x86-64 Assembly: Local Variables, Stack Frames & Alignment Explained

Quick but deep look at local variables on the stack in x86-64 assembly (YASM). We cover allocation with sub rsp, accessing via offsets, why the stack must be 16-byte aligned when calling libc functions like printf, and two practical ways to fix alignment crashes. Includes live segfault debugging and a full working example with a local array.

Great for anyone studying systems programming, computer architecture, or just trying to figure out why their assembly program randomly crashes on a library call.

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00:00 Introduction to Local Variables on the Stack
00:28 C++ Example of Function and Local Variables
01:06 Incoming Arguments in RDI vs Stack Variables
03:24 Pointers as Local Variables on Stack
04:34 Why the Stack ? Recursion and Multiple Calls
05:18 Visualizing Multiple Stack Frames
09:24 How Function Returns Adjust RSP
10:35 Stack Grows Downward in Memory
11:33 Program Setup ? Hybrid C++/Assembly
12:46 Assembly Module Overview
14:00 Function Prologue ? Register Push & Alignment
15:30 Allocating Stack Space for Local Array
17:45 Initializing Array in Loop
20:10 Printing Loop with printf
49:48 First Run ? Segfault Observed
51:00 16-Byte Stack Alignment Requirement
51:55 Fix 1 ? Extra Push/Pop in Prologue/Epilogue
53:15 Fix 2 ? Push/Pop Around Each printf Call
55:04 Testing Different Array Sizes
56:52 Debugging Alignment Behavior
58:54 Summary ? Creating Any Local Data on Stack
59:59 Closing Remarks & Subscribe Call

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