Editing X86 memory segmentation (section)

=== 80286 protected mode ===
The [[Intel 80286|80286]]'s [[protected mode]] extends the processor's address space to 2<sup>24</sup> bytes (16 megabytes), but not by adjusting the shift value used to calculate a segment address from the value in a segment register.  Instead, each 16-bit segment register now contains an index into a table of [[segment descriptors]] containing 24-bit base addresses to which offsets are added. To support old software, the processor starts up in "real mode", a mode in which it uses the segmented addressing model of the 8086.  There is a small difference though: the resulting physical address is no longer truncated to 20&nbsp;bits, so [[real mode]] pointers (but not 8086 pointers) can now refer to addresses from 100000<sub>16</sub> through 10FFEF<sub>16</sub>.  This nearly 64-kilobyte region of memory was known as the [[High Memory Area]] (HMA), and later versions of [[DOS]] could use it to increase the available "conventional" memory (i.e. within the first [[Megabyte|MB]]), by moving parts of DOS from conventional memory into the HMA.  With the addition of the HMA, the total address space is approximately 1.06&nbsp;MB.  Though the 80286 does not truncate real-mode addresses to 20&nbsp;bits, a system containing an 80286 can do so with hardware external to the processor, by gating off the 21st address line, the [[A20 line]].  The IBM&nbsp;PC&nbsp;AT provided the hardware to do this (for backward compatibility with software for the original [[IBM&nbsp;PC]] and [[IBM PC/XT|PC/XT]] models), and so all subsequent "[[IBM PC/AT|AT]]-class" PC clones did as well.

286 protected mode was seldom used as it would have excluded the large body of users with 8086/88 machines. Moreover, it still necessitated dividing memory into 64k segments like was done in real mode. This limitation can be worked around on 32-bit CPUs which permit the use of memory pointers greater than 64k in size, however as the Segment Limit field is only 24-bit long, the maximum segment size that can be created is 16MB (although paging can be used to allocate more memory, no individual segment may exceed 16MB). This method was commonly used on Windows 3.x applications to produce a flat memory space, although as the OS itself was still 16-bit, API calls could not be made with 32-bit instructions. Thus, it was still necessary to place all code that performs API calls in 64k segments.

Once 286 protected mode is invoked, it could not normally be exited except by performing a hardware reset. Machines following the rising [[IBM PC/AT]] standard could feign a reset to the CPU via the standardised keyboard controller, but this was significantly sluggish. Windows 3.x worked around both of these problems by intentionally triggering a [[triple fault]] in the interrupt-handling mechanisms of the CPU, which would cause the IBM AT-compatible hardware to reset the CPU, nearly instantly, thus causing it to
drop back into real mode.<ref>{{Cite web|url=http://blogs.msdn.com/b/larryosterman/archive/2005/02/08/369243.aspx|title = DevBlogs}}</ref>