Run Real Mode Code with Virtual 8086 Mode

Virtual 8086 mode

v8086 lets protected-mode kernels run real-mode BIOS calls safely by trapping sensitive instructions. Set the VM bit in EFLAGS via a task switch or IRET trick, and the CPU re-enters a sandboxed real-mode environment. For example, a monitor OS can invoke INT 0x10 without dropping global protection.

When to use it

c
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IOPL and I/O permission bitmap control port access
Exceptions: Many privileged ops fault to the monitor for emulation
Simpler alternative in hobby kernels: copy a tiny real-mode stub below 1 MB
Modern firmware favors UEFI native calls instead of v8086

I/O permission bitmap

The TSS can point to an I/O bitmap that traps IN/OUT from v8086 tasks. If the bitmap offset equals the TSS limit, all port I/O traps to the monitor. Many teaching kernels instead copy a 16-bit BIOS thunk below 1 MB and call it with a deliberate real-mode transition because full v8086 emulation is tedious. Document which approach your design uses so reviewers can follow the control flow.

Why for this exercise

You will configure a v8086 task or IRET frame to execute a real-mode BIOS interrupt from protected mode. This exercise requires setting the VM flag and handling traps if the CPU denies privileged instructions.

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

Virtual 8086 mode

When to use it

c
Loading…

IOPL and I/O permission bitmap control port access
Exceptions: Many privileged ops fault to the monitor for emulation
Simpler alternative in hobby kernels: copy a tiny real-mode stub below 1 MB
Modern firmware favors UEFI native calls instead of v8086

I/O permission bitmap

Why for this exercise

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

Topic Overview

Virtual 8086 mode

When to use it

I/O permission bitmap

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source

Run Real Mode Code with Virtual 8086 Mode

Topic Overview

Virtual 8086 mode

When to use it

I/O permission bitmap

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source