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LC

Transition from Protected Mode to Long Mode (x86_64)

hard7/8

Enter Ring 3 (User Mode) from Ring 0 Set Up the Task State Segment (TSS)

Language

Compiler

2.0s Limit 256MB

Submit and pass all tests to enable saving to GitHub.

Implementation

Simulate the transition to x86_64 long mode in C.

Requirements:

Show CPUID check for long mode support
Implement enable_pae() that sets CR4.PAE
Implement setup_pae_page_tables() with PML4, PDP, PD
Implement set_lm_bit() using EFER MSR
Show CR0.PG being set to enable paging + long mode
Show compatibility mode active
Show far jump to 64-bit segment
Print register sizes changing (32-bit -> 64-bit)
Show that segment limits are ignored in long mode
Print summary of all steps

Test:

Show CPUID check
Show PAE enabled
Show LM bit set
Show paging enabled
Show long mode active
Show 64-bit segment entered

Why This Matters

This exact PAE-then-EFER.LME-then-CR0.PG sequence is what Linux's arch/x86/boot/compressed/head_64.S and every 64-bit hobby OS on the OSDev wiki perform to escape 32-bit protected mode, and forgetting that long mode only activates once BOTH EFER.LME and CR0.PG are set (not either alone) is a classic bug that leaves the CPU stuck in compatibility mode. Every x86-64 machine on the planet boots through this identical multi-step dance.

Hints

0 / 3 revealed

Validation Suite

main.c

Workspace 1.0

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