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Build a Flat GDT for Protected Mode

hard2/8

Switch from Real Mode to Protected Mode and Display Text Set Up the Interrupt Descriptor Table in Protected Mode

Language

Compiler

2.0s Limit 256MB

Submit and pass all tests to enable saving to GitHub.

Implementation

Implement a flat GDT setup in C.

Requirements:

Define GDT entry structure (8 bytes, packed)
Implement encode_gdt_entry(base, limit, access, flags)
Create entries: null, kernel code, kernel data, user code, user data
Show selector calculations
Implement print_gdt_entry() that decodes all fields
Show GDTR structure and load_gdt() simulation
Print all 5 entries with decoded fields
Show privilege levels for each entry
Demonstrate that all segments use base=0, limit=4GB

Test:

Show null descriptor
Show kernel code segment
Show user data segment
Show selector values
Show GDT loaded

Why This Matters

Linux, Windows, and every BSD all run this exact flat-model GDT (base 0, limit 4GB) rather than true x86 segmentation, because paging — not segments — does the real memory protection work; the segment layer is kept alive purely because x86 mandates it. Getting the access byte wrong (0x9A vs 0x92, code vs data) is why a working kernel can suddenly triple-fault the instant it tries to execute code through a descriptor marked non-executable.

Hints

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Validation Suite

main.c

Workspace 1.0

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