The main function of the CPU is processing instructions given to the computer. This is why the CPU is known as the "brain" of the PC. Every operation that is carried out, including key strokes, opening and closing programs, and saving files, is carried out by the CPU. This function is separated into four functions or steps for every operation: fetch, decode, execute and store.
Typically, the main parts of a CPU responsible for carrying out the operations are the arithmetical logical unit and the control unit. The ALU carries out arithmetic and logical operations and makes decisions, whereas the CU fetches, decodes and executes processes.
The most popular CPU manufacturers, as of 2014, are AMD and Intel. Initially, a CPU contained only one processor; then, Intel came up with the dual-core processor, a CPU containing two processors, also known as two cores. For over two decades, as of August 2014, both Intel and AMD have continued to develop and produce multiple core processors. These are quad core, hexa core and octo core, or four, six and eight cores, respectively.
Other characteristics for functional performance of a CPU are the clock speed and the number of instructions it can perform in a second.