The Intel 80286 (also called iAPX 286), introduced on February 1, 1982, was a 16-bit x86 microprocessor with 134,000 transistors. Like its contemporary simpler cousin, the 80186, it could correctly execute most software written for the earlier Intel 8086 and Intel 8088. 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.
After the 6 and 8 MHz initial 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. 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.
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 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 DOS could utilize this additional RAM (extended memory) via BIOS call (INT 15h, AH=87h), or as RAM disk, or emulation of 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. Additionally, there was a performance penalty involved in accessing extended memory from real mode, as noted below.
The 286 was designed to run multitasking applications, including communications (such as automated PBXs), real-time process control, and multi-user systems.
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