1. basis concepts
- Micro-operations: We have already seen that the programs are executed as a sequence of instructions, each instruction consists of a series of steps that make up the instruction cycle fetch, decode, etc. Each of these steps are, in turn, made up of a smaller series of steps called micro-operations.
- Micro-operation execution: Each step of the instruction cycle can be decomposed into micro-operation primitives that are performed in a precise time sequence. Each micro-operation is initiated and controlled based on the use of control signals / lines coming from the control unit.
- Controller the data to move from one register to another
- Controller the activate specific ALU functions
- Micro-instruction: Each instruction of the processor is translated into a sequence of lower-level micro-instructions. The process of translation and execution are to as microprogramming
- Microprogramming: The concept of microprogramming was developed by Maurice Wilkes in 1951, using diode matrices for the memory element. A microprogram consist of a sequence of micro-instructions in a microprogramming.
- Microprogrammed Control Unit is a relatively logic circuit that is capable of sequencing through micro-instructions and generating control signal to execute each micro-instruction.
- Control Unit: The control Unit is an important portion of the processor.
The control unit issues control signals external to the processor to cause data echange with memory and I/O unit. The control Unit issues also control signals internal to the processor to move data between registres, to perform the ALU and other internal operations in processor. In a hardwired control unit, the control signals are generated by a micro-instruction are used to controller register transfers and ALU operations. Control Unit design is then the collection and the implementation of all of the needed control signals for the micro-instruction executions.
2. control unit design approaches
How can we use the concept of microprogramming to implement a Control Unit ? There are two approaches of Control Unit Design and implementation:
- Microprogrammed implementation
- Hardwired logic implementation
The figure 7.2 illustrated the control unit inputs. Two techniques have been used to implemente the CU. In a hardwired implementation, the control unit is essentially considered as a logic circuit. Its input logic signals are transformed into the set of ouput logic signals, which are the control signals. The approach of microprogrammed implementation is studied in this section.
Figure 7.2 Control unit with decoded inputs
2.1 approach of microprogrammed control unit
Principe:
- The control signal values for each microoperation are stored in a memory.
- Reading the contents of the control store in a prescribed order is equivalent to sequencing through the microoperations
- Since the “microprogram” of microoperations and their control signal values are stored in memory, this is a microprogrammed unit.
Remarks:
- Are more systematic with a well defined format?
- Can be easily modified during the design process?
- Require more components to implement
- Tend to be slower than hardwired units (due to having to perform memory read operations)