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

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

Learn by building

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

Learn by building