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Intel 80286

Intel 80286

Overview
The Intel 80286 introduced on 1 February 1982, was a 16-bit
16-bit
-16-bit architecture:The HP BPC, introduced in 1975, was the world's first 16-bit microprocessor. Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816. The Intel 8088 was program-compatible with the Intel 8086, and was 16-bit in that its registers were 16...

 x86 microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...

 with 134,000 transistors. Like its contemporary simpler cousin, the 80186, it could correctly execute most software written for the earlier Intel 8086
Intel 8086
The 8086 is a 16-bit microprocessor chip designed by Intel between early 1976 and mid-1978, when it was released. The 8086 gave rise to the x86 architecture of Intel's future processors...

 and 8088
Intel 8088
The Intel 8088 microprocessor was a variant of the Intel 8086 and was introduced on July 1, 1979. It had an 8-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers and the one megabyte address range were unchanged, however...

. 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.The 80286 is the first member of the family of advanced microprocessors with memory management
Memory management
Memory management is the act of managing computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and freeing it for reuse when no longer needed. This is critical to the computer system.Several...

 and wide protection abilities.
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Encyclopedia
The Intel 80286 introduced on 1 February 1982, was a 16-bit
16-bit
-16-bit architecture:The HP BPC, introduced in 1975, was the world's first 16-bit microprocessor. Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816. The Intel 8088 was program-compatible with the Intel 8086, and was 16-bit in that its registers were 16...

 x86 microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...

 with 134,000 transistors. Like its contemporary simpler cousin, the 80186, it could correctly execute most software written for the earlier Intel 8086
Intel 8086
The 8086 is a 16-bit microprocessor chip designed by Intel between early 1976 and mid-1978, when it was released. The 8086 gave rise to the x86 architecture of Intel's future processors...

 and 8088
Intel 8088
The Intel 8088 microprocessor was a variant of the Intel 8086 and was introduced on July 1, 1979. It had an 8-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers and the one megabyte address range were unchanged, however...

. 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.The 80286 is the first member of the family of advanced microprocessors with memory management
Memory management
Memory management is the act of managing computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and freeing it for reuse when no longer needed. This is critical to the computer system.Several...

 and wide protection abilities.

History


After the 6 and 8 MHz initial releases, it was subsequently scaled up to 12.5 MHz. (AMD
Advanced Micro Devices
Advanced Micro Devices, Inc. or AMD is an American multinational semiconductor company based in Sunnyvale, California, that develops computer processors and related technologies for commercial and consumer markets...

 and Harris
Harris Corporation
Harris Corporation is a Florida-based international communications equipment company that produces wireless equipment, electronic systems, and both terrestrial and spaceborne antennas for use in the government, defense, and commercial sectors. It is also the largest private-sector employer in...

 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
Instructions Per Cycle
In computer architecture, instructions per clock is a term used to describe one aspect of a processor's performance: the average number of instructions executed for each clock cycle...

. 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 mode
Addressing mode
Addressing modes are an aspect of the instruction set architecture in most central processing unit designs. The various addressing modes that are defined in a given instruction set architecture define how machine language instructions in that architecture identify the operand of each instruction...

s (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
Arithmetic logic unit
In computing, an arithmetic logic unit is a digital circuit that performs arithmetic and logical operations.The ALU is a fundamental building block of the central processing unit of a computer, and even the simplest microprocessors contain one for purposes such as maintaining timers...

, taking many cycles. Also, complex mathematical operations (such as MUL/DIV) took fewer clock cycles compared to the 8086.

It is a 68-pin microprocessor.It is available in PLCC(Plastic Leaded Chip Carrier), LCC(Lead Less Chip Carrier) and PGA(Pin Grid Array).
Having a 24-bit address bus, the 286 was able to address up to 16 MB
Megabyte
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000...

 of RAM, in contrast to 1 MB that the 8086 could directly access. While DOS
DOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...

 could utilize this additional RAM (extended memory
Extended memory
In DOS memory management, extended memory refers to memory above the first megabyte of address space in an IBM PC or compatible with an 80286 or later processor. The term is mainly used under the DOS and Windows operating systems...

