80286 CLASS CPUS

8/1/98

80286 Class CPUs: The Intel 80286 (1982) was introduced to the PC world in the IBM PC/AT. The 80286 is a 16 bit CPU packaged in 68 pin Pin Grid Array and Leadless Chip Carrier packages. It was originally available in a number of speeds from 6 to 12.5MHz. The PC/AT used an 8MHz CPU clock. 80286 based PCs were also sold using 10, 12 and 16 MHz versions of the CPU, Although the PC/AT represented a fairly major advance in PC architecture, the 80286 CPU was fairly quickly superseded by the 80386 CPU which offered far better software access to large amounts of memory.

The 80286 offered a 16 bit data interface and 24 bit (16MB) address range. Unlike the 8086/8088, the data and address were on separate pins so that it was no longer necessary to wait four clocks of move a single byte of data to or from memory over shared pins. For the most part, a three clock cycle was used to move 16 bit chunks of data. As a result, data transfers to/from the 80286 were roughly 2.5 times as fast as those to/from the 8088. 80286 instructions were streamlined resulting in many fewer clocks being required for some of the slower 8088 instructions. The short, frequently used, instructions could not be speeded up as much, but clock counts were trimmed. Intel claimed that an 80286 was "up to six times faster than an 8088" but that was comparing a 5MHz 8088 to an 8MHz 80286. At comparable clock speeds the performance improvement was more like 3.5 times. The MIPS rating on Intel's current website is 0.9 at 6MHz which works out to about 2.5 times the 8088.

The 80286 instruction set was largely identical with the 8088 some additional registers and instructions were added to support a "Protected" mode that computed 24 bit (16mb) external addresses using 16 bit indices to 8 bit prefixes which are then prefixed to normal 16 bit addresses. The 80286 could also operate in 8086 mode with 1mb of memory addressing. Unfortunately, while there was a provision for going from Real to Protected Mode there was no provision for going back to Real mode other than a time consuming reset of the CPU. Since I/O and MSDOS software required Real Mode, the CPU was never entirely satisfactory.

The Data Sheets in my Intel Microprocessor and Peripheral Handbook don't indicate 80286 power consumption and a web search failed to turn up a value. Given the small size of the package and the fact that it did not require heatsinking, it could not be more than a few watts. The 8087 math coprocessor was replaced with an 80287 with somewhat modified interfaces although the instruction set was largely compatible with the 8087.

Return To Index Copyright 1994-2002 by Donald Kenney.