Control Unit
The CU manages the execution of instructions. It Fetches the instruction,
decodes it and synchronises its execution. It then sends control signals to
other parts of the processor in order that the instructions are carried out. The
CU uses a bus called the control bus to send commands (carry signals) to
different parts of the processor
Buses
Signals are carried around the processor using pathways (like mini cables)
called busses. There are different types of busses that differ depending on the
type of data that they are expected to communicate.
System Buses: These are the Control, Address and Data bus.
(Each bus is a shared transmission medium, so that only one device can
transmit along a bus at one time).
The CPU is connected to main memory by three separate busses. When the
CPU wishes to access a main memory location, it sends this address to
memory on the address bus. The data in that location is then returned to the
CPU on that data bus. Control signals are sent along the control bus.
The Control Bus is a bi-directional bus meaning that signals can be carried in
both directions.

The data and address buses are shared by all components of the
system. Control lines must be therefore be provided to ensure that
access to and use of the data and address buses by the different
components of the system does not conflict.

The purpose of the control bus is to transmit command, timing and
specific status information between system components.

Timing signals specify operations to be performed.

Specific status signals indicate the state of a data transfer request, or
the status of a request by a component to gain control of the system
bus
Complete the following from page 54 Heathcote.
 Memory Write:
 Memory Read:
 I/O Write:
 I/O Read:
 Transfer ACK:
 Bus Request:
 Bus Grant:
 Interrupt request:
 Interrupt ACK:
 Clock:
 Reset: