Editing Intel 80286 (section)

===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.