Intel 80286

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Intel 80286
Intel 80386
Support status
Unsupported

The

address and data buses and also the first with memory management and wide protection abilities. The 80286 used approximately 134,000 transistors in its original nMOS (HMOS) incarnation and, just like the contemporary 80186,[6] it could correctly execute most software written for the earlier Intel 8086 and 8088 processors.[7]

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.[8]

History and performance

AMD 80286 (16 MHz version)

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, respectively. 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 reportedly measured to have a speed of about 0.21

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.[10]

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.[11]

Intel second sourced this microprocessor to Fujitsu Limited around 1985.[12]

Variants

Model number Frequency Technology Process Package Date of release Price USD[list 1]
80286-10[13] 10 MHz HMOS-III
1.5 μm
 July/August 1985 $155
80286-12[13] 12.5 MHz HMOS-III 1.5 μm July/August 1985 $260
MG80286[14] September/October 1985 $784
80286[15] 68 Pin PGA[list 2] January/February 1986
80286[15] 68 Pin PLCC[list 3] January/February 1986
  1. ^ In quantities of 100.
  2. ^ Sampling Q3 1985
  3. ^ Sampling Q2 1986

Architecture

Simplified 80286 microarchitecture
Intel 80286 die

Intel did not expect personal computers to use the 286.[16] The CPU was designed for

leadless chip carrier) and PGA (pin grid array) packages.[17]

The performance increase of the 80286 over the 8086 (or 8088) could be more than 100% per

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.[18]

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.[19] 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.[18] Additionally, there was a performance penalty involved in accessing extended memory from real mode as noted below.

Features

Siemens 80286 (10 MHz version)
IBM 80286 (8 MHz version)
Intersil 80286 (10 MHz version)

Protected mode

The 286 was the first of the x86 CPU family to support protected virtual-address mode, commonly called "

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,
  • 80287
    ).

By design, the 286 could not revert from protected mode to the basic 8086-compatible real address mode ("

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

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.

OS support

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 to a PC, but also because of 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

real-mode DOS programs with protected mode. They simply could not natively run in this new mode without significant modification. In protected mode, memory management and interrupt handling were done differently than in real mode. In addition, DOS programs typically would directly access data and code segments that did not belong to them, as real mode allowed them to do without restriction; in contrast, the design intent of protected mode was to prevent programs from accessing any segments other than their own unless special access was explicitly allowed. While it was possible to set up a protected-mode environment that allowed all programs access to all segments (by putting all segment descriptors into the GDT and assigning them all the same privilege level), this undermined nearly all of the advantages of protected mode except the extended (24-bit) address space. The choice that OS developers faced was either to start from scratch and create an OS that would not run the vast majority of the old programs, or to come up with a version of DOS that was slow and ugly (i.e., ugly from an internal technical viewpoint) but would still run a majority of the old programs. Protected mode also did not provide a significant enough performance advantage over the 8086-compatible real mode to justify supporting its capabilities; actually, except for task switches when multitasking, it actually yielded only a performance disadvantage, by slowing down many instructions through a litany of added privilege checks. In protected mode, registers were still 16-bit, and the programmer was still forced to use a memory map composed of 64 kB segments, just like in real mode.[20]

In January 1985,

IBM 4690 OS
, but the same limitations affected it.

The problems led to Bill Gates famously referring to the 80286 as a "brain-dead chip",[25][failed verification][when?] 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).

Other operating systems that used the protected mode of the 286 were Microsoft

80386 chip, Intel enhanced the protected mode to address more memory and also added the separate virtual 8086 mode, a mode within protected mode with much better MS-DOS compatibility, in order to satisfy the diverging needs of the market.[29]

Support components

This list of bus interface components that connects to Intel 80286 microprocessor.

See also

  • KR1847VM286 (Russian: КР1847ВМ286).[33]
  • iAPX, for the iAPX name
  • LOADALL – Undocumented 80286/80386 instruction that could be used to gain access to all available memory in real mode.
  • Windows/286

