Central Processing Unit

1. The portion of a computer that includes circuits controlling the interpretation and execution of instructions.

2. The portion of a computer that executes programmed instructions, performs arithmetic and logical operations on data, and controls input / output functions. Synonym central processor.

3. Abbreviation for communications processor unit. A computer embedded in a communications system, i.e., the portion of a digital communications switch that executes programmed instructions, performs arithmetic and logical operations on signals, and controls input/output functions.

The processor is truly the computer --- it is wired to compute arithmetic and related operations on numbers that it can hold in its data registers. A processor is also called a Central Processing Unit (CPU).

* The data registers hold numbers for computation, as noted earlier.

* There is a simple clock --- a pulse generator --- that helps the Control Unit do instructions in proper time steps.

* The arithmetic-logic unit (ALU) holds the wiring for doing arithmetic on the numbers held in the data registers. (Review the addition example above.)

* The control unit holds wiring that triggers the arithmetic operations in the ALU. How does the control unit know to request an addition or a subtraction? The answer is: it obtains instructions, one at a time, that have been stored in primary storage.

* The instruction counter is a register that tells the control unit where to find the instruction that it must do. (The details will be explained shortly.)

* The instruction register is where the instruction can be copied and held for study by the control unit,

* The address buffer and data buffer are two registers that are a ``drop-off'' point when the processor wishes to copy information from a register to primary storage (or read information from primary storage to a register). We study them later.

* The interrupt register is studied much later.