lecture2

Microprogrammed Control
An instruction is executed by carrying out a sequence of more rudimentary operations.
All operations and data transfers are controlled by the processor clock.
The ALU is a combinational circuit that has no internal storage.
To execute instructions, the processor must have some means of generating the control signals needed in the proper sequence.
Two categories: hardwired control and microprogrammed control
Hardwired system can operate at high speed; but with little flexibility.
Control Unit Organization
The previous organization cannot handle the situation when the control unit is required to check the status of the condition codes or external inputs to choose between alternative courses of action.
Use conditional branch microinstruction.
Organization of the control unit to allow conditional branching in the microprogram
Microinstructions
A straightforward way to structure microinstructions is to assign one bit position to each control signal.
However, this is very inefficient.
The length can be reduced: most signals are not needed simultaneously, and many signals are mutually exclusive.
All mutually exclusive signals are placed in the same group in binary coding.
Partial Format for the Microinstructions