Editing Protected mode (section)

=== The 286 ===
{{details|Intel 80286}}

The initial protected mode, released with the 286, was not widely used;<ref name="Memory usage"/> for example, it was used by [[Coherent (operating system)|Coherent]] (from 1982),<ref>{{cite web |url=http://textfiles.com/internet/FAQ/coherent.faq |title=General Information FAQ for the Coherent Operating System |date=January 23, 1993}}</ref> Microsoft [[Xenix]] (around 1984)<ref>{{cite press release |url=http://www.tenox.net/docs/xenix/microsoft_xenix_30_286_press_release.pdf |title=Microsoft XENIX 286 Press Release |publisher=Microsoft |access-date=2015-08-17 |archive-date=2014-10-21 |archive-url=https://web.archive.org/web/20141021111609/http://www.tenox.net/docs/xenix/microsoft_xenix_30_286_press_release.pdf |url-status=dead }}</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> Several shortcomings such as the inability to make BIOS and DOS calls due to inability to switch back to real mode without resetting the processor prevented widespread usage.<ref name="286 failings">{{cite book | last = Mueller | first = Scott | title = Upgrading and Repairing PCs, 17th Edition | date = March 24, 2006 | url = https://archive.org/details/upgradingrepairi0000muel_17thedition | type = Book | access-date = 2017-07-11 | edition = 17 | publisher = Que | isbn = 0-7897-3404-4 | chapter = P2 (286) Second-Generation Processors | chapter-url = http://www.informit.com/articles/article.aspx?p=481859&seqNum=13 | url-access = registration }}</ref> Acceptance was additionally hampered by the fact that the 286 allowed memory access in 64&nbsp;[[kilobyte]] segments, addressed by its four segment registers, meaning that only {{nobr|4 × 64 KB}}, equivalent to 256&nbsp;KB, could be accessed at a time.<ref name="Memory usage"/> Because changing a segment register in protected mode caused a 6-byte segment descriptor to be loaded into the CPU from memory, the segment register load instruction took many tens of processor cycles, making it much slower than on the 8086 and 8088; therefore, the strategy of computing segment addresses on-the-fly in order to access data structures larger than 128&nbsp;[[kilobyte]]s (the combined size of the two data segments) became impractical, even for those few programmers who had mastered it on the 8086 and 8088.

The 286 maintained backward compatibility with the 8086 and 8088 by initially entering [[real mode]] on power up.<ref name="Real mode on powered on" /> Real mode functioned virtually identically to the 8086 and 8088, allowing the vast majority of existing [[software]] for those processors to run unmodified on the newer 286. Real mode also served as a more basic mode to set up and [[Bootstrapping#Software loading and execution|bootstrap]] into protected mode. To access the extended functionality of the 286, the operating system would set up some tables in memory that controlled memory access in protected mode, set the addresses of those tables into some special registers of the processor, and then set the processor into protected mode. This enabled 24-bit addressing, which allowed the processor to access 2<sup>24</sup>&nbsp;bytes of memory, equivalent to 16&nbsp;[[megabyte]]s.<ref name="Address bus"/>