References

  1. ^ "CPU History - The CPU Museum - Life Cycle of the CPU". cpushack.com. Archived from the original on July 20, 2021. Retrieved September 6, 2021.
  2. ^ "1.5 µm lithography process - WikiChip". en.wikichip.org. Archived from the original on September 9, 2018. Retrieved January 21, 2019.
  3. ^ Ormsby, John, "Chip Design: A Race Worth Winning", Intel Corporation, Microcomputer Solutions, July/August 1988, page 18
  4. ^ "Microprocessor Hall of Fame". Intel. Archived from the original on July 6, 2007. Retrieved August 11, 2007.
  5. ^ iAPX 286 Programmer's Reference (PDF). Intel. 1983. page 1-1. Archived (PDF) from the original on August 28, 2017. Retrieved August 28, 2017.
  6. ^ A simpler cousin in the 8086-line with integrated peripherals, intended for embedded systems.
  7. ^ "Intel Museum – Microprocessor Hall of Fame". Intel.com. May 14, 2009. Archived from the original on March 12, 2009. Retrieved June 20, 2009.
  8. ^ Teixeira, Kevin, "What's Next For The 80286?", Intel Corporation, Microcomputer Solutions, November/December 1987, page 16
  9. ^ "Intel Architecure [sic] Programming and Information". Intel80386.com. January 13, 2004. Retrieved April 28, 2009.
  10. ^ "80286 Microprocessor Package, 1982". Content.cdlib.org. Archived from the original on March 6, 2019. Retrieved April 28, 2009.
  11. ^
    ISSN 0199-6649. Archived
    from the original on January 25, 2014. Retrieved December 25, 2021.
  12. ^ Intel Corporation, "NewsBits: Second Source News", Solutions, January/February 1985, Page 1.
  13. ^ a b Intel Corporation, "New Product Focus Components: 80286 Workhorses: Twice As Fast", Solutions, July/August 1985, Page 17.
  14. ^ Intel Corporation, "New Product Focus Components: Highest Ranking 16-bit Microprocessor Meets Military Objectives", Solutions, September/October 1985, page 13.
  15. ^ a b c Ashborn, Jim; "Advanced Packaging: A Little Goes A Long Way", Intel Corporation, Solutions, January/February 1986, Page 2
  16. ^ Gross, Neil; Coy, Peter (March 6, 1995). "The Technology Paradox". Bloomberg. Retrieved March 19, 2020.
  17. ^ "Intel 80286 microprocessor family". CPU-World. Archived from the original on March 31, 2012. Retrieved May 19, 2012.
  18. ^
    ISBN 978-8120339422. Archived
    from the original on February 27, 2017. Retrieved October 11, 2016.
  19. ^ Intel Corporation, "New Product Focus Components: Highest Ranking 16-bit Microprocessor Meets Military Objectives", Solutions, September/October 1985, page 13
  20. ^ Petzold, Charles (1986). "Obstacles to a grown up operating system". PC Magazine. 5 (11): 170–74. Archived from the original on February 27, 2017. Retrieved October 11, 2016.
  21. ^
    ISSN 0199-6649. Archived
    from the original on February 27, 2017. Retrieved October 11, 2016.
  22. Digital Research, Inc. November 1986. Archived
    (PDF) from the original on April 21, 2019. Retrieved August 14, 2018.
  23. Concurrent DOS 286, including STRUCT.H explicitly mentioning LOADALL
    for "8086 emulation".)
  24. ^ Calvo, Melissa; Forbes, Jim (February 10, 1986). "IBM to use a DRI operating system". InfoWorld. Vol. 8, no. 8. p. 12. Archived from the original on April 21, 2019. Retrieved September 6, 2011.
  25. ISBN 0-07-016638-2
    .
  26. ^ "Microsoft XENIX 3.0 Ready for 286" (PDF). Archived from the original (PDF) on January 7, 2014.
  27. ^ "An Introduction to Coherent: General Information FAQ for the Coherent Operating System". Archived from the original on June 4, 2016. Retrieved January 7, 2014.
  28. ^ "MINIX INFORMATION SHEET". Archived from the original on January 7, 2014.
  29. PC Magazine
    . pp. 150–152.
  30. ^ Ormsby, John, Editor, "New Product Focus: Components: Intel's 82X3X Chip-set Handles Logic Functions That Once Required The Services Of Sources Of Chips", Intel Corporation, Microcomputer Solutions, January/February 1988, page 13
  31. ^ Intel Corporation, "New Product Focus Components: The 82258 ADMA Boost iAPX 286 Family Performance", Solutions, November/December 1984, Page 14.
  32. ^ Intel Corporation, "NewsBits: Second Source News", Solutions, January/February 1985, Page 1
  33. ^ "Soviet microprocessors, microcontrollers, FPU chips and their western analogs". CPU-world. Archived from the original on February 9, 2017. Retrieved March 24, 2016.

External links

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