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LC

Run Real Mode Code with Virtual 8086 Mode

hard5/8

Enable Basic Paging in Protected Mode Enter Ring 3 (User Mode) from Ring 0

Language

Compiler

2.0s Limit 256MB

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Implementation

Simulate Virtual 8086 mode concepts in C.

Requirements:

Define EFLAGS structure with VM bit
Implement enter_v8086() that sets VM bit
Implement v8086_address_calc() with segment:offset
Show that paging still applies in V8086
Implement IOPL check for privileged instructions
Show VME concept
Simulate a DOS program running under monitor
Print EFLAGS before and after entering V8086
Show that interrupts can be virtualized
Demonstrate memory mapping via paging

Test:

Show V8086 entered
Show real mode style addressing
Show paging still active
Show IOPL check
Show monitor handling privileged instruction

Why This Matters

V8086 mode is exactly how Windows 3.x/9x ran legacy DOS boxes and how DOSBox-style emulators and some real-mode BIOS-call shims in early Linux VESA/VBE drivers executed 16-bit code without leaving protected mode. The IOPL-gated #GP-then-emulate trick you simulate here (CLI/STI/IN/OUT trapping to a monitor) is the same mechanism VME (CR4.VME) was added to accelerate in hardware.

Hints

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Validation Suite

main.c

Workspace 1.0

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