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Set Up the Global Descriptor Table

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Switch from Real Mode to Protected Mode Implement Interrupt Descriptor Table and Handlers

Language

Compiler

2.0s Limit 256MB

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Implementation

Implement a GDT setup simulation in C.

Requirements:

Define GDT_ENTRY structure matching x86 format (8 bytes)
Implement encode_gdt_entry() that packs base, limit, access, flags into 8 bytes
Create standard entries: null, kernel code, kernel data, user code, user data
Implement print_gdt_entry() to show decoded fields
Show GDTR structure (base + limit)
Implement load_gdt() that simulates lgdt instruction
Print selector values (0x08, 0x10, 0x1B, 0x23)
Show privilege levels for each entry
Demonstrate that all segments use flat model (base=0, limit=4GB)

Test:

Show null descriptor
Show kernel code segment with ring 0
Show user data segment with ring 3
Show selector calculations

Why This Matters

The flat-model GDT you're encoding here — ring 0 kernel segments, ring 3 user segments, all spanning 4GB — is exactly what every production x86 kernel sets up before touching paging, even though modern OSes rely on page tables rather than segmentation for real protection. Get an access byte wrong (0x9A vs 0x92, code vs data) and user code silently runs with kernel privileges instead of faulting.

Hints

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Validation Suite

main.c

Workspace 1.0

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