INTEL 286

The introduction of the Intel 286 processor originally featured in the IBM AT class computers should have solved the small vs. large memory model problem and let multi-tasking multi-user systems develop. The i286 featured 2 modes of operation. The classic mode (8086 compatible) became known as the 'real mode' and expanded instruction set and segmentless operation mode became known as the "protected mode." In the ideal world this processor should have been good for multi-tasking in that one would have hoped that a protected mode operating system could have been written which was logically similar to the MP/M II model for the 8080. The idea would be to periodically save the real mode processor state, use some external logic to page in some different memory, restore a different processor state, and proceed with a different task. The addition of the external logic would have allowed the I286 to behave as multiple 8086 processors on a time slicing basis.

Unfortunately, there was a limitation in the design of the i286 that made this virtualization impossible. Implicit in the design that I have described was a need to be able to freely switch from the real mode to the protected mode and back again, as you would be using a protected mode process to bank switch virtual 8086 processes in and out of a 'running position' and then jumping to the 'real mode' to run them. The fate of that bright idea was sealed when the i286 instruction set as finally released contained no command for switching from the protected mode to the real mode short of a processor reset. Observers of the time noted that MicroSoft which had only single tasking software and Intel were working pretty closely together at the time, and it was Digital Research who was ready to storm the market with a multi-tasking Dos but was unable to do so because of an 'I gotcha' in the i286 instruction set. Add some paranoia to the undisputed facts and it is pretty easy to see how some pretty nasty stories about who was doing what to whom circulated.

Loadall

At issue here was the absence of a proposed LOADALL instruction which would have allowed return to the real mode. Intel engineers left this instruction out on purpose, believing that once one had gone to the protected mode, it was irresponsible to return to the real mode, and that doing so would only lead to inherently unstable applications. The extent to which this decision was driven by marketing decisions as contrasted to engineering decisions will never be fullly known. The truth probably includes some of each, but although the ultimate impact of the decision was to sabotage Digital Research, to the extent marketing was involved in the decision, it may well have been driven by the urge of Intel to promote their own operating system which was a protected mode OS.

As a result, the i286 came and died without ever having an OS for it. Microsoft and IBM had OS/2 which was going to be ready any day now, but it simply never appeared, and most i286 computers were born and died with DOS on them. The only legacy of the I286 class computer that lives on today is the ISA bus, and it is fast on the way out. The original 286 brought as the AT class computer. This included a 1.2M 5.25" floppy drive (up from 360k on the 8 bit PC), the extended 16 bit ISA bus (up from an 8 bit). The bus at that time ran at CPU speed so the bus speed got up to 6 and 8 mhrz.