Micro Programmed Control

The control memory forms an integral part of the CPU. It generates a control signal in a proper sequence to execute an instruction. Micro Programmed Control is one of the two approaches the control memory generates the signal. 

The control unit in a digital computer initiates sequences of microoperations. A proper series of steps must be followed to transmit any signal. Thus, address sequencing forms a determining aspect of signal transmission. Firstly, the Micro Programmed Control stores the associated control signals as control words, which are inaccessible by the programmer. The signals generated are the same as that of machine language programmes. 

What are the technicalities on which a Micro Programmed Control works? 

The control memory implements the instructions by building a specific collection of signals at each system clock beat. With the help of a register transfer, each set of collection of output signals triggers a single micro-operation. The specific micro-operations produced are then saved in the memory. A single control signal handles all the bits of the microinstructions. The control signal becomes active once the bit signal is in the set condition. When the control signal is cleared, it becomes inactive.

The micro counter register generates the address supported by the control ROM. A multiplexer selects the output of an address ROM, a current address incrementer. It then saves the address saved in the following address field of the current microinstruction as inputs to the micro counter.

What are the steps through which a Micro Programmed Control executes the instructions? 

The Micro Programmed Control follows the sequence of steps as listed below – 

• The CPU breaks down the final instruction into sequential operations known as microinstruction 
• The control signals facilitate the execution of sequential micro-operations
• The CPU executes the instructions on the data direction by generating the control signals in the ROM. These control signals can direct the micro-operations associated with a microinstruction at any time step
• Following the sequence, it then generates the address of the next microinstruction to be executed
• Finally, it repeats the two preceding steps until the CPU executes all of the microinstructions associated with the instruction in the set

What are the categories of the Micro-programmed Control Unit? 

We can classify the Micro-programmed Control Unit into two types, namely horizontal and vertical, based on the type of Control Word stored in the Control Memory. They are – 

Horizontal Micro-programmed Control Unit: 

The CPU stores the control signals in a decoded binary format comprising 1 bit/CS. It also facilitates enabling more than one control signal simultaneously. 

• It allows for control words of a greater length
• The absence of any additional hardware (decoders) makes it faster than the Vertical Micro-programmed Control Unit
• It allows for a parallel transfer of data. CPU enables n CS simultaneously if the degree is n
• It has its applications in programmes that require ‘parallel processing’
• It is more adaptable and flexible than vertical micro-programmed systems

Vertical Micro-programmed Control Unit: 

Here, the CPU represents the control signals in the encoded binary format. It requires Log2(N) bits for N control signals. 

• It allows for control words of shorter length
• Because it necessitates the use of additional hardware (decoders) to generate control signals, it implies that it is slower than horizontal microprogramming
• It implements new control signals, making it more adaptable
• It is more flexible than a hardwired control unit but less flexible than a horizontal control unit
• It allows for a low degree of parallelism, with the degree of parallelism being either 0 or 1

Pros and Cons of Micro Programmed Control

Every system in the universe upholds its unique qualities. While some go in favour, some go against it! Let’s look at the advantages and disadvantages of Micro Programmed Control. 

What are the Upsides of Micro Programmed Control? 

• It allows for a more systematic design of the control unit
• It can facilitate an orderly and systematic design process
• It is more adaptable
• It is used to control functions implemented in software rather than hardware
• It can keep the control function’s underlying structure
• It is used to simplify the execution of complex operations
• It is easier to debug and modify
• It can significantly simplify the design of the control unit. As a result, it is less expensive and less prone to error

What are the downsides of Micro Programmed Control? 

• Adaptability comes at a higher cost
• It operates at a slower rate than a hardwired control unit

Conclusion

The Micro Programmed Control is a simple logical microprogramme present in the control memory that facilitates the generation of control signals. The main benefit of using a microprogrammed control is that no further changes can be made once the hardware configuration is set. If a different control sequence is to be used, a new set of microinstructions for the system must be written.