| Caption | An 8MHz Intel 80286 Microprocessor |
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| Manuf1 | Intel, IBM, AMD, Harris (Intersil), Siemens AG, Fujitsu |
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| Produced-start | 1982 |
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| Produced-end | early 1990s |
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| Slowest | 6 | slow-unit = MHz (4 MHz for a short time) |
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| Fastest | 25 | fast-unit = MHz |
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| Size-from | 1.5µm |
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| Arch | x86-16 (with MMU) |
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| Pack1 | PGA, CLCC and PLCC 68-pin |
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The Intel 80286 (also called iAPX 286), introduced on February 1, 1982, was a 16-bit x86 microprocessor with 134,000 transistors. Like its contemporary simpler cousin, the 80186, it could correctly execute most software written for the earlier Intel 8086 and Intel 8088. It was employed for the IBM PC/AT, introduced in 1984, and then widely used in most PC/AT compatible computers until the early 1990s.
History
After the 6 and 8 MHz initial releases, it was subsequently scaled up to 12.5 MHz. (AMD and Harris later pushed the architecture to speeds as high as 20 MHz and 25 MHz, respectively.) On average, the 80286 had a speed of about 0.21 instructions per clock. The 6 MHz model operated at 0.9 MIPS, the 10 MHz model at 1.5 MIPS, and the 12 MHz model at 1.8 MIPS.
Design
The 80286's performance per
clock cycle was more than twice that of its predecessors, the Intel 8086 and Intel 8088. In fact, the performance increase per clock cycle of the 80286 over its immediate predecessor may be the largest among the generations of x86 processors. Calculation of the more complex
addressing modes (such as
base+index) had less clock penalty because it was performed by a special circuit in the 286; the 8086, its predecessor, had to perform effective address calculation in the general
ALU, taking many cycles. Also, complex mathematical operations (such as MUL/DIV) took fewer clock cycles compared to the 8086.
Having a 24-bit address bus, the 286 was able to address up to 16 MB of RAM, in contrast to 1 MB that the 8086 could directly access. While DOS could utilize this additional RAM (extended memory) via BIOS call (INT 15h, AH=87h), or as RAM disk, or emulation of expanded memory, cost and initial rarity of software utilizing extended memory meant that 286 computers were rarely equipped with more than a megabyte of RAM. Additionally, there was a performance penalty involved in accessing extended memory from real mode, as noted below.
The 286 was designed to run multitasking applications, including communications (such as automated PBXs), real-time process control, and multi-user systems.
The later E-stepping level of the 80286 was a very clean CPU, 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).
Features
An interesting feature of this processor is that it was the first x86 processor with
protected mode. Protected mode enabled up to 16 MB of memory to be addressed by the on-chip linear
memory management unit (MMU) with 1 GB
logical address space. The MMU also provided protection from (crashed or ill-behaved) applications writing outside their allocated memory zones. By design, the 286 could not revert from protected mode to the basic 8086-compatible "
real mode" without a hardware-initiated reset. In the PC-AT, IBM added external circuitry as well as specialized code in the
ROM BIOS to enable special series of program instructions to cause the reset, allowing real-mode reentry (while retaining active memory and control). Though it worked correctly, the method imposed a huge performance penalty.
This limitation led to Bill Gates famously referring to the 80286 as a "brain dead chip", since it was clear that the new Microsoft Windows environment would not be able to run multiple MS-DOS applications with the 286. It was arguably responsible for the split between Microsoft and IBM, since IBM insisted that OS/2, originally a joint venture between IBM and Microsoft, would run on a 286 (and in text mode). To be fair, 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.
In theory, real mode applications could be directly executed in 16-bit protected mode if certain rules were followed; however, as many DOS programs broke those rules, protected mode was not widely used until the appearance of its successor, the 32-bit Intel 80386, which was designed to go back and forth between modes easily.
The 80286 provided built in memory protection mechanisms which was then almost exclusive to mainframes and minicomputers (CPUs like the NS320xx and M68000 needed additional components in order to implement MMU functions) and the large performance enhancements represented by the 80286 and many of its successors would pave the way for the x86 and the IBM PC architecture to extend from low performance personal computers all the way to high-end workstations and servers and even drive the market for other architectures.
See also
U80601 - Almost identical copy of the 80286 manufactured 1989/90 in the German Democratic Republic.
LOADALL - Undocumented 80286/80386 instruction that allowed access to all available memory in real mode.
References
External links
Intel Datasheets
Intel 80286 Programmer's Reference Manual 1987(txt). Hint: use e.g.
Hebrew (IBM-862) encoding.
Linux on 286 laptops and notebooks
Intel 80286 images and descriptions at cpu-collection.de
CPU-INFO: 80286, in-depth processor history
Overview of all 286 compatible chips
Category:1982 introductions
80286
