The Intel 80286

The 80286 was introduced to the world in 1982, and was an important little CPU. Based on the 8086 design, it added an address unit and separate address and data buses. It was made up of 134 000 transistors, fabricated at 1.5µm, manufactured into a 68-pin package, and clocked in at 5 - 12.5MHz. It could execute most code written for the 8086/8088 and was selected by IBM for their PC/AT desktop computer and, in fact, most personal computers in the 1980s.

The 286 was a 16-bit processor (x86-16), but featured a 24-bit address bus, and could execute instructions at a rate of around 0.21 instructions per clock (IPC) - which was very good at the time - though it could be much faster at code that was optimised for it. The core was made up of four functional units: the bus unit, the address unit, the instruction unit and the execution unit (the ALU).

The performance of the 286 could, in some cases, be more than 100% faster per clock cycle than the 8086 due to its improved design. The separate data and address buses contributed toward this, but of even greater influence was the separate address unit: where the 8086 had to calculate addresses using its general purpose ALU, the 80286 now had a dedicated adder for this purpose. The pipeline of the 286 was buffered, or “loosely coupled” as before, but with larger buffers and a more efficient prefetcher. A number of new instructions were added to the 80186 and 80286 designs, many of which were to support a new feature called protected virtual address mode, or “protected mode” - in essence, this was the introduction of virtual memory support to the x86 family.

There was also an 80287 math coprocessor for the 80286 family, to offload complex and floating point mathematical operations. It was very similar to the 8087, but was able to run at a higher clock rate.

AMD continued to second-source production for the 286 under the name Am286, and went on to produce 16, 20 and 25MHz models.

Codename:       -
Transistors:    134 000
Fabrication:    1.5µm
Die Size:       47 mm2
Clockspeed:     5 - 25MHz
L1 I-Cache:     -
L1 D-Cache:     -
L2 Cache:       -
FPU:            External 80287
Cores/Threads:  1
Decode Rate:    1
Dispatch Rate:  1
Issue Rate:     1
Execution Ports:1
ROB Entries:    -

Estimated IPC:  0.21
Estimated MIPS: 1.28MIPS @ 12MHz

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