21.2.2. Memory Organization of Micro-Programs

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. So, if you generate if you just take cell 1 memory row 1 memory row 2, memory 3 automatically control signals are generated as default because, they are already stored in the memory. 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.

The micro-programmed control unit is critical in managing the order in which control signals are produced. This sequence is essential because it determines how a computer executes instructions. Control signals are pre-defined and stored in memory cells, allowing them to be accessed in order. Understanding sequencing also requires recognizing how the system checks condition codes and status flags. These flags provide feedback that alters the flow of execution, such as determining which operation to perform next based on input from various sources like interrupts.

Think of a traffic light system at an intersection. The micro-programmed control unit is like the system controlling the lights, ensuring they change in a specific order (green to yellow to red) based on preset timings and conditions (like the presence of cars). Just as the signals are pre-programmed into the traffic light’s control unit, control signals in a computer are pre-stored and activated as needed.

So, 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. In conditional it is required to specify the address of the micro-program memory where the control must direct. So, means if it is some condition is not true you will go here, that is the next stage.

Branching decisions are essential for directing the flow of program execution based on certain conditions. Micro-instructions can be categorized as either conditional or unconditional. Conditional branches depend upon the evaluation of specific criteria, and if a condition is met, execution continues from a specified address in micro-program memory. If the condition is not met, execution can take an alternate path. This dynamic nature of branching enables complex logic in programming.

Imagine a choose-your-own-adventure book. As you read, you sometimes reach a point where the story presents options based on your decisions (like 'if you want to take the shortcut, turn to page 20'). This is akin to how conditional statements and branching work in micro-programs, guiding the flow based on conditions.

So, what I was saying you can just read in this slide. Basically, we have every instruction has 2 parts called fetch and execution phase. Execution phase means, here I am telling that decode do the operation and store. So, basically all for all instruction phase fetch phases is similar.

Every instruction processed by a micro-program goes through two primary phases: the fetch phase and the execution phase. In the fetch phase, the instruction is retrieved from memory, while in the execution phase, the instruction is decoded, and the relevant operations are performed. Both phases are typically similar across instructions, allowing the micro-program counter (MPC) to reference specific locations in memory that contain predefined instructions for fetching.

Consider ordering food at a restaurant. The fetch phase is like looking at the menu to decide what to order. The execution phase is when you actually place the order, process it (like cooking), and then serve the meal. Just as there are standard procedures for ordering food, micro-programs follow systematic steps for fetching and executing instructions.

So, 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.

The micro-program counter (MPC) serves as a guide for the sequence of instructions being executed. It normally increments to point to the next instruction to be processed. However, there are exceptions—like when a program encounters a branching instruction or an end instruction—or when the MPC must jump to a different part of memory. These points underscore the importance of control in managing how and when the MPC changes its value.

Think about navigating through a train system. The MPC is like a train schedule, moving sequentially from one stop to the next. If a train needs to change routes because of a detour (a branch), or if it reaches the final destination (an end instruction), it will pause or redirect instead of just moving to the next scheduled stop.

So, basically this will be your program micro program. So, basically the micro-program is nothing but they are all some parts of a memory.

Micro-program memory consists of various segments, each designated for specific functions—like storing control signals and branching decisions. This organization facilitates efficient micro-program execution, as different parts of memory are reserved for tasks such as executing commands or checking conditions. The thoughtful architecture of this memory structure ensures that instructions can be processed quickly and accurately based on pre-defined pathways.

Consider a library. The micro-program memory is like the library's collection, organized by sections (like fiction, non-fiction, and reference). Each section is designated for specific types of knowledge (or instructions), making it easier for patrons (the micro-program control unit) to locate and retrieve the necessary books (or control signals) for their needs.

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