Intel 80286

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Intel CPU 80286 in PGA housing for 8 MHz.
Production:	1982 to early 1990s
Producers: Intel AMD IBM Harris (Intersil) Siemens AG Fujitsu
Processor clock:	4 MHz to 25 MHz
Manufacturing :	1.5 µm, NMOS
Instruction set :	x86 (16 bit)
Base:	PGA, CLCC and PLCC 68-pin

The Intel 80286 was a microprocessor from Intel that was launched on February 1, 1982 as the successor to the 8086 . The 286 - as the 80286 is often called (pronounced eighty-six) - was initially installed in office microcomputer systems that ran under Unix . In 1984 the company IBM brought out the IBM PC AT based on the 80286 as the successor to the IBM PC XT . The 80286 was distributed almost exclusively through the AT computers and their replicas.

history

The Intel 80286 appeared on the market in 1982, shortly after the appearance of the IBM PC and four years after the Intel 8086 . It represents a complete revision of the 8086. It has about 3 to 4 times the performance of an 8086 with the same clock frequency. The first versions were clocked comparatively slowly with 4 MHz, 6 MHz and 8 MHz, later versions allowed clock frequencies up to 12.5 MHz. AMD and Harris later also produced CPUs with 16 MHz, 20 MHz and 25 MHz. In terms of technology, he is using an NMOS process with a 1.5 µm structure width for the last time . Later processors as well as replicas of the 80286 by Intersil and Fujitsu switched to the more energy-saving CMOS process .

architecture

Intel 80286 : Function Block Diagram

Fundamental differences in architecture to the 8086:

• no more multiplexed address / data bus (8086: data and address bus multiplexed)
• Faster memory accesses: 2 cycles (8086: 4 cycles)
• additional adder for address calculation (8086: normal ALU was used for address calculation, 4 additional pulses for simple address calculations [BP+SI], 8 pulses for double addresses [BP+SI+nn])
• Hardware multiplier, 16-bit multiplication in 21 cycles (8086: 110–120 cycles, Z80: 750 cycles in SW)
• faster shifter (but not yet a barrel shifter), 1 cycle per shift (8086: 4 cycles)
• Additional commands: Shift immediate, MUL immediate, PUSH immediate, PUSHA / POPA, ENTER / LEAVE, INS / OUTS, BOUND
• Protected mode
• 4 more address lines
• Commands for the 80287 no FWAITlonger require synchronization using .

Features

Protected mode

Processor core (die) of an Intel 80286 (8 MHz)

With regard to its instruction set, the 80286 is backwards compatible with the 8086, but it introduced an additional operating mode: the protected mode . The operating mode that is downwardly compatible with the 8086 was given the designation Real Mode . The designation real mode became so common that even today it is often said that the 8086 and its brother 8088 would run in real mode, because in real mode the 80286 was almost completely software compatible with the 8086 - including the limitation to 1  MiB Address space .

Protected mode, on the other hand, allows up to 16 MiB to be addressed via the 24-bit address bus . The restriction to a maximum of 64 KiB segments remained in this operating mode - all address registers were still only 16 bits wide. However, a segment could be assigned to almost any address in the 24-bit address space. This was the only way to open up the entire address space for program code. Due to the memory management option of the protected mode, a virtual memory of almost 1 GiB (16383 segments of max. 64 KiB) is available.

Protected mode supports multitasking , memory protection and other extensions that are not available in real mode. Although protected mode was more difficult to program than real mode, it offered a multitude of advantages and new possibilities. Intel developers believed that old programs that followed certain rules would run unchanged in the new mode. In practice, however, it was found that due to many problems with the PC architecture and the MS-DOS operating system, almost no real-mode programs could run in protected mode. Bill Gates also referred to the 80286 as a brainless chip because the CPU in a Windows environment could not process several MS-DOS programs at the same time. This was only made possible in the successor 80386 , which got its own operating mode, the Virtual 8086 Mode .

The CPU could be switched to Protected Mode relatively easily - the way back was only possible via a reset or by using an undocumented command (LOADALL 0x0F05 - load all processor registers from the memory). There has been much speculation as to why Intel didn't offer an easier way to return from Protected Mode to Real Mode - the most common reasons are:

• Intel considered protected mode to be so superior that no one would want to switch back to real mode.
• A corresponding order was provided, but it did not work properly and was temporarily shut down before the market launch.
• A design flaw in the micro-architecture of the 80286 did not allow a problem-free downshift without profound changes to it.
• The implementation of the command was simply forgotten.

• 

  Intel 80286 for 10 MHz (PLCC housing)

• 

  Siemens SAB 80286 (PLCC housing)

• 

  IBM 80286 for 8 MHz (PGA housing)

Memory extender

In 1987, with Windows 2.1 , Microsoft implemented a so-called memory extender for the first time , which used the protected mode of the 80286 as an aid. With such memory extenders, memory above the 1 MB limit and high memory , the so-called extended memory , can be used from DOS .

In 1988 this access was standardized. The standard was called XMS . The memory extenders were called XMS extenders. The best-known among them was the HIMEM.SYS extender, which was supplied with DOS 5.0 from 1990 onwards .

DOS extenders and Windows

From 1990 onwards, DOS extenders also appeared to improve the use of the protected mode of the 80286 and 80386 processors.

Until Windows 3.0 in real mode of the PC / XT ( 8086 / 88 supports) for Windows 3.1 standard mode (d. E., The 16-bit protected mode) of the AT (80286). Windows 3.11 and Windows 95, however, only ran in extended mode (i.e. 32-bit protected mode), which requires an 80386 processor or higher.

Technical specifications

• Max. Addressable memory: 16  MiB
• Processing width: 16 bits
• Data bus: 16 bit
• Address bus: 24 bit
• L1 cache: does not exist
• L2 cache: nonexistent
• Design: PGA , PLCC or LCC with 68 pins
• Operating voltage ( VCore ): 5 V
• Release DATE: 1982
• Manufacturing technology: 1.5 µm
• Die size : 47 mm² with 134,000 transistors
• Max. Clock frequencies:
  • 04 MHz: only available for a short time, external clock: 8 MHz
  • 06/8/10/12/16 MHz
  • 20 MHz: only Harris / Intersil CS80C286-20 and AMD N80C286-20
  • 25 MHz: only Harris / Intersil CS80C286-25

Peripheral modules

• Intel 82284 : Clock
• Intel 82288 : bus controller
• Intel 82289 : bus arbiter

See also

• Am286 , copy by AMD , produced from 1986.
• U80601 , GDR replica from the Microelectronics Combine Erfurt , produced from 1989.

literature

• C. Vieillefond: Programming the 80286 . SYBEX Verlag, 1987, ISBN 3-88745-668-8 .

Web links

Commons : Intel 80286  - collection of pictures, videos and audio files

• Intel 80286 - Information and photos of the different versions at cpu-collection.de (English)
• On the occasion of 20 years of PC programming: How do I get main memory?

Individual evidence

1. ↑ The history of × 86 CPUs - Intel 286
2. ↑ blogs.msdn.com