Editing Intel 80286 (section)

==Features==
[[File:80286-processor-made-in-austria.jpg|thumb|upright|Siemens 80286 (10&nbsp;MHz version)]]
[[File:KL IBM 80286.jpg|thumb|upright|IBM 80286 (8&nbsp;MHz version)]]
[[File:Intersil MG80C286 10 883 1.jpg|thumb|upright|Intersil 80286 (10&nbsp;MHz version)]]

===Protected mode===
The 286 was the first of the x86 CPU family to support ''protected virtual-address mode'', commonly called "[[protected mode]]". In addition, it was the first commercially available microprocessor with on-chip [[memory management unit]] (MMU) capabilities (systems using the contemporaneous [[Motorola 68010]] and [[NS320xx]] could be equipped with an optional MMU controller). This would allow IBM compatibles to have advanced multitasking OSes for the first time and compete in the [[Unix]]-dominated<ref>{{Cite web |title=DOS Days - IBM OS/2 |url=https://dosdays.co.uk/topics/os2.php |access-date=2025-05-19 |website=dosdays.co.uk}}</ref> [[network server|server]]/[[workstation]] market.

Several additional instructions were introduced in the protected mode of 80286, which are helpful for multitasking operating systems.

Another important feature of 80286 is the prevention of unauthorized access. This is achieved by:
* Forming different segments for data, code, and stack, and preventing their overlapping.
* Assigning privilege levels to each segment. Segments with lower privilege levels cannot access segments with higher privilege levels.

In 80286 (and in its co-processor [[Intel 80287]]), arithmetic operations can be performed on the following different types of numbers:
* unsigned [[packed decimal]],
* unsigned binary,
* unsigned unpacked decimal,
* signed binary,
* [[floating-point number]]s (only with an [[80287]]).

By design, the 286 could not revert from protected mode to the basic 8086-compatible ''real address mode'' ("[[real mode]]") without a hardware-initiated reset. In the PC/AT introduced in 1984, IBM added external circuitry, as well as specialized code in the [[ROM BIOS]] and the [[8042]] keyboard microcontroller to enable software to cause the reset, allowing real-mode reentry while retaining active memory and returning control to the program that initiated the reset. (The BIOS is necessarily involved because it obtains control directly whenever the CPU resets.) Though it worked correctly, the method imposed a huge performance penalty.

In theory, real-mode applications could be [[protected mode#Real mode application compatibility|directly executed in 16-bit protected mode]] if certain rules (newly proposed with the introduction of the 80286) were followed; however, as many DOS programs did not conform to those rules, protected mode was not widely used until the appearance of its successor, the [[32-bit computing|32-bit]] [[Intel 80386]], which was designed to go back and forth between modes easily and to provide an emulation of real mode within protected mode. When Intel designed the 286, it was not designed to be able to multitask real-mode applications; real mode was intended to be a simple way for a bootstrap loader to prepare the system and then switch to protected mode; essentially, in protected mode the 80286 was designed to be a new processor with many similarities to its predecessors, while real mode on the 80286 was offered for smaller-scale systems that could benefit from a more advanced version of the 80186 CPU core, with advantages such as higher clock rates, faster instruction execution (measured in clock cycles), and unmultiplexed buses, but not the 24-bit (16 MB) memory space.

To support protected mode, new instructions have been added: ARPL, VERR, VERW, LAR, LSL, SMSW, SGDT, SIDT, SLDT, STR, LMSW, LGDT, LIDT, LLDT, LTR, CLTS. There are also new exceptions (internal interrupts): invalid opcode, coprocessor not available, [[double fault]], coprocessor segment overrun, stack fault, segment overrun/general protection fault, and others only for protected mode.

