Switch from Real Mode to Protected Mode and Display Text

Topic Overview

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 simulate the transition on paper: a GDT descriptor, CR0.PE, the far jump, and segment reloads, driven by a boot-step script from stdin. This exercise requires the exact order: disable interrupts, load GDT, set PE, far jump, reload segments, and it faults exactly where real silicon faults.

Simulate the real-mode to protected-mode transition in C, driven by a boot-step script on stdin.

Protocol (all commands one per line, numbers in hex unless noted):

• a20 enable the A20 line, print "A20=1"
• gdt <base> <n> load GDTR: base (hex), n entries (decimal); print "GDTR=0x%08X:%04X" with limit = n*8-1
• pe set CR0 bit 0 and enter protected mode; print "CR0=0x%08X"
• jmpi <idx> far jump through GDT index: real mode prints "JMP REAL CS=0x%04X"; in protected mode a missing GDTR or an index with idx*8+7 beyond the GDT limit prints nothing and halts with RESULT=TRIPLE_FAULT, otherwise print "CS=0x%04X"
• movs <idx> load DS: in real mode DS=idx directly; in protected mode index 0 or an index past the limit halts with RESULT=TRIPLE_FAULT, otherwise print "DS=0x%04X"
• int10 BIOS video call: real mode prints "BIOS CALL OK"; in protected mode it faults (16-bit real-mode ISR) and halts with RESULT=GP_FAULT

After all steps (or the first fault), print the summary: MODE=REAL|PROTECTED CS=0x%04X DS=0x%04X A20=%u RESULT=OK|TRIPLE_FAULT|GP_FAULT