The Processor: Nerve Center

THE CONTROL UNIT

Just as the processor is the nucleus of a computer system, the control unit is the nucleus of the processor. It has three primary functions:
1. To read and interpret program instructions
2. To direct the operation of internal processor components
3. To control the flow of programs and data in and out of RAM
During program execution, the first in a sequence of program instructions is moved from RAM to the control unit, where it is decoded and interpreted by the decoder. The control unit then directs other processor components to carry out the operations necessary to execute the instruction.
The control unit contains high-speed working storage areas called registers that can store no more than a few bytes (see Figure 2-5). Registers handle instructions and data at a speed about 10 times faster than that of cache memory and are used for a variety of processing functions. One register, called the instruction register, contains the instruction being executed. Other general-purpose registers store data needed for immediate processing. Registers also store status information. For example, the program register contains the RAM address of the next instruction to be executed. Registers facilitate the movement of data and instructions between RAM, the control unit, and the arithmetic and logic unit.