Switch from Real Mode to Protected Mode

Leaving Real Mode

Protected mode enables paging, segmentation with descriptors, and ring separation. Switch requires loading a GDT, setting CR0.PE, far jumping to 32-bit code segment, and initializing segments.

Mode Switch Sequence

Steps:

• Load GDT with lgdt
• Set PE bit in CR0
• Far jump to 32-bit code selector
• Reload segment registers with data segment selectors

For example, after the far jump, CS points at a descriptor with D/B flags for 32-bit operations while DS/ES/SS use flat data selectors.

Why This Matters in Production

Every OS you've ever booted on real x86 hardware , from DOS-era systems to modern Linux , passes through this exact real-mode-to-protected-mode transition, and forgetting the far jump after setting CR0.PE is a rite-of-passage bug that leaves the CPU executing stale real-mode instructions until it triple-faults. This is also the turning point where early teaching kernels like xv6's x86 predecessors first became genuinely 32-bit.

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 implement the mode transition assembly and verify 32-bit code runs. This exercise requires explaining why a near jump immediately after setting PE is illegal on x86.