Micro-programmed Control Unit

A Micro-programmed Control Unit is a control unit that has binary control values saved as words in its memory.

It executes the instructions that are supposed to be implemented by constructing a fixed collection of signals at regular intervals of each system clock beat. Every single output signal generates one micro-operation which includes a register transfer as well. Therefore, definite micro-operations generate sets of control signals which can then be saved in the memory.

All bits that form the microinstruction are linked to their respective control signals. The control signal is active when the bit is set and it is inactive when the bit is cleared. We can choose to save these microinstructions in a definite sequence in the internal ‘control’ memory. The computer inside a computer is the control unit of a microprogram-controlled computer.

The steps followed by the microprogrammed control are listed below:

1. The microprogrammed control can execute any given instructions. Once the instructions are received, the CPU divides them down into a set of sequential operations called microinstruction. These microinstructions require the control signals to execute.
2. Control signals which are saved in the read-only memory are initialized in order to execute the instructions in the data direction. At any given time-step, these control signals can control the micro-operations concerned with a microinstruction that is to be performed. This is followed by the generation of the address of the next microinstruction to be executed.
3. The above-mentioned steps keep on repeating in a loop until all the microinstructions associated with the instruction in the set are executed.
4. From the micro counter, the register originates the address which is supported to the control ROM.
5. Inputs to the micro counter are provided by a multiplexer that selects the output of an address ROM, a current address incrementer, and an address that is stored in the next address field of the current microinstruction.

The micro programmed control unit has the following advantages:

• There can be a more systematic design of the control unit.

• It is much easier to debug and change.

• It retains the underlying structure of the control function.

• It simplifies the design of the control unit, making it less prone to error.

• It can be organized and systematic.

• Essentially, it controls software-based functions, not hardware-based ones.

• It offers greater flexibility.

• It facilitates the execution of complex functions.

It also has a few disadvantages:

• The cost of adaptability is higher.

• Compared to a hardwired control unit, it is slower.

Types of Micro-programmed Control Units

Control Words are categorized into two sorts based on how they are stored in the Control Memory (CM):

1. Horizontal Micro-programmed control Unit

The control signals are provided in a 1 bit/CS decoded binary format. For example, if 27 Control signals are present in the processor then 27 bits are required. More than 1 control signal can be enabled at a time. 

• It supports longer control words.

• We can also use it in parallel processing applications.

• A higher degree of parallelism is supported by it.

• No additional decoders are needed which makes it faster than Vertical Microprogrammed.

• It provides more flexibility than Vertical Microprogrammed.

     2. Vertical Micro-programmed control Unit 

• The encoded binary format is used to represent the control signals. - Log2(N) bits are required for N control signals.

• Shorter control words are supported.

• It is more adaptable since it facilitates the addition of new control signals.

• It facilitates a low degree of parallelism i.e., the degree of parallelism is either 0 or 1.                    

• It is slower than horizontal microprogramming because it requires additional hardware (decoders) to create control signals.

• It has less flexibility than a horizontal control unit, but it is more adaptable than a hardwired control unit.

Hardwired Control Unit

The Finite State Machine (FSM) is used in hardwired control systems to generate the control signals. Circuits of this type are sequential logic circuits. Physical connection of components such as gates, flip flops, and drums creates the final circuit. Hardwired controllers are therefore called so. Hardwired control consists of a combinational circuit that outputs desired control signals for the decoding and encoding functions. An instruction decoder decodes the instruction that is loaded into the IR. The instruction decoder generates 28 (256) lines if the IR is an 8-bit register.

Instruction step decoders, external inputs, and condition codes provide inputs to the encoder. Individual control signals are generated for each of these inputs. After all instructions have been executed, the end signal is generated. Additionally, it resets the control step counter, ready for a generation of the following instruction's control step.

Implementing the hardwired control has the main objective of minimizing the circuit's cost and improving the speed of the operation. Several methods have been devised for designing hard-wired control logic, including the following:

• Sequence Counter Method: this is the most practical way for designing a somewhat complicated controller.

• Delay Element Method: for creating the sequence of control signals, this approach relies on the usage of timed delay components.

• State Table Method: the standard algorithmic technique to designing the Notes controller utilizing the classical state table method is used in this method.

Authors: Gargi Andhale, Harsh Uike, Harshita Jain, Hrishikesh Zope, Hruday Jain

               Vishwakarma Institute of Technology, Pune