) via BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....

 call (INT 15h, AH=87h), or as RAM disk
RAM disk
A RAM disk or RAM drive is a block of RAM that a computer's software is treating as if the memory were a disk drive...

, or emulation
Emulator
In computing, an emulator is hardware or software or both that duplicates the functions of a first computer system in a different second computer system, so that the behavior of the second system closely resembles the behavior of the first system...

 of expanded memory
Expanded memory
In DOS memory management, expanded memory is a system of bank switching introduced April 24, 1985 that provided additional memory to DOS programs beyond the limit of conventional memory. Expanded memory uses parts of the address space normally dedicated to communication with peripherals for program...

, 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
Computer multitasking
In computing, multitasking is a method where multiple tasks, also known as processes, share common processing resources such as a CPU. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is actively executing instructions for...

 applications, including communications (such as automated PBXs), real-time
Real-time computing
In computer science, real-time computing , or reactive computing, is the study of hardware and software systems that are subject to a "real-time constraint"— e.g. operational deadlines from event to system response. Real-time programs must guarantee response within strict time constraints...

 process control
Process control
Process control is a statistics and engineering discipline that deals with architectures, mechanisms and algorithms for maintaining the output of a specific process within a desired range...

, and multi-user
Multi-user
Multi-user is a term that defines an operating system or application software that allows concurrent access by multiple users of a computer. Time-sharing systems are multi-user systems. Most batch processing systems for mainframe computers may also be considered "multi-user", to avoid leaving the...

 systems.

The later E-stepping level
Stepping level
The term stepping level in the context of CPU architecture or integrated circuitry is a version number.Stepping level refers to the introduction or revision of the lithographic mask or masks within the set of plates that generate the pattern that produces the CPU or integrated circuit...

 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
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units...

. Protected mode enabled up to 16 MB of memory to be addressed by the on-chip linear memory management unit
Memory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...

 (MMU) with 1 GB logical address
Logical address
In computing, a logical address is the address at which an item appears to reside from the perspective of an executing application program....

 space. The MMU also provided protection from (crashed or ill-behaved) applications writing outside their allocated memory zones.The MMU and protection mechanisms are enabled only in case of protected mode keeping them disabled in real mode of operation.The use of additional instructions which are helpful for multitasking operating systems are allowed in protected mode of 80286.
Another important feature of 80286 is Prevention of Unauthorised Access.This is achieved by:
  • Forming different segments for data,code,stack and preventing their overlapping
  • Assigning Privilege levels to each segment.Segment with lower privilege level cannot access the segment with higher privilege level.


The 80286 provided built in memory protection mechanisms which was then almost exclusive to mainframes and minicomputers (CPUs like the NS320xx
NS320xx
The 320xx or NS32000 was a series of microprocessors from National Semiconductor . They were likely the first 32-bit general-purpose microprocessors on the market, but due to a number of factors never managed to become a major player...

 and M68000 needed additional components in order to implement MMU
MMU
MMU may refer to:* Manchester Metropolitan University in England* Manned Maneuvering Unit, a NASA rocket pack that snaps onto the back of a spacesuit* Marymount University, a Catholic university in Arlington, Virginia...

 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 workstation
Workstation
A workstation is a high-end microcomputer designed for technical or scientific applications. Intended primarily to be used by one person at a time, they are commonly connected to a local area network and run multi-user operating systems...

s and servers and even drive the market for other architectures.

In 80286, arithmetic operations can be performed on following different types of numbers:
unsigned packed decimal,unsigned binary, unsigned
unpacked decimal,signed binary , and floating point numbers.

By design, the 286 could not revert from protected mode to the basic 8086-compatible "real mode
Real mode
Real mode, also called real address mode, is an operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 20 bit segmented memory address space and unlimited direct software access to all memory, I/O addresses and peripheral hardware...

