5.4. Conclusion

So, this is overall the basic comp, CPU components which are involved in a procedure call. All the components are quite similar to all other functions and all other instructions we have discussed till now, but stack pointer is a register which plays a very special role over here.

In this chunk, we learn that procedure calls in a CPU involve components similar to other functions or instructions. However, the stack pointer is highlighted as a crucial register that manages the memory used during these calls. It holds the address of the top of the stack, helping the CPU know where to save and retrieve data during function calls and returns.

Imagine the stack pointer as a bookmark in a book. It tells you where you left off each time you open the book, just like the stack pointer tells the CPU where to continue executing after a function has completed.

It is basically only 3 very simple steps: you call a procedure, before calling a procedure place everything in this stack, call the procedure, jump to the instruction where the first instruction of the procedure is; after completing the procedure, return back to the main program from where the procedure is called.

This chunk explains the three fundamental steps in executing a procedure call: 1) Saving the current context (like the program counter and relevant variables) onto the stack, 2) Making the call to the procedure and jumping to the procedure's starting address, and 3) After the procedure executes, bringing back the saved context to return to where the call was made.

Think of this process similar to making a phone call. First, you jot down the important things you need to discuss (saving context), then you dial the number and chat (the procedure call), and finally, you hang up and get back to your previous conversation (returning to the main program).

The stack goes downwards; that means, you will be going downwards as a stack implementation. This part of the program memory is allocated for a stack when your stack will be saved.

The stack memory is implemented such that it grows downwards, meaning that new elements are added at lower memory addresses. This design allows for efficient use of memory and ensures that there is enough space for storing data during nested calls and returns. Whenever a procedure is called, relevant data is pushed downwards onto the stack.

Visualize a stack of plates where you can only add or remove plates from the top. As you stack more plates, you are effectively 'going down' to store more, much like how the stack memory operates.

We are at 1CB, so at this point the program main program is calling subroutine A which with is located at location 300. Procedure A is going to call procedure B.

Here, we observe how nested procedure calls work. When the main program reaches a certain point it calls Procedure A. While executing, Procedure A may further call another Procedure B. Each call results in saving the current execution context onto the stack, allowing the CPU to return to the exact point in the previous procedure after the completion of the nested call.

Imagine a relay race. The head runner (main program) passes the baton (control) to the next runner (Procedure A), who may also pass it down to a third runner (Procedure B). At every baton pass, there's a record (stack) of where each runner was in case they need to return after completing their leg of the race.

After completing procedure B you have to return to the point in procedure A from where procedure B was called, and then to the main program.

Completion of nested procedures involves returning back through each level. After Procedure B gets completed, control transfers back to Procedure A at the specific point right after the call was made. Once Procedure A completes its execution, it returns control back to the main program. This sequence ensures all operations get completed without losing previous states.

Consider baking a cake that has several layers. Once you finish baking the top layer (Procedure B), you need to place it on the bottom layer (Procedure A), and finally you present the entire cake (main program) to guests. You cannot skip to the presentation without completing the prior layers.

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