22.4.1. Unconditional vs Conditional Jumps

So, we will take some elaborate examples, which will actually clear out the whole theory for you. This is the figure which will actually clear what I was saying, you have to take it in slightly elaborate manner, and let us look at it what happens. We basically have a memory. So, we have actually 3 fields. So, this is the control function field. So, this is very very important the control signals are like program counter in, program counter out all these things will be there basically that is the main part of it. There are 2 other parts basically one is called the condition select and one in the branch address field. Branch address fields means, it says that from this instruction if some conditions are true or something then the next address may be say 101. So, this is going to say that I am not going maybe at this present in the instruction called say 001. So, I will go to 101, if some condition is true. That is what this is saying and where I have to go, I have to go to 101. So, here we are telling which are the signals, here we are telling which is the instruction location you have to go, and we are saying that what is basically the condition it has to be met.

In the context of computer programming, jump instructions are crucial for controlling the flow of a program. Jump instructions can be categorized into unconditional jumps and conditional jumps. An unconditional jump occurs regardless of any conditions; it always transfers control to the specified address. Conditional jumps, on the other hand, depend on specific conditions being met. For example, if a condition in the program evaluates to true, the program counter (PC) updates to a new address (e.g., 101). This section discusses how memory and control fields work together to determine the next instruction based on these jump types.

Think of a GPS system that directs you to a destination. An unconditional jump is like a GPS instruction that tells you to drive straight to a particular address regardless of other factors, such as traffic. A conditional jump is like getting a directive based on traffic conditions: if the road is clear, you proceed; if not, you might take an alternate route.

Now, how can I tell that I have to go to this branch? This is very simple 101 means you have to update the value of micro-program counter micro-program 𝑃𝐶 to 101. Otherwise it will become from 001 to it will 010 it will increment, but you are telling no I don’t want to go to 010 rather I want to go to 101, which is basically 5. So, if you look at it. So, this part of the control signals will directly go to the ALU the 𝑃𝐶 in and all these ports, but basically the branch address field is going to be basically one input to the 𝑀𝑃𝐶. So, if you allow or if the jump condition is true basically the 𝑀𝑃𝐶 will directly take the value of 101.

To implement jump instructions, the micro-program counter (MPC) plays a critical role. The MPC is responsible for keeping track of the address of the next instruction to execute. If a conditional jump is triggered (i.e., the specified condition is met), the MPC is updated with the new address (e.g., 101). If the condition is not met, the MPC continues incrementing to the next address (e.g., from 001 to 010). This process ensures that the program can navigate through different instructions based on the logical conditions defined within the code.

Imagine a teacher who is navigating students through a lesson plan. If a student answers a question correctly, the teacher might skip ahead to a more advanced topic (like jumping to 101). If the student struggles, the teacher continues with the current material (incrementing the lesson plan).

How to decide that whether this line will be 011, that is going to be decided by this condition select field. So, for that they are actually be having a multiplexer-based implementation. You can have any other implementation they have a micro programmed based implementation. So, they actually say that say for example, I have only 4 condition codes. So, you have a 4:1 multiplexer as simple as that. If you have many conditions to be checked that is going to increase.

The decision process for whether to perform a jump or not often relies on a component called the condition select field, which utilizes a multiplexer (MUX) to determine the appropriate control signals based on the conditions present. For example, with a 4:1 multiplexer, several conditions are evaluated, and based on the active selection code (like 01, 10, or 11), the corresponding signals are sent to control the MPC. Depending on these conditions, the appropriate jump logic can be executed, leading to either an increment or a jump to a specific instruction address.

Consider a traffic light system as analogous to the multiplexer. Depending on the light's colors (conditions), drivers receive different signals: red for stop, green for go, and yellow for caution (conditional logic). The decision to proceed or stop is dependent entirely on these signals.

So, if I want to go for jump unconditional then, somehow irrespective of this basically if I want to have basically a jump unconditional that means, I just want a 1 over here and whatever address I put it should go over there; that means, here the condition should not matter anything or whatever condition I put over here should not matter anything.

An unconditional jump allows the program to skip to a specified address without evaluating any conditions. By setting the control signal to always provide a '1', the MPC loads the predetermined address (e.g., 101) regardless of the execution state or any conditions present. This functionality is crucial in scenarios requiring immediate jumps, such as in loops, function calls, or error handling, where certain paths must be executed without conditional checks.

Imagine a person with a map who decides to go to a marked destination without checking the weather or traffic—they simply proceed to the indicated location. This approach can sometimes be useful, just as unconditional jumps are used in programming.

So, this is basically the organization of a micro-programmed control unit. So basically whatever I have told you is actually written in the slide. So, to implement this logic we actually this use a multiplexer. So, the multiplexer corresponds to jump unconditional, jump conditional and just increment basically the, that is what is the case.

The micro-programmed control unit orchestrates the operation of control signals using multiplexers to manage jump types (unconditional and conditional). This organization allows the processor to execute multiple types of jumps and increments by dynamically selecting which path to take based on coded conditions. Multiple inputs and their corresponding outputs simplify and streamline control decisions, ultimately leading to efficient execution of instructions.

Think of a diner with multiple menus. Depending on what customers order (conditions), the server directs the kitchen to prepare the selected dish (jump instruction). If a customer orders something at random, just like in an unconditional jump, the kitchen complies without needing additional input.

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