" without a hardware-initiated reset. In the PC/AT introduced in 1984, 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.

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
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....

 Intel 80386
Intel 80386
The Intel 80386, also known as the i386, or just 386, was a 32-bit microprocessor introduced by Intel in 1985. The first versions had 275,000 transistors and were used as the central processing unit of many workstations and high-end personal computers of the time...

, which was designed to go back and forth between modes easily. 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 January 1985, Digital Research
Digital Research
Digital Research, Inc. was the company created by Dr. Gary Kildall to market and develop his CP/M operating system and related products. It was the first large software company in the microcomputer world...

 previewed Concurrent DOS 286 in cooperation with Intel. The product would function strictly as an 80286 native mode operating system, allowing users to take full advantage of the protected mode to perform multi-user, multitasking operations while running 8086 emulation. This worked on the B-1 prototype step of the chip, but Digital Research discovered problems with the emulation on the production level C-1 step in May, which would not allow Concurrent DOS 286 to run 8086 software in protected mode. The release of Concurrent DOS 286 was delayed until Intel would develop a new version of the chip. In August, after extensive testing E-1 step samples of the 80286, Digital Research acknowledged that Intel corrected all documented 286 errata, but said there were still undocumented chip performance problems with the prerelease version of Concurrent DOS 286 running on the E-1 step. Intel said, the approach, Digital Research wished to take in emulating 8086 software in protected mode, differed from the original specifications, nevertheless they implemented minor changes in the microcode
Microcode
Microcode is a layer of hardware-level instructions and/or data structures involved in the implementation of higher level machine code instructions in many computers and other processors; it resides in special high-speed memory and translates machine instructions into sequences of detailed...

 that would allow Digital Research to run emulation mode much faster, incorporated into the E-2 step. Named IBM 4680 OS, IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

 originally chosed DR Concurrent DOS 286 as the basis of their IBM 4680 computer for IBM Plant System products and Point-of-Sale terminals in 1986. The same limitations affected Digital Research's FlexOS 286 version 1.0, a derivation of Concurrent DOS 286, developed in 1986, introduced in January 1987, and later adopted by IBM for their IBM 4690 OS
IBM 4690 OS
IBM 4690 Operating System, sometimes shortened to 4690 OS or 4690, is IBM's specially designed Point of Sale operating system. 4690 is widely used by IBM's retail customers to drive retail systems running their own applications as well as IBM's Application Client Server Environment , Supermarket...

.

The problems led to Bill Gates
Bill Gates
William Henry "Bill" Gates III is an American business magnate, investor, philanthropist, and author. Gates is the former CEO and current chairman of Microsoft, the software company he founded with Paul Allen...

 famously referring to the 80286 as a "brain dead chip", since it was clear that the new Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

 environment would not be able to run multiple MS-DOS
MS-DOS
MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...

 applications with the 286. It was arguably responsible for the split between Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 and IBM, since IBM insisted that OS/2
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...

, originally a joint venture between IBM and Microsoft, would run on a 286 (and in text mode).

See also

  • U80601
    U80601
    The U80601 was a 16-bit microprocessor made in 1989-1990 in the former German Democratic Republic of East Germany and thus outside Intel's power to stop it being made or sold...

     – Almost identical copy of the 80286 manufactured 1989/90 in the German Democratic Republic
    German Democratic Republic
    The German Democratic Republic , informally called East Germany by West Germany and other countries, was a socialist state established in 1949 in the Soviet zone of occupied Germany, including East Berlin of the Allied-occupied capital city...

    .
  • LOADALL
    LOADALL
    LOADALL is the common name for two different, undocumented machine instructions of Intel 80286 and Intel 80386 processors, which allow access to areas normally outside of the IA-32 API scope, like descriptor cache registers...

    – Undocumented 80286/80386 instruction that allowed access to all available memory in real mode.

External links