21.1.4. Incrementing the Micro-Program Counter

So, basically the micro-programmed control unit ensures that the signal will be generated in correct sequence. So, sequencing here is a bit tricky, because generating control signals is directly they are encoded, directly they are actually put in the memory location.

The micro-programmed control unit is responsible for generating control signals in the correct order. This is important because these control signals dictate how the CPU operates and carry out tasks. The signals are organized in such a way that they are stored in memory locations and executed sequentially. This sequencing is essential for the proper functioning of the CPU.

Think of a micro-programmed control unit as a conductor of an orchestra. Just as a conductor ensures that each musician plays their part at the right time to create beautiful music, the control unit ensures that each control signal is generated at the right time to ensure proper CPU operation.

Sequencing is actually very important here because, many times we will depend on the condition codes and status flags. Based on some storage that is some signal from the memory WMFC based on some case like some interrupt etcetera.

Sequencing goes beyond just following memory instructions; it also relies on condition codes and status flags. These flags (like zero or carry flags) are indicators of the CPU's current status and dictate the next steps to follow. When specific conditions arise, the micro-program counter may not just increment linearly but may need to jump to a non-consecutive instruction based on these flags.

Imagine a traffic light system. Each light (red, yellow, green) represents a condition that changes the flow of traffic. Depending on conditions (like a pedestrian crossing), the lights will signal different actions. Similarly, the CPU uses condition codes to decide whether to proceed to the next instruction or take a different branch based on the current state.

You have to have also arrangement for something called a branch decision. In right normal programs we have micro instructions where actually which is conditional, and also, we have something which is unconditional.

The control unit must make branch decisions between conditional and unconditional micro-instructions. For conditional instructions, it requires a specific address to direct the micro-program memory when a certain condition is met. This is crucial for control flow in program execution, as it enables the CPU to react to different scenarios during instruction processing.

Consider a GPS navigation system. It gives you instructions based on your current location and the chosen destination. If you miss a turn (a conditional situation), it recalculates and gives you a different route. Similarly, the micro-program counter changes paths based on the conditions evaluated by the CPU.

So, micro-program memory architecture and normal memory architecture there is not much difference, they are almost the same thing.

In terms of structure, micro-program memory is similar to standard memory. However, it is specifically allocated for storing micro-programs or instructions that correspond to higher-level macro instructions. This memory architecture allows the CPU to retrieve the correct sequence of micro-instructions needed for each specific operation.

Think of a library where books are stored by genre. The overall structure is similar, whether you are looking for fiction or non-fiction. In this case, the library represents the computer's memory, while the sections for different genres symbolize how micro-program memory is organized for specific CPU functions.

So, whenever instruction has to be fetched the sequence of micro instructions is always similar. So, whenever instruction has to be fetched you can directly invoke that part of the memory, micro-program memory which has the micro instructions corresponding to fetch.

The instruction fetching process is standardized, meaning that each instruction requires a similar sequence of micro-instructions for fetching. This process involves invoking the relevant portion of micro-program memory that contains the fetching instructions, ensuring efficiency and consistency in instruction retrieval.

Imagine a vending machine. When you choose a drink, the machine follows a predetermined sequence to deliver it (checking availability, dispensing the item, and collecting payment). In this analogy, the vending machine represents the CPU, and the consistency of its operation relates to the standardized instructions for fetching.

Now, if you look at this slide which actually tells in more formal manner which I was saying that during the execution of a micro-programmed control, the 𝑀𝑃𝐶 is always incremented. So it goes excepting the branch condition.

Typically, the micro-program counter (MPC) is incremented with each executed micro-instruction. However, there are exceptions such as when an end instruction is reached or a branch condition must be evaluated. In these cases, the MPC will not simply increase but may redirect based on the encountered conditions.

Think of a film script where the actors follow a sequence of lines (micro-instructions). Sometimes, a director calls 'cut' (end instruction), and the scene stops, or they might change the scene entirely (branch), requiring the actors to jump to a different script. Similarly, the MPC may not always increment; it could pause or redirect.

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