Implementation of Micro Instructions Sequencer

An Overview

Digital processor processes can be flexibly managed by microprogrammed control units. The micro instruction sequencer, a vital component responsible for retrieving and carrying out micro instructions, is at the Centre of a microprogrammed control unit. In this article, we will examine the design of a micro instruction sequencer and see how it is put into practice.

Micro Instructions Sequencer

A circuit that manages the sequencing of microinstructions in a computer with microprogramming is known as a micro-instruction sequencer. It is in charge of creating the addresses for the subsequent micro-instruction to be executed and making sure that they are executed in the proper order.

The following elements are commonly included in a micro-instruction sequencer −

• A control memory where the microinstructions are kept.

• The address of the current microinstruction is stored in a programme counter (PC).

• A controller that produces addresses for the subsequent microinstructions to be performed.

• A timing circuit that guarantees the execution of the microinstructions in the proper sequence.

The micro-instructions are sequentially stored in the control memory. The address of the first microinstruction in the sequence serves as the PC's initialization point. The address for the following microinstruction to be executed is generated by the sequence controller. The timing circuit makes sure that the micro-instructions are carried out in the appropriate sequence.

Implementation

The process of putting a micro instruction sequencer into use entails creating the parts and logic that a microprogrammed control unit needs to fetch and execute micro instructions. Here is a general description of the implementation procedure −

Design the Control Memory

• The quantity of micro instructions needed to implement the appropriate set of control signals should be determined.

• Utilise an appropriate memory technology, such as ROM or PLA (Programmable Logic Array), while designing the control memory.

• Each micro instruction's address and related control signals should be assigned.

Computer programme counter

• Create a programme counter that keeps track of the location of the following micro instruction to be fetched.

• Using the quantity of micro instructions in the control memory, calculate the programme counter's width.

Fetching Micro Instructions

• To access the control memory and get the micro instruction located at the specified address, use the programme counter.

• For decoding and the creation of control signals, the fetched micro instruction is often placed in a micro instruction register (MIR).

Generation of Control Signals

• To obtain the control signals needed for various CPU components, decode the fetched micro instruction.

• Create logic circuits or a control signal generator based on the decoded micro instruction to generate the required control signals.

• Combinational logic circuits, multiplexers, or other suitable parts can be used to generate the control signals.

Logic Sequencing

• Establish the logic for the sequencing to manage the flow of micro instructions.

• Depending on the complexity of the microprogram, this logic can be implemented using a counter-based technique or a finite state machine (FSM).

• Add control flow instructions to the programme, such as conditional branches and jumps, to change the order of the micro instructions based on certain circumstances.

Calculating the Next Address

• Identify the logic that will be used to determine the address of the following micro instruction to be obtained.

• The program counter may need to be increased in order to retrieve the following sequential micro instruction, or it may need to be modified in response to control flow instructions like branches and jumps.

Execution oversight

• Create the timing and control circuits required to guarantee the correct execution of micro instructions.

• To make sure that control signals are generated at the proper time and for the right amount of time, coordinate the timing and sequencing of micro instructions.

For each micro instruction, repeat these steps

• For each micro instruction stored in the control memory, repeat steps 3 through 7, making sure to produce and execute all required control signals in the proper order.

Conclusion

One crucial component of microprogrammed control units is the development of a micro instruction sequencer. Through the retrieval, decoding, and generation of the required control signals, it makes it easier for micro instructions to be executed effectively. Designers can build a strong micro instruction sequencer that efficiently controls the operations of a microprogrammed processor by carefully designing the control memory, programme counter, sequencing logic, and control signal generation. The micro instruction sequencer is crucial in attaining control flexibility, allowing the processor to efficiently carry out varied activities and carry out complicated commands.