Editing X86 (section)

===Real mode===
{{Main|Real mode}}
{{More citations needed section|date=January 2014}}

Real Address mode,<ref>{{cite book|url=http://bitsavers.org/components/intel/80286/210498-001_iAPX_286_Programmers_Reference_1983.pdf|title=iAPX 286 Programmer's Reference|at=Section 1.2, "Modes of Operation"|publisher=Intel|year=1983|access-date=January 27, 2014|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> commonly called Real mode, is an operating mode of [[8086]] and later x86-compatible [[Central processing unit|CPUs]]. Real mode is characterized by a 20-bit segmented memory address space (meaning that only slightly more than 1 [[Mebibyte|MiB]] of memory can be addressed{{Efn|Because a segmented address is the sum of a 16-bit segment multiplied by 16 and a 16-bit offset, the maximum address is 1,114,095 (10FFEF&nbsp;hex), for an addressability of 1,114,096&nbsp;bytes&nbsp;{{=}} 1&nbsp;MB&nbsp;+ 65,520&nbsp;bytes. Before the 80286, x86 CPUs had only 20 physical address lines (address bit signals), so the 21st bit of the address, bit 20, was dropped and addresses past 1&nbsp;MB were mirrors of the low end of the address space (starting from address zero). Since the 80286, all x86 CPUs have at least 24 physical address lines, and bit 20 of the computed address is brought out onto the address bus in real mode, allowing the CPU to address the full 1,114,096 bytes reachable with an x86 segmented address. On the popular IBM PC platform, switchable hardware to disable the 21st address bit was added to machines with an 80286 or later so that all programs designed for 8088/8086-based models could run, while newer software could take advantage of the "high" memory in real mode and the full 16&nbsp;MB or larger address space in protected mode—see A20 gate.}}), direct software access to peripheral hardware, and no concept of [[memory protection]] or [[computer multitasking|multitasking]] at the hardware level. All x86 CPUs in the [[Intel 80286|80286]] series and later start up in real mode at power-on; [[Intel 80186|80186]] CPUs and earlier had only one operational mode, which is equivalent to real mode in later chips. (On the IBM PC platform, direct software access to the IBM [[BIOS]] routines is available only in real mode, since BIOS is written for real mode. However, this is not a property of the x86 CPU but of the IBM BIOS design.)

In order to use more than 64&nbsp;KB of memory, the segment registers must be used. This created great complications for compiler implementors who introduced odd pointer modes such as "near", "far" and "huge" to leverage the implicit nature of segmented architecture to different degrees, with some pointers containing 16-bit offsets within implied segments and other pointers containing segment addresses and offsets within segments. It is technically possible to use up to 256&nbsp;KB of memory for code and data, with up to 64&nbsp;KB for code, by setting all four segment registers once and then only using 16-bit offsets (optionally with default-segment override prefixes) to address memory, but this puts substantial restrictions on the way data can be addressed and memory operands can be combined, and it violates the architectural intent of the Intel designers, which is for separate data items (e.g. arrays, structures, code units) to be contained in separate segments and addressed by their own segment addresses, in new programs that are not ported from earlier 8-bit processors with 16-bit address spaces.