Intel 80286: Difference between revisions

| caption = An Intel A80286-8 processor with a gray ceramic heat spreader

The [[Intel]] '''80286'''<ref>{{cite web |title=Microprocessor Hall of Fame |url=http://www.intel.com/museum/online/hist%5Fmicro/hof/ |publisher=Intel |access-date=August 11, 2007 |archive-url=https://web.archive.org/web/20070706032836/http://www.intel.com/museum/online/hist_micro/hof/ |archive-date=July 6, 2007}}</ref> (also marketed as the '''iAPX 286'''<ref name="i286">{{cite book|url=http://bitsavers.org/components/intel/80286/210498-001_iAPX_286_Programmers_Reference_1983.pdf|title=iAPX 286 Programmer's Reference|at=page 1-1|publisher=Intel|year=1983|access-date=August 28, 2017|archive-date=August 28, 2017|archive-url=https://web.archive.org/web/20170828232803/http://www.bitsavers.org/components/intel/80286/210498-001_iAPX_286_Programmers_Reference_1983.pdf|url-status=live}}</ref> and often called '''Intel 286''') is a [[16-bit computing|16-bit]] [[microprocessor]] that was introduced on February 1, 1982. It was the first 8086-based CPU with separate, non-[[Bus (computing)#Bus_multiplexing|multiplexed]] [[address bus|address]] and [[bus (computing)|data buses]] and also the first with [[memory management]] and wide protection abilities. It had a data size of 16 bits, and had an address width of 24 bits, which could address up to 16MB of memory with a suitable operating system such as Windows compared to 1MB for the 8086. The 80286 used approximately 134,000 transistors in its original [[NMOS logic|nMOS]] ([[depletion-load NMOS logic|HMOS]]) incarnation and, just like the contemporary [[Intel 80186|80186]],<ref>A simpler cousin in the 8086-line with integrated peripherals, intended for [[embedded system]]s.</ref> it can correctly execute most software written for the earlier [[Intel 8086]] and [[Intel 8088|8088]] processors.<ref>{{cite web |url=http://www.intel.com/museum/online/hist_micro/hof/index.htm |title=Intel Museum – Microprocessor Hall of Fame |publisher=Intel.com |date=May 14, 2009 |access-date=June 20, 2009|archive-url=https://web.archive.org/web/20090312081932/http://www.intel.com/museum/online/hist_micro/hof/index.htm |archive-date=March 12, 2009}}</ref>

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>

* [[Windows 2.1x#Windows.2F286|Windows/286]]