Microprogrammed Control Unit
In this course, we will study the hardwired control unit, microprogrammed control unit, the types of the microprogrammed control unit, and the difference between hardwired and microprogrammed.
What is the microprogrammed control unit?
The microprogrammed control unit uses a sequence of microinstructions in microprogramming language. It acts as a midway between the hardware and software. It generates a set of control signals. It is easy to design, implement, and test. It is also flexible to modify.
This microprogrammed control unit consists of a set of control signals, control variables, control words, control memory, microinstructions, and microprogram.
- Control Word: it is a word whose individual bits represent various control signals. They are the strings of 0’s and 1’s.
- Micro-routine: It is a sequence of control words that correspond to the control sequence of machine instruction.
- Micro-instruction: They are the individual control words in this micro-routine. They specify control signals for the execution of micro-operations.
- Micro-program: It is a sequence of micro-instructions. It is stored in a ROM or RAM called a Control Memory (CM).
- Control Store: It is the special memory in which the micro-routines for all instructions in the instruction set of a computer are stored.
- Control Signals: they are nothing but groups of bits that are used to select the path.
What is a hardwired control unit?
The control hardware can be seen as a state machine that changes from one state to another in every clock cycle. It depends on the contents of the instruction register, the condition codes, and the external inputs. The control signals are the outputs of the state machine.
What are the types of microprogrammed control units?
Horizontal Micro-programmed Control Unit :
The control signals are expressed in the decoded binary format. Example: If 53 Control signals are present in the processor then 53 bits are required. Multiple control signal can be enabled at a time.
- It supports longer control words.
- It is used in parallel processing applications.
- It allows a higher degree of parallelism. If the degree is n, n CS is enabled at a time.
- It requires no additional hardware(decoders). It means it is faster than Vertical Microprogrammed.
- It is more flexible than vertical microprogrammed
Vertical Micro-programmed Control Unit :
The control signals are represented in the encoded binary format. For N control signals- Log2(N) bits are required.
- It supports shorter control words.
- It supports the easy implementation of new control signals therefore it is more flexible.
- It allows a low degree of parallelism i.e., the degree of parallelism is either 0 or 1.
- Requires additional hardware (decoders) to generate control signals, it implies it is slower than horizontal microprogrammed.
- It is less flexible than horizontal but more flexible than that of a hardwired control unit.
What is the difference between hardwired and microprogrammed control unit?
|HARDWIRED CONTROL UNIT
||MICROPROGRAMMED CONTROL UNIT
|It is a circuitry approach.
||It is implemented by programming
|The instructions are of RISC style
||The instructions are of CISC style
|Modification is difficult.
||Modification is easy
|It is more efficient for simple instructions
||It is more efficient for complex instructions
|It is a costly approach
||It is a less costly approach
|There is no requirement of control memory
||Control memory is required