Switch from Real Mode to Protected Mode and Display Text

Entering protected mode

Protected mode unlocks 32-bit operands, privilege levels, and optional paging. The CPU stays in real mode until CR0.PE is set, but the prefetch queue still holds real-mode decode until a far jump loads CS from the GDT. For example, after LGDT and setting PE, jmp 0x08:protected_entry flushes the pipeline with the new code selector.

Minimum sequence

nasmCopy
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• Load flat data/stack selectors into DS, ES, FS, GS, SS
• A20 must be enabled before touching high memory
• Set up stack pointer in 32-bit mode before calling C code
• IDT can wait until after basic PM bring-up

Post-switch checklist

After the far jump, execute only 32-bit instructions; mixing 16-bit operands requires operand-size prefixes. Load ESP before calling any pushed-argument function. If video output still uses VGA text at 0xB8000, identity mapping must cover that region. Take a snapshot of GDT and IDT pointers in a struct for later long-mode upgrades.

Why for this exercise

You will build a GDT, switch CR0.PE, far-jump to 32-bit code, and print text. This exercise requires the exact order: disable interrupts, load GDT, set PE, far jump, reload segments.