Editing Protected mode (section)

=== Virtual 8086 mode ===
{{main|Virtual 8086 mode}}

With the release of the 386, protected mode offers what the Intel manuals call ''virtual 8086 mode''. Virtual 8086 mode is designed to allow code previously written for the 8086 to run unmodified and concurrently with other tasks, without compromising security or system stability.<ref name="V8086 mode">{{ cite book | title = Intel 64 and IA-32 Architectures Software Developer's Manual Combined Volumes 3A, 3B, 3C, and 3D: System Programming Guide | publisher = [[Intel]] | at = Section 20.2 Virtual 8086 Mode |date=May 2019 | url = https://software.intel.com/en-us/download/intel-64-and-ia-32-architectures-sdm-combined-volumes-3a-3b-3c-and-3d-system-programming-guide }}</ref>

Virtual 8086 mode, however, is not completely backward compatible with all programs. Programs that require segment manipulation, privileged instructions, direct hardware access, or use [[self-modifying code]] will generate an [[exception handling|exception]] that must be served by the operating system.<ref name="V8086 limitations">{{ cite book | title = Intel 64 and IA-32 Architectures Software Developer's Manual Combined Volumes 3A, 3B, 3C, and 3D: System Programming Guide | publisher = [[Intel]] | at = Section 20.2.7 Sensitive Instructions |date=May 2019| url = https://software.intel.com/en-us/download/intel-64-and-ia-32-architectures-sdm-combined-volumes-3a-3b-3c-and-3d-system-programming-guide }}</ref> In addition, applications running in virtual 8086 mode generate a [[trap (computing)|trap]] with the use of instructions that involve [[input/output]] (I/O), which can negatively impact performance.<ref name="V86 performance">{{ cite web | url = http://osdev.berlios.de/v86.html | title = Virtual 8086 Mode | access-date = 2007-07-25 | last = Robinson | first = Tim | date = August 26, 2002 | format = Guide | publisher = berliOS | quote = A downside to using V86 mode is speed: every IOPL-sensitive instruction will cause the CPU to trap to kernel mode, as will I/O to ports which are masked out in the TSS. | url-status = dead | archive-url = https://web.archive.org/web/20021003235610/http://osdev.berlios.de/v86.html | archive-date = October 3, 2002 }}</ref>

Due to these limitations, some programs originally designed to run on the 8086 cannot be run in virtual 8086 mode. As a result, system software is forced to either compromise system security or backward compatibility when dealing with [[legacy system|legacy software]]. An example of such a compromise can be seen with the release of [[Windows NT]], which dropped backward compatibility for "ill-behaved" DOS applications.<ref name="DOS and NT">{{cite book | last = Dabak | first = Prasad |author2=Millind Borate  | title = Undocumented Windows NT | type = Book |date=October 1999 | publisher = Hungry Minds | isbn = 0-7645-4569-8}}</ref>