit that executes computer programs. The primary responsibility of a computer processor is to execute a sequential set of instructions that constitute a program. CPU operation can be divided into four basic steps, namely, fetch, decode, execute, and writeback. During the 'fetch' step, the processor retrieves program instructions from memory. In the decode step, the instruction is broken down into parts. The instruction set architecture of the CPU defines the way in which an instruction is decoded. In the 'execute' step, CPU performs the operation implied by the program instruction. During the 'writeback' step, the CPU writes back the results of execution, to the computer's memory.