The control unit of the processor doesn't correspond to registers for holding data; it is the circuitry that connects the registers and components in the processor and controls the sequence of operations.

The control bus has several "bits" (wires or signals). These different signals are named by different vendors according to their function.

Six such typical signals are:

INTR - External interrupt signal (sent to the processor)
INTA - Interrupt acknowledgement (sent from the processor)
IOR - Read operation from some device port (device register)
IOW - Write operation to some device port (device register)
MEMR - Read a memory location
MEMW - Write a memory location