A microprogrammed control unit (MCU) is a digital circuit that controls the operations of a computer’s central processing unit (CPU).
There are two main types of MCUs:
A hardwired control unit is a type of MCU that uses fixed, hardwired logic circuits to implement the control signals needed to execute instructions.
In this approach, the control unit’s control signals are directly generated by the logic gates and flip-flops. This type of control unit is simple and fast, but it is difficult to modify or update once it is designed and implemented.
On the other hand, a microprogrammed control unit is a type of MCU that uses microcode to generate the control signals needed to execute instructions. In this approach, the control signals are stored in a memory called a control store, which can be easily modified or updated. This type of control unit is more flexible and easier to modify or update, but it is slower than a hardwired control unit.
An address sequencer circuit is a component of a microprogrammed control unit that generates the sequence of addresses that are used to read microinstructions from the control store. The address sequencer circuit typically consists of a counter and a decoder. The counter generates the addresses in sequence, while the decoder translates the addresses into the appropriate control signals that are used to fetch and execute instructions.
During the execution of an instruction, the address sequencer circuit generates a sequence of addresses that are used to read the corresponding microinstructions from the control store. Each microinstruction contains the control signals needed to execute a specific part of the instruction. The microinstructions are executed in sequence, with each microinstruction generating the control signals needed for the next step in the instruction execution process. Once all of the microinstructions for an instruction have been executed, the control unit generates the necessary signals to fetch the next instruction from memory, and the process repeats.