5.3.2. Pop Operations Implementation

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.

Implementing a procedure call involves three main steps. First, you prepare the stack by saving necessary data before the procedure call. This ensures that when the procedure is completed, the program can return to the exact point it left off. The second step is to call the procedure, which means the program jumps to the defined location of the procedure instructions in memory. Finally, after the procedure finishes executing, control returns to the main program, restoring the previous program's context using the data saved in the stack.

Think of this process like going to a meeting. Before leaving your office (calling the procedure), you would save your work (placing everything in the stack) to ensure you do not forget anything important. During the meeting (the procedure), you focus on the discussions. After the meeting, you return to your office (returning to the main program), and pick up where you left off.

We have a processor whose memory address range is 000 to FFF. The memory contains portions for the stack... The main program is located from 1100, 110 to 2CF, procedure A 300 to 3B0, and procedure B is located from memory location 3C1 to 3EF.

The memory of the processor is organized with address ranges from 000 to FFF, which means it can store various pieces of data and instructions. Specific ranges are set aside for different uses, including procedures and stacks. The main program and two procedures, A and B, are stored at unique memory locations, allowing the CPU to locate and execute them as needed. This structured approach enables efficient execution of nested procedures where one procedure can call another.

Imagine a library with assigned sections for each type of book. The main program is in the fiction section, while procedure A is in the non-fiction section, and procedure B is in the reference area. This organization makes it easy to find what you need and ensures that whatever section you're in, you can quickly locate and access the information.

The stack pointer at present is at F20... Whenever from A to you will be calling procedure B, you have to do the same thing you have to save the value of 37A.

The stack pointer (SP) helps keep track of the current position in the stack where values are stored. When a procedure calls another, such as when procedure A calls procedure B, the SP is updated to ensure that values needed for returning to the original procedure are saved correctly. This ensures that when procedures finish executing, they can return to the right places in the code, maintaining the reliability of the program's flow.

Continuing with the meeting analogy, imagine if you took notes (values) during a meeting. You have a notepad (stack) placed next to you. Each time you switch to a different discussion (call another procedure), you jot down where you left off (update the stack pointer). This way, you can return to that point after the discussion ends, ensuring that you don’t miss any crucial points.

First PC = 1CD... the value of 1CD is saved over here.

Before a procedure is called, the program counter (PC), which points to the next instruction to be executed, must be saved. This is crucial because after the procedure completes, the program needs to continue executing from the exact instruction it left off. By saving the PC along with other important contexts like the program status word and register values onto the stack, the system ensures data integrity and proper function flow through nested calls.

Think of this as saving your progress in a video game before trying a challenging level. When you hit 'save', the game notes your current position (program counter) and the resources you've collected (context values), allowing you to return precisely to that moment should you fail or need a break.

After completing procedure B... return instruction is executed, it will first look at the stack pointer.

Upon finishing a procedure, the return instruction is executed, signaling the processor to jump back to the point where the procedure was called. The stack pointer helps retrieve the necessary information from the stack, which includes the return address and saved context. This process is essential for ensuring that the program can continue seamlessly as if it were never interrupted.

Imagine finishing a meeting and needing to report back to your team. You check your notes (stack) where you had recorded the main points discussed and the action items. Using this information, you can confidently report back just as if you hadn't left at all.

--