26.5.2. Condition Select and Branch Address

Now, let us again now actual I will tell you two things, two things are very important over here. So, as I told you in the last class that in this unit we will also see how a complete macro instruction is executed in terms of micro instructions. Because in the last unit we just saw that how to do a fetch here we will see how a total instruction is executed. Secondly, as I told you we also give some idea that how micro instructions can be macro instruct micro instructions corresponding to different type of similar macro instructions can be clubbed.

This chunk introduces the core concepts of micro and macro instructions. Macro instructions are high-level commands that perform specific operations, while micro instructions are the lower-level instructions that the CPU executes to perform these commands. By clubbing similar types of macro instructions, like addition and subtraction, we can optimize the execution process. This section aims to clarify that while executing a macro instruction, we can view the sequence of micro instructions that it comprises, thus providing a more detailed understanding of how CPUs operate at a low level.

Imagine a recipe (macro instruction) that contains multiple steps (micro instructions). Just as certain steps can be combined or executed in a sequence to streamline cooking, similar computer instructions can be grouped to improve performance in executing programs.

So, this type of branching will be actually called the implicit jumps we have to do, because you are writing a common micro program for different macro programs. So, this is not a requirement of the macro program. So, for example, if I have add 𝑅1 and 𝑅2. So, if this instruction is there you do not require to explicitly write jump instructions in the micro routine because this is this is not a jump instruction.

In this section, implicit jumps are introduced as a part of optimizing micro instruction routines. Implicit jumps allow programs to move from one instruction to another without explicitly stating a jump command. For instance, when executing a simple addition, the micro program does not need to specify jumps because the next instruction is understood based on the flow of the program, continuing the execution as intended.

Think of a flow of activities in a day, like going from breakfast to work. You don’t need to announce that you are 'jumping' to work after breakfast; it's understood as the next step, just like in programming where the next instruction follows naturally.

So, there are basically 2 parts. One is called the explicit that was because of the instruction type if it is add, sub this will not be there, and there will be some implicit type which will be coming in to the micro routines because you are doing an optimization.

This chunk explains that explicit jumps are deliberate commands in the micro instruction set that manage program flow based on specific conditions, such as checking flags. For example, the instruction "jump if zero" (e.g., JMPZ) requires an explicit micro instruction to check the zero flag and decide whether to jump to a specific memory address. This is different from implicit jumps, which happen automatically based on program flow.

Consider a traffic signal at an intersection. When the light turns red, drivers must stop (explicit instruction), as opposed to simply driving straight through without needing to think about each stop.

So, this is actually the implicit jump routine, which will be there in the micro program, and then another type of jumps which will be there which are actually explicitly mention in the macro; macro instruction corresponding to that right for example, 𝐽𝑀𝑃𝑍. So, it says that if the 0 flag is set you jump to memory location 𝑀, that is an explicit jump instruction which has to be in built in the micro routine.

In this section, control signals are discussed in the context of a multiplexer (MUX) configuration that directs the flow of execution. Different control signals will determine if the program should jump to a new address or continue executing. When a jump instruction like JMPZ is executed, the zero flag is checked, and if it indicates a condition to branch, the program counter is updated to point to the new address. The multiplexers play a critical role in selecting the correct address based on the control signals generated in the CPU.

Think of a decision-making flowchart where at each step (a control signal) you choose a path (branching). The MUX acts like a traffic guide, directing which way to go based on current conditions and rules, ensuring you take the right path based on the traffic light status.

So, assume that we have we are going to write a common routine corresponding to jump on 𝑍 to 𝑀 and jump on carry to 𝑀. So, this figure if you look at. So, this is already we have discussing in the last unit just a refresh.

This chunk concludes the discussion with specific examples of commonly used jump instructions, focusing on jumps conditioned on specific flags (like jump on zero or carry). The goal is to demonstrate how these different instructions, though they may share similar structure in terms of micro programs, lead to distinctly different execution paths depending on the state of the system flags.

Imagine a fork in a road: one path leads to a destination that's only accessible when the traffic light is green (jump on zero), while the other path is taken when a certain condition about traffic is met (jump on carry). Just like the conditional paths, programmed instructions decide the course of action based on specific conditions.

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