22.5.1. Fetching Instructions

We 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.

In computing, the 'control function field' plays a crucial role in guiding the operations of memory and processing units. It typically includes signals like 'program counter in' and 'program counter out', which define how the program counter (PC) behaves. The PC keeps track of the next instruction to be executed. Additionally, there are two vital parts: the 'condition select' which determines if certain conditions are met before executing the next command, and the 'branch address field' which specifies the next address the program should jump to based on those conditions.

Think of the control function field like a navigation system in a car. Just like the navigation system indicates whether to keep going straight or take a turn based on current location and destination, the control function field guides the PC on whether to follow the next instruction in the sequence or branch to a different one based on existing conditions.

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.

The branch address field's primary function is to dictate where control flows in a program. When executing instructions, certain conditions might alter the expected sequence. For example, if a condition is true, the program is told to move from its current instruction (say at address 001) to a different one (like 101). This capability allows for more dynamic and flexible programming, enabling loops, conditionals, and decision-making processes.

Imagine you're cooking and following a recipe. If a recipe says, 'If the sauce isn't thick enough, stir for two more minutes,' that’s a branching instruction. If the sauce is thick enough (condition met), you move on to the next step. But if not, you 'branch' back to stirring and make adjustments before progressing.

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.

Updating the micro-program counter (MPC) is essential for determining the flow of instructions executed by the processor. If the branch condition is satisfied, the current value of the MPC is overridden; rather than simply incrementing from 001 to 010, the MPC directly jumps to 101. This immediate update allows the processor to bypass irrelevant instructions and execute only the ones pertinent to the branching condition.

Think of the MPC as a reader following pages in a book. If the reader reaches a page (001) that says 'If you want to go to the next chapter (100), turn to page 101 instead,' the reader will skip to that page directly instead of going to page 100, which they may not need. This ensures they are only reading the content that is relevant.

Here you see this is very interesting there is a control to 𝑀𝑃𝐶. So, what is the control to 𝑀𝑃𝐶 one is load and increment bar; that means, if this line is 1. So, what is this going to do it is going to load the value which is available at this port so; that means, if this is 1 you are going to take the value of 101 in the micro-program control and you are going to jump to 101, but if this line is 0 then actually what is going to happen it is just going to make it as +1.

The control mechanism for the micro-program counter (MPC) determines whether the counter will load a new address or simply increment its current value. If a control signal associated with MPC is activated (set to 1), the MPC will load a specified address, such as 101. Conversely, if this control signal is not activated (set to 0), the MPC will merely increment its current value by 1. This control system is fundamental to executing the correct sequence of instructions.

Consider a traffic control signal at an intersection. If the light is green (control is '1'), cars can proceed to the next street (load a new address). If the light is red (control is '0'), cars must stop and wait (simply stay in the current position and increment their waiting time). Understanding when to move forward or remain still is similar to the MPC's decision-making process.

How to decide that whether this line will be 0 and 1 then my job will be done. How to decide that whether this line will be 0 and 1 then my job will be done. So, for that they are actually be having a multiplexer-based implementation.

The condition select field is integral in deciding which state the MPC should transition to based on current conditions. It uses a multiplexer to manage different input conditions. The multiplexer effectively routes the relevant control signal to determine if the MPC should load a new address or increment its value. By selecting different connection states based on incoming signals, the multiplexer offers flexibility in decision-making.

Think of a multiplexer as a restaurant menu. You can only choose one dish from a list of options. Depending on whether you're feeling hungry, adventurous, or want something quick, you might select a different dish (line in the multiplexer). Just as your menu choice determines what you order, the condition select field in the MPC determines the next action to take.

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