From Real Mode to Protected Mode: Building Custom GDT & IDT for x86 Security

The journey from 16-bit real mode into 32-bit protected mode. Hand-built Global Descriptor Table and Interrupt Descriptor Table — the foundation every modern OS rests on.

August 13, 2025

Harrison Guo

Video by: HarrisonSecurityLab

Published on YouTube: 2025-08-13

Bootloader Protected Mode GDT IDT x86 Operating Systems NanoBoot

The single most consequential transition in x86: 16-bit real mode → 32-bit protected mode. Once you cross it, you get segment privilege levels, larger address space, and the foundation every OS layer above you takes for granted.

This video walks the GDT and IDT setup from scratch, then performs the lgdt / far jump that flips the CPU into the new mode. Hand-coded, no OS underneath.

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