What is in a CPU?

Answer 1

A computer contains memory, input devices, output devices and a CPU , or Central Processing Unit.

The CPU is the bit that executes the program. The program is a set of instructions written by a human. The CPU reads, interprets and carries out those instructions. It is truly the heart of the computer. The components of a computer system can be combined and permuted almost without limit.

There is really just one limit; if the CPU is removed then it's not a computer anymore. Without the CPU there is nothing to send to the printer, nothing to read what is being typed on the keyboard, nothing to carry out the instructions in the program or even to know what the instructions were.

What is in a CPU is quite easy to list, but would need a book to explain properly. A CPU contains

• a read only memory (ROM) containing some instructions for initial switch-on
• an address bus, which specifies a memory location to be read from or written to
• a main data bus, which carries data to or from memory.
• an instruction register, containing the instruction now being executed
• an address register containing the memory address wanted next
• an instruction counter, keeping track of where the next instruction will be found
• an arithmetic and logic unit (ALU) which does arithmetic and logical functions
• an instruction decoder, which, working in conjunction with the microprogram unit containing another ROM, sorts out what is done when and by which unit.
• various units moving data to and from the outside world via an input/output bus
• a clock and a clock-phase generator which ensures that different units work in sync.