24.3.2. Branching in Micro Programs

For each instruction of the CPU there is a corresponding micro program for generating the control signal. For each macro instruction there will be a micro instruction, there will be a micro program. Each micro program is a sequence of micro instructions, and it is nothing but zeros and ones which is in a memory as we have already seen.

In computer architecture, when a CPU (Central Processing Unit) runs an instruction (known as a macro instruction), it relies on a sequence of lower-level instructions called micro instructions. Each micro instruction consists of binary signals (0s and 1s) stored in memory, which instruct the CPU on specific tasks to perform. Essentially, for every high-level command the CPU processes, there is a detailed step-by-step set of instructions that it follows to execute that command.

Think of making a recipe. The macro instruction is like the recipe title (like 'Bake a Cake'), while the micro instructions are the individual steps (mixing flour, adding eggs, preheating the oven). Each step has specific requirements, just as each micro instruction has specific signals that the CPU must follow to complete the overall task.

The Op-Code actually corresponds to the micro program. The Op-Code corresponding to the micro program is loaded into the control memory.

The Op-Code is a part of the instruction that signifies what action the CPU should perform. Once the CPU fetches an instruction, the decoding phase determines which micro program is to be executed by using the Op-Code. This phase is crucial, as it decides the specific micro instructions that will need to be executed to carry out the command represented by the macro instruction.

Imagine you have a control panel with different buttons (Op-Codes). Each button corresponds to a different machine function, like 'print' or 'scan.' When you press a button, it tells the machine what to do next, similar to how an Op-Code tells the CPU which set of micro instructions to follow.

But actually here branching is 2 that is one thing you have to emphasize; one is a normal branch, but none one branch means basically what corresponds to the macro program. So, if the macro program says that you have to do a branch based on the condition, the micro program will branch correspondingly.

In micro programming, branching refers to the ability to take different paths within a program based on certain conditions. This can happen in two ways: normal branching, which follows the instructions in the macro program, and inherent branching, where micro instructions may diverge from one another based on the specific command being executed. This allows for optimized programs by avoiding redundant instructions and only executing what is absolutely necessary based on the command.

Consider navigating through a city. You start from one location (the fetch part) and receive directions (instructions) that might say 'go left' or 'go right' depending on traffic conditions (branching based on conditions). Instead of having a separate route for every possibility, you adjust your path based on the current situation, similar to how a micro program branches depending on whether it needs to execute an addition or a subtraction.

So, therefore, we require many branches in a micro program execution compared to a macro program.

Micro programs are designed to optimize memory use by allowing certain sections of code to be shared across multiple instructions. For instance, the common processes involved in executing several different arithmetic operations can be grouped together, and only the differing parts (like the operation type) need to have branches. This not only saves memory space but also makes it quicker to execute common functionalities without rewriting the same lines of code over and over for each instruction.

Think of a group project where members are assigned different roles. Each member may complete similar tasks (common processes) but with different focuses; for example, one might draft while another verifies facts. Instead of having separate action plans for each member, they can share a common checklist for tasks they all have to do, along with specific instructions for their individual roles.

Without branching, it will be a very unoptimized solution, and in fact, if there are branch instructions; obviously, micro program has to be have follow a branching path.

The functionality of branching is critical in micro programming for efficiency and performance. Without a well-implemented branching structure, a program may be less efficient, resulting in excessive memory use or slower execution. Managing the branching properly allows for more organized and maintainable code while improving runtime as the CPU does not waste cycles on unnecessary tasks.

Imagine a traffic system. Without traffic lights (branching points), cars would continue to flow regardless of whether they are facing oncoming traffic or not, possibly leading to chaos. Similarly, branching points in a micro program help direct the flow of operations, ensuring that each instruction is executed seamlessly based on the context provided.

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