Intel 80286

The Intel 80286 (officially called the iAPX 286, also known as the i286 or 286) is a 16-bit microprocessor of the x86 family, which was launched by Intel on February 1, 1982. It has 134,000 transistors. Like its contemporary cousin, the 80186, it can successfully run most of the software written for the Intel 8086 and Intel 8088. Initial versions of the i286 ran at 7 and 8 MHz, but it eventually reached speeds of up to 25 MHz. It was the microprocessor chosen to equip the IBM Personal Computer/AT, introduced in 1984, which caused it to be the most widely used in AT-compatibles until the early 1990s.

Despite its great popularity, there are few computers left with the i286 running today. The successor to the i286 was the 32-bit Intel 80386.

History

AMD 80286 (version of 16 MHz)

After initial versions at 6 MHz and 8 MHz, Intel released a 12.5 MHz model. AMD and Harris expanded that speed to 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 at 1.5 MIPS, and the 12 MHz at 1. 8 MIPS.

Design

Microarchitecture of 80286

The 80286's performance per clock cycle is more than double that of its predecessors, the Intel 8086 and Intel 8088. In fact, the performance increase per clock cycle may be the largest among different processor generations x86. The calculation of the more complex addressing modes (such as base + index) used fewer clock cycles because it was done by special circuitry in the 286; the 8086 had to perform the effective address computation on the general ALU, which took many cycles. Also, complex math operations (such as MUL/DIV) took fewer cycles than on the 8086.

Having a 24-bit address bus, it is capable of addressing up to 16 MiB of RAM, while the 8086 can only address 1 MiB. Although additional RAM (extended memory) can be used by MS-DOS through a BIOS call INT 15h, AH=87h, or as a RAM disk, or through expanded memory emulation with extended memory previously enabled in software, the cost of memory and the rarity of software that used extended memory and that few i286-based computers had more than 1 MiB of memory. Additionally, there was a performance penalty involved when accessing extended memory from real mode, as noted below.

The i286 was designed to run multi-tasking applications, including communications (such as automated PBXs), real-time process control, and multi-user systems.

The last E-stepping level of the 80286 was a very clean CPU, free of several significant bugs that caused problems for programmers and operating system writers on early B-step and C-step CPUs (common in ATs). and AT clones).

Features

One of the interesting features of this processor is that it was the first x86 processor with protected mode, in which there were four execution rings and memory division using segment tables. 16-bit versions of the OS/2 operating system worked in this mode. In this protected mode all memory was allowed to be used directly, enabling up to 16 MiB of memory to be addressed with the chip's linear memory management unit (MMU) and with 1 GiB of logical address space. The MMU also offered cross-application protection to prevent accidental (or malicious) data writing outside of the allocated memory area. By design, once the processor entered protected mode, it could not return to 8086-compatible real mode without a hard reset. On the IBM PC/AT, IBM added external circuitry as well as specialized code into the ROM BIOS to enable a special series of program instructions to cause a reset, allowing reentry to real mode while keeping memory active. Although this worked correctly, the method imposed a huge performance penalty.

This limitation led to Bill Gates' famous reference 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 on the 286. He was arguably responsible for the split between Microsoft and IBM, since IBM insisted that OS/2, originally a joint venture between IBM and Microsoft, run on a 286 (and in text mode). To be fair, when Intel designed the 286, it wasn't designed to be able to multitask real-mode applications; real mode was intended as a simple way for a bootstrap loader to prepare the system and then switch to protected mode.

In theory, real-mode applications could be run directly in 16-bit protected mode if certain rules were followed; however, as many DOS programs broke these rules, protected mode was not widely used until the 80286's successor, the 32-bit Intel 80386, was designed to easily switch back and forth between modes.