21.1.5. Branch Decision Logic

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.

The micro-programmed control unit is responsible for generating control signals in the correct sequence, which is crucial for the execution of instructions in the CPU. These control signals are stored in memory locations and are automatically accessed when needed. For instance, each instruction has a corresponding set of control signals which can be thought of as a blueprint that tells the CPU how to carry out that instruction. The challenge lies in ensuring the correct sequence is followed, especially in complex operations.

Imagine following a recipe that tells you how to bake a cake. Each step in the recipe corresponds to a control signal that guides you through the process. If you were to bake the cake without following the recipe in the correct order, you would likely end up with a disastrous result. Similarly, in computing, following the correct sequence of control signals is crucial for the execution of instructions.

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. Because of some case and also some status flag like zero flag, carry flag so all these things we must take into picture, and then you have to decide whether it will be the next phase, or it will go to some other instruction which is not consecutive.

In computer systems, branching decisions are made based on condition codes and status flags, such as the zero flag or carry flag. This means that during instruction execution, the control unit must evaluate certain conditions to determine the next step. If a specific condition is met, the control might branch out to a different instruction that is not necessarily the next in line. This allows the CPU to respond dynamically to various states and interrupts, making the system more flexible.

Think of a traffic light system. The traffic light follows a set pattern (green, yellow, red) but it can change based on the presence of vehicles or pedestrians. If a car approaches while the light is red and there is a pedestrian waiting to cross, the system needs to decide whether to turn green immediately or follow the long-set pattern. Similarly, computers must evaluate conditions and decide whether to continue sequentially or branch off to a new instruction.

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. So, whenever a new instruction has to be executed first actually micro-program corresponding to fetch will be executed.

Each instruction comprises two main phases: the fetch phase and the execution phase. During the fetch phase, the instruction is retrieved from memory. This is a standardized process where the CPU accesses the micro-program memory that contains instructions for fetching the data. After fetching, the instruction is decoded to understand what action needs to be taken before executing the operation and storing the result. This consistent approach simplifies the execution process for different instructions.

Think of this process like a teacher giving students assignments. First, they call the students to collect the assignments (fetch). After the collection, they review the assignments to check if any corrections are needed (decode), and then, the teacher provides feedback or grades them (execution). Just as the assignment collection process is the same for each student, the instruction fetch phase in the CPU remains consistent for all instructions.

So, apart from branch address these micro programs can specify which status flags conditions etcetera has to be taken for the condition check. So, that that means, in a very nutshell basically you just go micro-program counter increases one by one, but whenever there is a condition which depends on the input, depends on the flags, depends on some I/O input.

The micro-program counter (MPC) usually increments sequentially after each instruction execution. However, if certain conditions based on input flags or I/O data arise, the control logic specifies different actions. This means the MPC may not just move sequentially; instead, it may jump to a new memory location based on these specific conditions. The branch address must be explicitly defined for this reason, indicating where the control should jump next if a condition is satisfied.

Imagine you're on a road trip and using a GPS. Normally, you'd follow the suggested route step by step (increments). But if you see a sign indicating a road closure or heavy traffic ahead, the GPS will provide an alternative route (branch). Just as you must decide whether to continue on your current path or take a detour based on real-time information, the CPU must also adjust its execution path based on the conditions it encounters.

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. So, I am filling the values 0110 something like this has been filled. Now, I have to reserve some part here and some part I will tell you why these things are to be reserved, because otherwise it will go in sequence.

Micro-program memory is organized into different segments to handle control signals and branching conditions efficiently. Some parts of the memory are reserved for storing control signals, while others are dedicated to specify branch addresses for condition checks. This design allows the processor to quickly reference the necessary control signals and conditions, facilitating a smooth operation for both sequential execution and branching without confusion.

Consider a library where various sections are dedicated to different genres of books. There’s a section for fantasy, another for non-fiction, and so on. If you need to find a fantasy book, you know to head directly to the fantasy section rather than searching blindly through the entire library. Similarly, in micro-program memory, each part is designated for specific tasks, which helps the CPU access information efficiently.

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