Mumbai University > Computer Engineering > Sem 5 > Microprocessor

Marks: 5M

Year: May 2016

1. Virtual 80386 mode (also called V86 mode) is a dynamic mode in the sense that the processor can switch repeatedly and rapidly between V86 mode and protected mode.
2. The CPU enters V86 mode from protected mode to execute an 8086 program, then leaves V86 mode and enters protected mode to continue executing a native 80386 program.
3. An 8086 program runs in this environment as part of a V86 (virtual 8086) task. V86 tasks take advantage of the hardware support of multitasking offered by the protected mode.
4. Not only can there be multiple V86 tasks, each one executing an 8086 program, but V86 tasks can be multi-programmed with other 80386 tasks.
5. The purpose of a V86 task is to form a "virtual machine" with which to execute an 8086 program.
6. 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:
7. 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.
8. 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.
9. Software that helps implement virtual 8086 machines is called a V86 monitor.