How Debuggers "Actually" Step Through Code

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How do debuggers actually step through code? In this Terminal episode, Ryan, Casey, Prime, and TJ break down the surprisingly complex world behind step over, step into, int3 traps, breakpoints, CPU interrupts, and why debugging is way harder than most developers realize.

We cover:
• How debuggers map source lines to machine instructions
• Why optimized builds make stepping unpredictable
• How int3 works and why debug traps slow everything down
• Kernel ring transitions (ring 3 → ring 0 → ring 3)
• Why recursive functions break naïve stepping
• Why conditional breakpoints are painfully slow
• Overwriting and restoring machine code at runtime
• How user-level int3 could revolutionize debugging performance
• Why stepping requires control-flow analysis, disassembly, and tons of edge-case handling

Plus: magnets, recursive treadmills, banking apps, and ordering coffee through SSH. Welcome to Terminal.


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📌 Why It Matters

Stepping through code isn’t “next line” — every press of F10 or F11 is a dance between the debugger, the kernel, CPU interrupt tables, JIT’d trap handlers, and compiler-generated machine code. This episode is a clear breakdown of the invisible machinery every programmer relies on.

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🛠 Tools & Tech Mentioned
• x86 interrupt table (IDT)
• int3 instruction / trap
• DWARF
• PDB
• LLDB / GDB
• Thread stepping
• Instruction pointer (IP / RIP)
• Stack pointer / call stack
• CPU context switching
• JIT patching for debug traps
• Function inlining, optimizing compilers
• Recursive stepping logic
• Kernel ring transitions (ring 0 ↔ ring 3)

⏱ Topics Covered / SEO Keywords

Debugger stepping explained, how step over works, how step into works, int3 instruction, interrupt handling, CPU trap instructions, x86 interrupts, DWARF debug info, PDB debug info, breakpoint performance, conditional breakpoints slow, how debuggers patch instructions, how CPUs handle interrupts, stepping through optimized code, recursive stepping bug, kernel vs user-mode debugging, debugger design, programming podcast, software engineering deep dive, how compilers work with debug info, low-level debugging, kernel dev, operating systems, stepping algorithm, instruction pointer mapping, control flow analysis, runtime disassembly, jump instructions, relative vs absolute jumps, debugger architecture, Terminal podcast, Primeagen, Casey Muratori, Ryan Fleury, TJ, Better Software Conference, Wukash.