===OS support===
The protected mode of the 80286 was not routinely utilized in PC applications until many years after its release, in part because of the high cost of adding extended memory to a PC, but also because of the need for software to support the large user base of 8086 PCs. For example, in 1986 the only program that made use of it was VDISK, a [[RAM disk]] driver included with [[PC DOS]] 3.0 and 3.1. A [[DOS]] could utilize the additional RAM available in protected mode ([[extended memory]]) either via a [[BIOS]] call (INT 15h, AH=87h), as a [[RAM disk]], or as [[emulator|emulation]] of [[expanded memory]].<ref name=Bahadure2010/>

The difficulty lay in the incompatibility of older [[real-mode]] DOS programs with protected mode. They could not natively run in this new mode without significant modification. In protected mode, memory management and interrupt handling were done differently than in real mode. In addition, DOS programs typically would directly access data and code segments that did not belong to them, as real mode allowed them to do without restriction; in contrast, the design intent of protected mode was to prevent programs from accessing any segments other than their own unless special access was explicitly allowed. While it was possible to set up a protected-mode environment that allowed all programs access to all segments (by putting all segment descriptors into the [[Global Descriptor Table]] (GDT) and assigning them all the same privilege level), this undermined nearly all of the advantages of protected mode except the extended (24-bit) address space. The choice that OS developers faced was either to start from scratch and create an OS that would not run the vast majority of the old programs, or to come up with a version of DOS that was slow and ugly (i.e., ugly from an internal technical viewpoint) but would still run a majority of the old programs. Protected mode also did not provide a significant enough performance advantage over the 8086-compatible real mode to justify supporting its capabilities; actually, except for task switches when multitasking, it yielded a performance disadvantage, by slowing down many instructions through a litany of added privilege checks. In protected mode, registers were still 16-bit, and the programmer was still forced to use a memory map composed of 64&nbsp;kB segments, just like in real mode.<ref name=PCMAG1986>{{cite journal |author-last=Petzold |author-first=Charles |author-link=Charles Petzold |title=Obstacles to a grown up operating system |journal=PC Magazine |volume=5 |issue=11 |pages=170–74 |year=1986 |url=https://books.google.com/books?id=pDGnxFyejN4C&pg=PA170 |access-date=October 11, 2016 |archive-date=February 27, 2017 |archive-url=https://web.archive.org/web/20170227081813/https://books.google.com/books?id=pDGnxFyejN4C&pg=PA170 |url-status=live }}</ref>

Intel had not expected the lack of [[virtual machine]] support for 8086 software to be a problem, because it thought that new software using all of the 80286's capabilities would quickly appear. [[Bill Gates]] referred to the 80286 as a "brain-damaged" chip, because it cannot use virtual machines to multitask multiple [[MS-DOS]] applications<ref name="dewarsmosna1990">{{Cite book |last=Dewar |first=Robert B. K. |url=https://archive.org/details/microprocessorsp00robe/page/110/mode/1up?view=theater |title=Microprocessors: A Programmer's View |last2=Smosna |first2=Matthew |publisher=New York: McGraw-Hill |year=1990 |isbn=0-07-016638-2 |page=110 |url-access=registration}}</ref> with an operating system like [[Microsoft Windows]]. It was arguably responsible for the split between [[Microsoft]] and IBM, since IBM insisted that [[OS/2]], originally a joint venture between IBM and Microsoft, would run on a 286 (and in text mode).
{{r|dewarsmosna1990}}

