Chapter 15 Virtual 8086 Mode

The purpose of a V86 task is to form a "virtual machine" with which to execute an 8086 program. A complete virtual machine consists not only of 80386 hardware but also of systems software. Thus, the emulation of an 8086 is the result of cooperation between hardware and software:

• The hardware provides a virtual set of registers (via the TSS), a virtual memory space (the first megabyte of the linear address space of the task), and directly executes all instructions that deal with these registers and with this address space.
• The software controls the external interfaces of the virtual machine (I/O, interrupts, and exceptions) in a manner consistent with the larger environment in which it executes. In the case of I/O, software can choose either to emulate I/O instructions or to let the hardware execute them directly without software intervention.

• 15.1 Executing 8086 Code
• 15.1.1 Registers and Instructions
• 15.1.2 Linear Address Formation
• 15.2 Structure of a V86 Task
• 15.2.1 Using Paging for V86 Tasks
• 15.2.2 Protection within a V86 Task
• 15.3 Entering and Leaving V86 Mode
• 15.3.1 Transitions Through Task Switches
• 15.3.2 Transitions Through Trap Gates and Interrupt Gates
• 15.4 Additional Sensitive Instructions
• 15.4.1 Emulating 8086 Operating System Calls
• 15.4.2 Virtualizing the Interrupt-Enable Flag
• 15.5 Virtual I/O
• 15.5.1 I/O-Mapped I/O
• 15.5.2 Memory-Mapped I/O
• 15.5.3 Special I/O Buffers
• 15.6 Differences From 8086
• 15.7 Differences From 80286 Real-Address Mode

• Figure 15-1. V86 Mode Address Formation
• Figure 15-2. Entering and Leaving the 8086 Program