5.2. Stack Implementation Details

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.

A procedure call consists of three main steps. First, before calling a procedure, relevant data needs to be saved onto a stack. This stack temporarily holds data that needs to be returned later. Second, the system jumps to the procedure's starting address in memory, allowing it to execute. Finally, once the procedure completes, control returns to the main program, retrieving saved data from the stack to continue execution smoothly.

Think of it like writing a note to yourself (the stack) before you leave your house for a meeting (calling a procedure). You save important information on the note, attend the meeting (execute the procedure), and afterward, you return home to check your note and carry on with your day (return to the main program).

We have a processor whose memory address range is 000 to FFF. A part of this is reserved for the stack, which is going downwards; that means, you will be going downwards as a stack implementation.

The processor's memory is organized from address 000 to FFF. Within this range, a specific segment is reserved for the stack, typically at the higher end. This stack grows downwards into lower memory addresses as items are pushed onto it, contrary to how regular lists might grow upward. Understanding this layout is critical for proper memory management.

Imagine a stack of boxes where new boxes (data) are added at the top. As you put more boxes on top, the stack builds downward, which can seem counterintuitive if you think of a stack growing upward. Referring to addresses as growing downwards helps visualize how data is structured in memory.

While I am executing the main program, it will call procedure A at memory location 1CD. Then, while executing procedure A, it will call procedure B from the instruction at location 37A.

In a nested procedure call, the main program first calls Procedure A, which occupies a specific memory location. While Procedure A is running, it may need to call another Procedure B. Each of these calls utilizes the stack to manage returns, allowing each procedure to keep its context intact before returning back to the previous level.

Consider a nested scenario like a restaurant: you (the main program) order an appetizer (Procedure A). While waiting, you also request a drink (Procedure B). The waiter (stack) keeps track of both orders, ensuring everything is returned to you in the right order once all items are ready.

The stack pointer at present is F20; that is the top of the stack it is assumed.

The stack pointer (SP) is a register that points to the topmost item on the stack. In this example, it's assumed to start at address F20. As data is pushed onto the stack, the SP decreases to point to the new top item, ensuring that the correct memory address is used for retrieval or management of data during procedure calls.

Think of the stack pointer as the position marker on a stack of plates. When you add a plate (push), you raise the marker to the top plate's height. When you need a plate (pop), the marker indicates where to take the top plate from. If you lose track of your marker, you risk grabbing the wrong plate or not knowing when the stack is low.

Before jumping to the procedure, save the values of the program status word, the program counter, and the variables which you have saved before going to this one.

Before executing a procedure call, it’s essential to save the current state of the program – including the program status word (which holds flags), the program counter (which points to the next instruction), and other important variable states. This process ensures that once the procedure is finished, the program can resume exactly where it left off, preserving the flow of execution.

It's akin to pausing a movie (current state) to take a phone call (procedure call). You jot down where you paused (state) so you can return to the same spot later without missing a second of the movie.

The procedures for push, pop, call, and return involve specific micro-operations that manage how data is placed and retrieved from the stack effectively.

Executing stack operations like push (adding to the stack), pop (removing from the stack), call (starting a new procedure), and return (ending a procedure and going back) involves several micro-operations. Each operation modifies the stack pointer accordingly, ensuring that data is correctly managed and available when needed.

Imagine a library system (the stack) where books are stored. To 'push' a book onto a shelf, you need to record where it goes (memory address) and put it in a specific spot. To 'pop' it back off the shelf, you need to check your records (stack pointer) to find the right book before retrieving it.

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