24.3. Execution of Machine Instructions

A full program is written in terms of machine instruction that is macro program how it is executed. For each machine instruction when it is in the decoding cycle based on the Op-Code, the corresponding micro program is loaded into the memory. For the first phase, it means a simple fetch.

In the execution of machine instructions, the very first step is the fetch phase. This phase involves retrieving the instruction from memory and loading it into the processor. The term macro instructions refers to high-level commands like 'ADD' or 'SUBTRACT' that we write as part of a program. Each of these macro instructions has an associated micro program in the control memory, which dictates how the processor should execute it. During the decoding cycle, the Op-Code (the part of the instruction that specifies the operation to be performed) determines which micro program to load. For example, if the Op-Code represents an addition operation, the fetch phase will bring in the relevant micro program that contains the specific instructions to carry out that addition.

Think of the fetch phase as checking the index of a cookbook to find the recipe for a dish you want to make. Just like you look up a recipe title to find out where in the book it is located, the processor looks up the Op-Code to find the corresponding micro program in memory.

After the fetch, the instruction register actually decodes what the exact instruction is. If it is add, if it is store, if it is load accordingly the MPC will be pointed to a different part.

Once the instruction has been fetched, the next step is the execution phase. During this phase, the instruction register decodes the fetched instruction to determine its type — for instance, whether it involves addition ('ADD'), storage ('STORE'), or loading a value ('LOAD'). Based on this decoding, the micro program counter (MPC) updates its reference to point to the appropriate section of the micro program memory. For example, if the instruction decodes to 'ADD', the MPC will redirect to the set of micro instructions that will execute the addition logic using the relevant data.

Imagine you're at a restaurant. After looking at the menu (fetching), the waiter (instruction register) takes your order and communicates it to the kitchen staff. Each dish has its own set of preparations, so the kitchen (the MPC) needs to know what to do next based on your specific order.

One the present micro program is complete, the next micro program is loaded based on the new instruction. Whenever a new instruction has to be executed, it will first load the MPC correspond to the micro program memory address.

Branching is a crucial element in the execution of machine instructions, especially in optimizing the efficiency of a micro program. After completing one micro program (for instance, adding two numbers), the MPC resets to point back to the initial memory address that fetches another instruction. This constant loading and execution can be quite dynamic, as the type of operation will dictate which micro program is invoked next. Branching ensures the micro program can 'jump' back to the fetch phase or to different instructions efficiently without duplicating memory space.

Consider a traffic light system. Once the light turns green and cars start moving (completing the current instruction), when it changes back to red, it triggers the light to switch back to green for the next sequence of cars that need to get through the intersection (branching to the next instruction).

So, what is the solution? ... wherever there are diversions we can put jumps, so that how basically it works.

To optimize the handling of instructions and minimize the size of the control memory, designers utilize various strategies. For example, instead of creating a distinct micro program for each instruction type (like 'add' or 'subtract'), common sections can be consolidated. In practice, both 'ADD' and 'SUBTRACT' may share much of their logic, and only require different control signals for certain operations. By facilitating jumps (or branches) within the micro program, it allows the execution path to diverge based on the instruction type without duplicating entire sections of code.

Think of a multi-tasking chef in a kitchen. Rather than having a separate recipe for every variation of a dish, a chef can use a base recipe for a sauce and simply adjust one or two ingredients (like switching from tomato to curry spices) depending on whether they are making pasta or curry. This saves time and effort while ensuring both dishes taste great.

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