- Intel 80286
Infobox Computer Hardware Cpu
name = 286
caption = An Intel 80286 Microprocessor
manuf1 = Intel
manuf2 = AMD
manuf3 = Harris Corporation
manuf4 = Siemens AG
produced-start = 1982
produced-end = early 1990s
slowest = 6 | slow-unit = MHz
fastest = 25 | fast-unit = MHz
size-from = 1.5
pack1 = PLCC 68-pin
Intel286 [cite web |title=Microprocessor Hall of Fame |url=http://www.intel.com/museum/online/hist%5Fmicro/hof/ |publisher=Intel |accessdate=2007-08-11] , introduced on February 1, 1982, (originally named 80286, and also called iAPX 286 in the programmer's manual) was an x86 16-bit microprocessorwith 134,000 transistors.
It was widely used in
IBM PC compatible computers during the mid 1980sto early 1990s.
After the 6 and 8
MHzinitial 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. [ [http://www.intel80386.com Intel Architecure Programming and Information ] ] 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. [ [http://content.cdlib.org/ark:/13030/kt7h4nc9c2/?layout=metadata&brand=calisphere 80286 Microprocessor Package, 1982 ] ]
The 80286's performance was more than twice that of its predecessors (the
Intel 8086and Intel 8088) per clock cycle. 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
DOScould utilize this additional RAM ( extended memory) via BIOScall (INT 15h, AH=87h), or as RAM disk, or emulationof 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. As well, there was a performance penalty involved in accessing extended memory from real mode, as noted below.
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).
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 addressspace. The MMU also provided some degree of prevention from (crashed or ill-behaved) applications writing outside their allocated memory zones. However, the 286 could not revert to the basic 8086-compatible " real mode" without resetting the processor, which imposed a performance penalty (though some very clever programmers did figure out a way to re-enter real mode via a series of software instructions which would execute the reset while retaining active memory and control). The Intel 8042 keyboard controller at IBM PC/AThad a function to initiate a "soft boot" which resets a host CPU only.
This limitation led to
Bill Gatesfamously referring to the 80286 as a 'brain dead chip' [Microprocessors: A Programmer's View, Robert B. K. Dewar and Matthew Smosna, New York: McGraw-Hill, 1990, ISBN:0-07-016638-2] , since it was clear that the new Microsoft Windowsenvironment would not be able to run multiple MS-DOSapplications with the 286. It was arguably responsible for the split between Microsoftand 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. See Protected Mode for more info.
The 80286 provided the first glimpse into the world of the protection mechanisms then exclusive to the world of mainframes and minicomputers which would pave the way for the x86 and the IBM PC architecture to extend from the personal computer all the way to high-end servers, drive the market for other architectures all the way down to only the highest-end servers and mainframes, and blur the differences between microcomputers and mainframes, a fact which presumably gave the
IBM PC/ATits name.
LOADALL- famous undocumented 80286/80386 instruction that allowed to access all available memory in real mode
Notes and references
* [http://www.ragestorm.net/downloads/286intel.txt Intel 80286 Programmer's Reference Manual 1987(txt)]
* [http://tuxmobil.org/286_mobile.html Linux on 286 laptops and notebooks]
* [http://www.cpu-collection.de/?tn=1&l0=cl&l1=80286 Intel 80286 images and descriptions at cpu-collection.de]
* [http://www.cpu-info.com/index2.php?mainid=286 CPU-INFO: 80286, in-depth processor history]
* [http://www.cpu-info.com/index2.php?mainid=Collection&ID=80286&type=cpu Overview of all 286 compatible chips]
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