24.1.1. Organization and Optimization of Microprogrammed controlled Control Unit

Welcome to the unit 8 on the Module of Control. So, what we are discussing as of now in this module that how basically we can execute the controls, if the methodology is a micro program control.

In this section, we introduce the concept of Microprogrammed Control Units. The microprogrammed control mechanism provides a way to execute control signals that govern the operation of a computer's CPU using a sequence of microinstructions. This approach allows for flexibility and easier modification when compared to hardwired control systems.

Imagine a conductor leading an orchestra. Just like a conductor uses different hand signals to guide musicians to play their instruments at specific times, a microprogrammed control unit sends control signals to different parts of the CPU to perform instructions.

We have seen how to optimize them using a vertical and hybrid micro program. In this basically what we will try to do we will be mainly focusing on optimization of micro programmed control unit in more depth and by taking more specific examples.

Optimization in microprogrammed control units involves reducing the redundancy in microinstruction sets. Vertical and hybrid microprogramming are techniques to achieve this. By grouping common tasks and only changing the variables that differ (like the specific operation performed), we can save memory and improve efficiency.

Think of a cooking recipe that requires the same base ingredients. Instead of writing a new recipe for every dish based on the same ingredients, you can create a master recipe that outlines the common steps, only modifying it for unique ingredients. This reduces the amount of recipe writing needed.

For each machine instruction when it is in the decoding cycle based on the Op-Code, the corresponding micro program is loaded into the memory.

When a machine instruction is executed, it first enters the decoding phase where the Opcode (operation code) is analyzed. Based on this Opcode, the CPU determines the specific microprogram that needs to be loaded into control memory. This microprogram will dictate all subsequent operations for executing that instruction.

Imagine a teacher preparing a lesson plan based on the subject taught. Each lesson plan is tailored to the topic (Opcode) and specifies the activities (microinstructions) to administer that day. This ensures the correct content is delivered effectively.

The micro program counter fetches each micro instruction in sequence and then generates the required signal or if required jumps will also be there.

The Micro Program Counter (MPC) is crucial in controlling the sequence of microinstruction execution. It retrieves microinstructions one after the other and can also jump to different parts of the microprogram if needed (e.g. for branching operations). This enables conditional execution of certain tasks within a macro instruction.

Think of the MPC as a navigation system in a car. The system follows a planned route but can divert if there's roadwork or a detour (jumping to a different microinstruction when certain conditions are met).

So, therefore, basically we can write a single micro program for add and sub with just a branching instruction.

Branching instructions in microprograms allow for flexible execution paths based on the input instructions. For instance, instead of writing separate microprograms for each instruction type (like add and subtract), a single program can be created with branches that direct the flow based on the operation type. This saves memory and reduces the complexity of the control unit.

Picture a flowchart decision. Instead of having separate charts for each scenario, one chart can lead to different paths (branches) based on the decision made (e.g., add or subtract), allowing for clearer and more concise data processing.

In fact, as I told you jump branch to incorporate branching, here also you will require to check the contents of the code control status flag etc.

Optimizing memory in microprogrammed control units requires careful management of branching. By incorporating techniques such as status flag checking (which indicates the current state of the system), unnecessary microinstructions can be avoided, further enhancing the efficiency of the control unit.

Think of a librarian using a catalog system. Instead of going through every book to find a specific one, they check the catalog to see if it's available ('status flag') before searching a specific section. This saves time and resources.

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