DDCA Ch6 - Part 12: The Stack

Stack Overview

In this section, the concept of a stack in computer architecture is introduced, likened to a stack of dishes where items are added and removed in a last-in-first-out manner.

Understanding the Stack

• The stack is part of main memory used to temporarily store variables, operating on a last-in-first-out principle. ()

• It expands when more memory space is needed and contracts when no longer required. The stack grows downward from the highest memory location to a lower address. ()

• The stack pointer (sp) is a register that points to the top of the stack, indicating the current position for adding or removing data. ()

Function Calls and Register Handling

This segment delves into managing registers during function calls, emphasizing the importance of preserving and restoring register values for seamless execution.

Register Management in Function Calls

• Before modifying registers during function calls, it's crucial to save their values to prevent unintended side effects. ()

• Saving registers involves moving the stack pointer down by an appropriate number of bytes to create space for storing register values temporarily. ()

• After executing functions that alter register values, it's essential to restore the saved register values back from the stack before returning from the function call. ()

Preservation of Registers Across Function Calls

This part focuses on preserving specific registers across function calls to maintain data integrity and ensure smooth program execution.

Preserving Registers

• S registers represent variables; hence, they need preservation across function calls to avoid corruption of local variables' data. ()

• Certain registers like s registers, stack pointer (sp), and return address (ra) should remain unchanged after function calls unless explicitly saved and restored by callee functions if needed. ()

Optimizing Register Handling in Functions

Optimizing register handling within functions can streamline operations by minimizing unnecessary saving and restoration processes.

Efficient Register Management

• By optimizing register usage based on preservation requirements, functions can reduce unnecessary saving and restoring actions, enhancing efficiency. ()

Function Calls and Stack Frames

In this section, the speaker discusses function calls, stack frames, and the management of registers during function execution.

Function Execution in f1 and f2

• In function f1, various operations are performed, potentially affecting s4 and s5 as internal variables. After completion, the return address is retrieved from the stack, the stack pointer is adjusted back to its original position, and other relevant variables are restored.

• Function f2 is simpler compared to f1. It primarily utilizes s4 as a saved register without calling other functions. The process involves saving s4 on the stack, adjusting the stack pointer downwards by four bytes, executing tasks, restoring s4 from the stack, and returning to the initial address.

Stack Management during Function Calls

• When calling f1 or f2, specific registers such as a0, a1, ra (return address), s4, and s5 are stored on the stack to create a stack frame for each function.

• As functions are called sequentially like f1 followed by f2, each function allocates space on the stack for its saved registers. Upon completion of a function's execution (e.g., f2), its allocated space is deallocated before moving back up through the call chain.

Register Handling in Function Calls

• Prior to calling another function (callee), a caller saves necessary registers including ra and temporary/a registers. Arguments are placed in designated argument registers (a0-a7) before invoking the callee using jump and link instructions.