80186 introduced some instructions for HLL features. 80286 introduced some instructions for protected mode, and provided some multitasking ability with external MMU which was never intended for PCs but servers/workstations, but the protection mode and addressing line was implemented in a rather different way from the 80386 family. So what has really been inherited from 80286? Shouldn't we regard 80386 as the direct descend of 8086/80186, rather than 80286?

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    Intel defines the line as 8086->286->386 with the 186 as a sidestep as a SoC for embedded application. 'heritage' other than that isn't a factual issue. And musings about some philosophical heritage is simply asking for opinion – Raffzahn Feb 19 at 12:57
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    I think this could be answered as-is, but I’m still not sure what is meant by the question. The 386 removes no features of the 286, and in fact builds upon them quite extensively. The 32-bit protected mode is a direct extension of 16-bit protected mode, selectors, descriptor tables, protection rings and everything. The virtual 8086 mode builds upon 286’s hardware task-switching features. That operating systems preferred to use paging and software task-switching is not a matter of Intel abandoning this design, certainly not as early as with the 386. – user3840170 Feb 19 at 13:16
  • @user3840170 The same way the 486 is an extension of the 386 by incocoperationg the FPU to enable a tighter coupling and resource sharing. – Raffzahn Feb 19 at 13:26
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    I'm inclined to agree that how one regards the inheritance, given the factual composition of each processor, is largely a matter of opinion. Do you have any reasoning for your concern as to the descendancy? – Chenmunka Feb 19 at 14:06
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    Is there a good source for the claim "was never intended for PCs but servers/workstations"? – Tommy Feb 19 at 20:54

You are getting everything wrong.

The 286 has integrated segmentation unit and protected modes to allow multitasking OSes and more memory (up to 16MB). It was actually used for that purpose in early versions of OS/2 and Windows.

The definition of "workstation" is arbitrary. 640kB DOS was becoming cramped, and the 286 allowed to use more memory.

The 386 had two important enhancements : 32bits registers and a pagination unit. The pagination unit allows virtual memory and is more suitable for common UNIX-like OSes which aren't well adapted to x86 segmentation. i386 paged MMU is architected between the segmentation unit addresses and physical memory.

The 386 supports all of 286 protected modes, they can be used in 16bits environments (and many, many 386 ran for years in popular 16bits OSes such as Windows 3.1), and in new 32bits protected modes (such as the WIN32 extension for Windows 3.1).

Since the introduction of AMD 64bits mode, and a bit before, segmentation concepts (with complex stuff like call gates, segment limits...) were a bit obsoleted as pagination is good enough for multitasking OSes, and more compatible with RISC CPUs which have no concept of segments. (rough simplification)

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    It may be worth to mention that the 286 was used for Unixoide systems as early as 1982, half a decade before OS/2 (1987) or even the IBM PC-AT (1982). – Raffzahn Feb 19 at 21:01

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