In January 1985, [[Digital Research]] previewed the [[Concurrent DOS 286]] 1.0 operating system developed in cooperation with Intel. The product would function strictly as an 80286 native-mode (i.e. protected-mode) operating system, allowing users to take full advantage of the protected mode to perform multi-user, multitasking operations while running 8086 emulation.<ref name="Infoworld_1985_Super"/><ref name="DRI_1986_FlexOS286"/><ref name="DRI_1986_CDOS68K-2"/> This worked on the B-1 prototype step of the chip, but Digital Research discovered problems with the emulation on the production level C-1 step in May, which would not allow Concurrent DOS 286 to run 8086 software in protected mode. The release of Concurrent DOS 286 was delayed until Intel would develop a new version of the chip.<ref name="Infoworld_1985_Super"/> In August, after extensive testing on E-1 step samples of the 80286, Digital Research acknowledged that Intel corrected all documented 286 errata, but said that there were still undocumented chip performance problems with the prerelease version of Concurrent DOS 286 running on the E-1 step. Intel said that the approach Digital Research wished to take in emulating 8086 software in protected mode differed from the original specifications. Nevertheless, in the E-2 step, they implemented minor changes in the [[microcode]] that would allow Digital Research to run emulation mode much faster.<ref name="Infoworld_1985_80286"/> Named [[IBM 4680 OS]], [[IBM]] originally chose DR Concurrent DOS 286 as the basis of their [[IBM 4680]] computer for IBM Plant System products and [[point-of-sale]] terminals in 1986.<ref>{{cite magazine|url=https://books.google.com/books?id=oi8EAAAAMBAJ&pg=PA12|title=IBM to use a DRI operating system|first1=Melissa |last1=Calvo |first2=Jim |last2=Forbes |magazine=InfoWorld|date=February 10, 1986|access-date=September 6, 2011|page=12|archive-date=April 21, 2019 |volume=8 |issue=8 |archive-url=https://archive.today/20190421185117/https://books.google.de/books?id=oi8EAAAAMBAJ&pg=PA12&redir_esc=y%23v=onepage&q&f=false|url-status=live}}</ref> Digital Research's [[FlexOS 286]] version 1.3, a derivation of Concurrent DOS 286, was developed in 1986, introduced in January 1987, and later adopted by IBM for their [[IBM 4690 OS]], but the same limitations affected it.

Other operating systems that used the protected mode of the 286 were Microsoft [[Xenix]] (around 1984),<ref>{{cite web| url=http://www.tenox.net/docs/microsoft_xenix_30_286_press_release.pdf |title= Microsoft XENIX 3.0 Ready for 286 |archive-url=https://web.archive.org/web/20140107072346/http://www.tenox.net/docs/microsoft_xenix_30_286_press_release.pdf |archive-date=January 7, 2014}}</ref> [[Coherent (operating system)|Coherent]],<ref>{{cite web |url=http://textfiles.com/internet/FAQ/coherent.faq |title=An Introduction to Coherent: General Information FAQ for the Coherent Operating System |access-date=January 7, 2014 |archive-date=June 4, 2016 |archive-url=https://web.archive.org/web/20160604042547/http://textfiles.com/internet/FAQ/coherent.faq |url-status=live }}</ref> and [[Minix]].<ref>{{cite web| url=http://minix.net/minix/minix.html |title=MINIX INFORMATION SHEET | archive-url=https://web.archive.org/web/20140107074722/http://minix.net/minix/minix.html |archive-date=January 7, 2014}}</ref> These were less hindered by the limitations of the 80286 protected mode because they did not aim to run MS-DOS applications or other real-mode programs.

When designing the 80386 Intel engineers were aware of, and agreed with, the 80286's poor reputation.<ref name="inteloh20081202">{{Cite interview |last=Crawford |first=John |interviewer=Jim Jarrett |title=Intel 386 Microprocessor Design and Development Oral History Panel |last2=Hill |first2=Gene |last3=Leukhardt |first3=Jill |last4=Prak |first4=Jan Willem |last5=Slager |first5=Jim |url=https://archive.computerhistory.org/resources/access/text/2015/06/102702019-05-01-acc.pdf |access-date=2025-05-15 |publisher=Computer History Museum |place=Mountain View, California |language=en-US}}</ref> They enhanced the 80386's protected mode to address more memory, and also added the separate [[virtual 8086 mode]], a mode within protected mode with much better MS-DOS compatibility.<ref>{{cite magazine |first=Charles |last=Petzold |author-link=Charles Petzold |title=Intel's 32-bit Wonder: The 80386 Microprocessor |magazine=[[PC Magazine]] |date=November 25, 1986 |pages=150–152}}</ref>