The Intel 80286 (also marketed as the iAPX 286 and often called Intel 286) is a 16-bit microprocessor that was introduced on February 1, 1982. It was the first 8086-based CPU with separate, non-multiplexed address and 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 (HMOS) incarnation and, just like the contemporary 80186, it can correctly execute most software written for the earlier Intel 8086 and 8088 processors.
The 80286 was employed for the IBM PC/AT, introduced in 1984, and then widely used in most PC/AT compatible computers until the early 1990s. In 1987, Intel shipped its five-millionth 80286 microprocessor.
Intel's first 80286 chips were specified for a maximum clockrate of 5, 6 or 8 MHz and later releases for 12.5 MHz. AMD and Harris later produced 16 MHz, 20 MHz and 25 MHz parts. Intel, Intersil and Fujitsu also designed fully static CMOS versions of Intel's original depletion-load nMOS implementation, largely aimed at battery-powered devices. Intel's CMOS version of the 80286 was the 80C286.
On average, the 80286 was said to have a speed of about 0.21 instructions per clock on "typical" programs, although it could be significantly faster on optimized code and in tight loops, as many instructions could execute in 2 clock cycles each. The 6 MHz, 10 MHz, and 12 MHz models were reportedly measured to operate at 0.9 MIPS, 1.5 MIPS, and 2.66 MIPS respectively.
The later E-stepping level of the 80286 was free of the several significant errata that caused problems for programmers and operating-system writers in the earlier B-step and C-step CPUs (common in the AT and AT clones). This E-2 stepping part may have been available in later 1986.
Intel second sourced this microprocessor to Fujitsu Limited in about 1985.
Intel expected the 286 to be used primarily in industrial automation, transaction processing, and telecommunications, instead of in personal computers.
The CPU was designed for multi-user systems with multitasking applications, including communications (such as automated PBXs) and real-time process control. It had 134,000 transistors and consisted of four independent units: the address unit, bus unit, instruction unit, and execution unit, organized into a loosely coupled (buffered) pipeline, just as in the 8086. It was produced in a 68-pin package, including PLCC (plastic leaded chip carrier), LCC (leadless chip carrier) and PGA (pin grid array) packages.
The performance increase of the 80286 over the 8086 (or 8088) could be more than 100% per clock cycle in many programs (i.e., a doubled performance at the same clock speed). This was a large increase, fully comparable to the speed improvements seven years later when the i486 (1989) or the original Pentium (1993) were introduced. This was partly due to the non-multiplexed address and data buses, but mainly to the fact that address calculations (such as base+index) were less expensive. They were performed by a dedicated unit in the 80286, while the older 8086 had to do effective address computation using its general ALU, consuming several extra clock cycles in many cases. Also, the 80286 was more efficient in the prefetch of instructions, buffering, execution of jumps, and in complex microcoded numerical operations such as MUL/DIV than its predecessor.
The 80286 included, in addition to all of the 8086 instructions, all of the new instructions of the 80186: ENTER, LEAVE, BOUND, INS, OUTS, PUSHA, POPA, PUSH immediate, IMUL immediate, and immediate shifts and rotates. The 80286 also added new instructions for protected mode: ARPL, CLTS, LAR, LGDT, LIDT, LLDT, LMSW, LSL, LTR, SGDT, SIDT, SLDT, SMSW, STR, VERR, and VERW. Some of the instructions for protected mode can (or must) be used in real mode to set up and switch to protected mode, and a few (such as SMSW and LMSW) are useful for real mode itself.
The Intel 80286 had a 24-bit address bus and as such had a 16 MB physical address space, compared to the 1 MB address space of prior x86 processors. It was the first x86 processor to support virtual memory supporting up to 1 GB via segmentation. However, memory cost and the initial rarity of software using the memory above 1 MB meant that until late in its production, 80286 computers rarely shipped with more than 1 MB of RAM. Additionally, there was a performance penalty involved in accessing extended memory from real mode as noted below.
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 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:
floating-point numbers (only with an 80287).