Editing Protected mode (section)

=== Real mode application compatibility ===

According to the ''Intel 80286 Programmer's Reference Manual'',<ref name="286 compatibility">{{ cite book | title = 80286 and 80287 Programmer's Reference Manual | url = http://bitsavers.org/components/intel/80286/210498-005_80286_and_80287_Programmers_Reference_Manual_1987.pdf | date = 1987 | publisher = Intel | location = Santa Clara, CA | at = Section 1.2 Modes of Operation }}</ref>

{{ quote
| the 80286 remains upwardly compatible with most 8086 and 80186 application programs. Most 8086 application programs can be re-compiled or re-assembled and executed on the 80286 in Protected Mode.
}}

For the most part, the binary compatibility with real-mode code, the ability to access up to 16 MB of physical memory, and 1 GB of [[virtual memory]], were the most apparent changes to application programmers.<ref name="286 programmer ref">{{ cite book | title = 80286 and 80287 Programmer's Reference Manual | url = http://bitsavers.org/components/intel/80286/210498-005_80286_and_80287_Programmers_Reference_Manual_1987.pdf | date = 1987 | publisher = Intel | location = Santa Clara, California | at = Section 1.3.1 Memory Management }}</ref> This was not without its limitations. If an application utilized or relied on any of the techniques below, it would not run:<ref name="Compatibility limitations">{{ cite book | title = 80286 and 80287 Programmer's Reference Manual | url = http://bitsavers.org/components/intel/80286/210498-005_80286_and_80287_Programmers_Reference_Manual_1987.pdf | date = 1987 | publisher = Intel | location = Santa Clara, California | at = Appendix C 8086/8088 Compatibility Considerations }}</ref>

* Segment arithmetic
* Privileged instructions
* Direct hardware access
* [[Self-modifying code|Writing to a code segment]]
* Executing data
* Overlapping segments
* Use of BIOS functions, due to the BIOS interrupts being reserved by Intel<ref name="BIOS only available through workarounds">{{ cite web | url = http://www.freepatentsonline.com/6105101.html | title = Memory access control method and system for realizing the same | work = US Patent 5483646 | access-date = 2007-07-25 | date = May 6, 1998 | format = Patent | quote = This has been impossible to-date and has forced BIOS development teams to add support into the BIOS for 32 bit function calls from 32 bit applications. }}</ref>

In reality, almost all [[DOS]] application programs violated these rules.<ref name="Incompatibilities">{{ 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 = ...&nbsp;secondly, protected mode was also incompatible with the vast amount of real-mode code around at the time. | 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, [[virtual 8086 mode]] was introduced with the 386. Despite such potential setbacks, [[Windows 3.0]] and its successors can take advantage of the binary compatibility with real mode to run many Windows 2.x ([[Windows 2.0]] and [[Windows 2.1x]]) applications in protected mode, which ran in real mode in Windows 2.x.<ref name="Windows protected mode usage">{{ 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 | url-status = dead | archive-url = https://web.archive.org/web/20021003235610/http://osdev.berlios.de/v86.html | archive-date = October 3, 2002 }}</ref>