Understand the x86 Boot Sequence

From Reset Vector to Your Code

x86 PCs start in real mode at 0xFFFFFFF0, executing firmware that eventually loads a boot sector to 0x7C00 and jumps there. Your bootloader's job is to find the kernel and switch to protected mode later.

Stage Checklist

Firmware path:

• POST hardware init
• MBR or GPT load of stage1
• Boot sector code at 0x7C00 with 0xAA55 signature

For example, a 512-byte sector ending with boot signature loads in one INT 13h read of cylinder 0, head 0, sector 1.

Why This Matters in Production

This is the exact path every x86 hobby kernel referenced by the OSDev wiki takes, and it's the same MBR-at-0x7C00-then-GRUB flow that boots real Linux installations before the kernel takes over. Getting a single stage wrong , like jumping to the wrong load address , is why 'kernel panics before printing anything' is one of the most common early OS-dev debugging sessions.

Debugging Checklist

Before you call the implementation done, walk failure modes on purpose. Test empty structures, single-element edge cases, maximum concurrency, and errno paths that must not crash the program. OS code usually fails in production when happy-path tests pass but invariants break under contention or memory pressure.

Implementation Notes

Keep structures small and name fields after kernel counterparts when possible. That lets you read man pages and kernel source side by side while you work. Print observable events during development; remove noisy logs once tests pass reliably.

For This Exercise

You will trace the reset vector through your bootloader's first instructions. The task asks you to label each register state (CS:IP) at handoff points in a boot map diagram.