Intel 80286

Lore & Background

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, with 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 was the 80C286.

On average, the 80286 was said to have a speed of about 0.21 instructions per clock on typical programs, though it could be significantly faster on optimized code and in tight loops. 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 several significant errata that caused problems for programmers and operating-system writers in the earlier B-step and C-step CPUs. Intel second sourced this microprocessor to Fujitsu Limited in about 1985.

The CPU was designed for multi-user systems with multitasking applications, including communications and real-time process control. It consisted of four independent units: the address unit, bus unit, instruction unit, and execution unit, organized into a loosely coupled pipeline. It was produced in a 68-pin package, including PLCC, LCC, and PGA packages. The performance increase of the 80286 over the 8086 could be more than 100% per clock cycle in many programs, partly due to the non-multiplexed address and data buses and the dedicated address calculation unit.

Reader's Guide

The 80286 was the first x86 CPU to support protected virtual-address mode, commonly called protected mode, and the first commercially available microprocessor with on-chip memory management unit capabilities. This allowed IBM compatibles to have advanced multitasking operating systems for the first time and compete in the Unix-dominated server/workstation market. Several additional instructions were introduced for protected mode, and the CPU prevented unauthorized access by forming different segments for data, code, and stack and assigning privilege levels to each segment.

By design, the 286 could not revert from protected mode to real mode without a hardware-initiated reset. In the PC/AT, IBM added external circuitry and specialized code in the ROM BIOS and the 8042 keyboard microcontroller to enable software to cause the reset, though the method imposed a huge performance penalty. In theory, real-mode applications could be directly executed in 16-bit protected mode if certain rules were followed; however, as many DOS programs did not conform to those rules, protected mode was not widely used until the appearance of the 32-bit Intel 80386, which was designed to go back and forth between modes easily and provide an emulation of real mode within protected mode.

The protected mode of the 80286 was not routinely utilized in PC applications until many years after its release, in part because of the high cost of adding extended memory and the need for software to support the large user base of 8086 PCs. For example, in 1986 the only program that made use of it was VDISK, a RAM disk driver included with PC DOS 3.0 and 3.1. The difficulty lay in the incompatibility of older real-mode programs with protected mode, as they could not natively run without significant modification.

Did You Know?

• The 80286 was the first 8086-based CPU with separate, non-multiplexed address and data buses.
• It could address up to 16 MB of memory, compared to 1 MB for the 8086.
• Intel second sourced the 80286 to Fujitsu Limited in about 1985.