5.3.4. Return Operations Implementation

Now, we have talked enough theory and this is actually a nutshell what I have told you completes the theory of a procedure call 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 there jump to that memory location, after completing the procedure return back to the main program from where the procedure is called.

This chunk explains the basic steps involved in a procedure call. It is broken down into three simple actions:

1. First, before calling a procedure, all necessary context or information is saved onto the stack. The stack is a region in the memory where temporary data can be stored.
2. Once the information is saved, the program jumps to the memory location where the procedure is defined to execute it.
3. After completing the procedure, the program must return to the original calling point, restoring any saved context from the stack.

Think of the stack as a notepad that you use while reading a book. When you start reading a new chapter (call a procedure), you note down where you are in the book (save context) so you can return to that exact page (return back) after you're done with the chapter.

So, we have a processor whose memory address range is 000 to FFF. So, it is an address bus will be 12 bits... the main program is located from 1100, 110 to 2CF, procedure A 300 to 3B0, procedure B is located from memory location 3C1 to 3EF.

This section discusses how memory is organized for different procedures in a CPU architecture. The processor has a memory range from 000 to FFF, and different procedures are allocated specific ranges of addresses.

• The main program occupies a block of memory ranging from 110 to 2CF.
• Procedure A is allocated memory between 300 and 3B0 while Procedure B occupies addresses from 3C1 to 3EF. This structured allocation ensures that each procedure has its own space in memory and aids in efficient retrieval of instructions during execution.

Imagine a library where each section (main program, procedure A, procedure B) is allocated specific shelves (memory addresses), making it easier to find and read books (execute instructions) without confusion.

The stack pointer at present is F20 that is the top of the stack it is assumed... we are going to see how the stack is implemented or how the stack is modified when such a code is executed using jump instructions in a nested procedure.

In this chunk, the concept of the stack pointer is introduced, which marks the top of the current stack. The stack pointer begins at a predefined memory address (F20) and changes as procedures are called and return. When a procedure is invoked, the current state (such as memory addresses and values) gets saved to the stack, and the stack pointer adjusts accordingly to reflect the new top of the stack. This dynamic adjustment allows multiple nested calls without losing track of where to return to.

Think of the stack as a stack of plates in a cafeteria. The stack pointer tells you where the top plate (the most recent procedure saved) is. Every time you add a plate (call a new procedure), you add it to the top, and when you take one away (return from a procedure), you take the top plate off.

So, this part of the memory has been allocated for the stack... So, this is a very very simple way in how a procedure is basically implemented.

This section outlines a practical example of how nested procedures work in a given memory allocation. It describes how Procedure A calls Procedure B, emphasizing the need to save the return location and any required context on the stack as each call occurs. This helps in keeping track of where to return when a procedure completes. By managing the stack rigorously, the CPU can execute complex calculations divided into smaller tasks.

Imagine you are on a road trip (main program) and you stop at a friend's house (Procedure A) before heading to a restaurant (Procedure B). You need to remember the route you took to your friend's place because you have to return back the same way after your meal. This method of remembering each stop ensures a smooth trip.

So, now we will see how the stack is implemented... So, when I will be returning I will be returning back to this point.

This chunk discusses the importance of saving context before jumping to another procedure. As the program moves to call Procedure A, it saves the program counter and other necessary values onto the stack. This ensures that when the called procedure is done executing, it can return to the exact spot in the previous procedure without losing any information. The saved context will include the program status word and general registers needed for proper execution flow.

Consider playing a video game where you can pause (saving context) before you try a new level (calling a new procedure). If you succeed in the new level, you can resume playing from where you paused, making sure you don’t lose your progress.

After procedure B is completed... the main program will start executing from 1CE.

This section explains the return process from procedures. When Procedure B finishes, it looks at the stack to find out where to return to (the last saved address in the program counter). It uses the stack pointer to retrieve this information and updates the program counter to the next instruction in Procedure A or back to the main program. The stack pointer then increments to point to the next item on the stack, facilitating the continuation of execution from the correct location.

Going back to the earlier example, it’s like after finishing dessert at the restaurant (Procedure B) and checking the directions saved on your phone to navigate back to your friend's house (Procedure A) and then home (main program). Each time you go back, it remembers exactly where you were in the journey.

So, now we will see a slight because we are... call, return, push and pop are implemented in a micro level.

This chunk delves into the micro-level operations executed in the CPU during call, return, push, and pop actions. Each instruction comprises several micro-operations that dictate how data is saved on the stack or retrieved. For example, during a push, the CPU needs to load values into the stack pointer before actually storing the data in memory. Similarly, for returns, it employs several read operations to fetch previously saved data, updating registers accordingly before executing the next instruction.

Imagine a factory assembly line (CPU), where each worker (micro-operation) has specific tasks to complete for a production project (the main operation). Each operation—like assembling parts (push), disassembling parts (pop), and finishing tasks (call and return)—requires careful coordination among several workers to ensure the end product is right.

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