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.

Microprogrammed Control Unit

This microprogrammed control unit consists of a set of control signals, control variables, control words, control memory, microinstructions, and microprogram.

  1. Control Word: it is a word whose individual bits represent various control signals. They are the strings of 0’s and 1’s.

  2. Micro-routine: It is a sequence of control words that correspond to the control sequence of machine instruction.

  3. Micro-instruction: They are the individual control words in this micro-routine. They specify control signals for the execution of micro-operations.

  4. Micro-program: It is a sequence of micro-instructions. It is stored in a ROM or RAM called a Control Memory (CM).

  5. Control Store: It is the special memory in which the micro-routines for all instructions in the instruction set of a computer are stored.

  6. 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.

Vertical Micro-programmed Control Unit :

The control signals are represented in the encoded binary format. For N control signals- Log2(N) bits are required.

What is the difference between hardwired and 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