21.1.3. Instruction Fetch and Execution

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.

The micro-programmed control unit is responsible for generating control signals in a specific order. These control signals are essential for executing instructions properly and are stored in memory locations. This means that the correct sequence of instructions is critical for overall operation, as each signal must be generated in the right order.

Think of the micro-programmed control unit like a conductor in an orchestra. Just as the conductor ensures that each musician plays their instrument at the right time, the control unit ensures that each part of the computer's instruction set is executed in the correct sequence, maintaining harmony in operation.

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.

Sequencing is crucial because the outcome of the instruction execution often depends on various conditions, such as the status flags (like zero or carry flags). These flags determine if the next instruction should be executed immediately or if a different instruction is needed based on memory signals or interrupts.

Imagine a traffic light system. The traffic light changes based on the conditions of traffic and time of day. Similarly, the sequencing of instructions in a micro-programmed control unit changes based on the conditions set by status flags, determining which instruction should come next.

So, you have to have also arrangement for something called a branch decision. So, in right normal programs we have micro instructions where actually which is conditional, and also, we have something which is unconditional.

Branch decisions refer to the need to determine which instruction to execute next. This can be conditional (if a certain flag is true) or unconditional (always go to this instruction). When a conditional instruction is executed, the system checks the relevant condition and then either follows the next instruction in sequence or jumps to a different instruction based on the result.

Think of a choose-your-own-adventure book, where at the end of each page, the reader must decide which page to turn to based on the choices made. Similarly, when a computer encounters a branch instruction, it must choose which instruction to execute next based on the conditions provided by previous instructions.

So, first is first phase is called the fetch an instruction. So, we have discussed many many times that for most of the instruction fetch phase is similar.

The first phase in executing an instruction is the fetch phase, where the instruction that needs to be executed is obtained from memory. This phase is similar for most instructions, as it generally follows the same steps to retrieve the instruction code.

This is similar to how a waiter in a restaurant fetches an order slip from the kitchen. Regardless of what the order is, the waiter must first retrieve it before any cooking or serving can take place.

After that the add will be decoded, and then different types of activities has to take place. So, whenever 𝐴𝐷𝐷 has been decoded, then it is a register to register operation.

Once the fetch phase is complete, the instruction is decoded to understand what operation it requires. For instance, if the instruction is an ADD operation, the decoder interprets it as needing to add values stored in two registers. Based on the decoded instruction, the appropriate micro-instructions are executed to carry out the operation.

Think of it like following a recipe in cooking. After retrieving the recipe (fetch), a chef reads and understands what needs to be done (decode) and then starts preparing the ingredients and cooking (execution) based on the instructions provided in the recipe.

So, that is what has been actually is the job of a micro-programmed control unit. So, apart from branch address these micro programs can specify which status flags conditions etcetera has to be taken for the condition check.

The micro-programmed control unit maintains the micro-program counter (MPC), which points to the current instruction or micro-instruction to be executed. It manages jumps and sequencing based on conditions checked by the control signals and status flags.

Imagine a teacher using a lesson plan (MPC) to guide a class. The teacher keeps track of which topic to teach based on what was covered previously (like status flags). If a certain topic needs more time or a review, the teacher can decide to branch back to that topic instead of moving linearly through the lesson.

So, whenever it reaches the end instruction it does not go that is obvious.

When the sequence of micro-instructions for a particular operation reaches its end, the MPC will not automatically increment to the next instruction location. Instead, it must wait for the next instruction to be actively fetched, as some micro-instructions may not lead to a further operation until another set of conditions are met.

This can be likened to a story that ends on a cliffhanger. The reader must wait for the next book in the series to continue. Similarly, when the current operation completes, the system must pause and wait for the subsequent instruction before moving on.

So basically, this micro-program actually has one part of the address, which is the, which one part of the memory, which is actually called or in a row.

The micro-program is structured to manage multiple parts. One part is dedicated to generating control signals, another for conditional checks, and a branch address field to specify where to go next if a condition is met. This organization allows the system to handle various instruction paths efficiently.

Consider a filing system in an office where files are stored in different folders based on certain criteria. The main folder contains standard documents, while others hold conditional documents that change based on specific requirements or circumstances. This structured approach ensures that information can be accessed and processed quickly.

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