The Control Unit
The control unit is the “policeman” or “traffic manager” of the CPU. It monitors
the execution of all instructions and the transfer of all information. The control
unit extracts instructions from memory, decodes these instructions, making sure
data is in the right place at the right time, tells the ALU which registers to use,
services interrupts, and turns on the correct circuitry in the ALU for the execution
of the desired operation. The control unit uses a program counter register to find
the next instruction for execution and a status register to keep track of overflows,
carries, borrows, and the like. Section 4.7 covers the control unit in more detail.