The
control unit is responsible for maintaining the steady cycle of fetch-and-execute, and the ALU provides the
hardware for arithmetic operations, value comparisons (greater than, less than, equal to), and logical functions
(AND, OR, NOT, etc.).
Both the control unit and the ALU include special, very high-performance memory cells called registers.
Registers are intimately connected to the wiring of the control unit and the ALU; some have a special purpose,
and some are general purpose. One special-purpose register is the program counter (PC).
The PC keeps track of the address of the instruction to execute next. When the control unit begins
a fetch??“execute cycle, the control unit moves the instruction stored at the address saved in the PC to another
special register called the instruction register (IR). When such a fetch of the next instruction occurs, the control
unit automatically increments the PC, so that the PC now ???points??? to the next instruction in sequence.
The control unit then decodes the instruction in the IR, and executes the instruction. When execution is
complete, the control unit fetches the instruction to which the PC now points, and the cycle continues.
Other registers of the ALU are general purpose.
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