How to 386 Your AT: Intel Inboard 386/AT

A more ‘recent’ post compared to others here: an article by CPUShack on the Intel Inboard 386/AT, a plug-in board for the PC-XT/AT that ‘upgrades’ the processor from the 8086/80286 to the 80386. Some cards also featured faster RAM and even (gasp) cache memory. :slight_smile:

With the release of the 32-bit Intel 386 processor in 1986, owners of IBM PC/XT and AT type systems (8088 and 80286 systems) were left a bit in the dust. This was a concern (or opportunity) for Intel as well. They designed an upgrade solution at the same time as the 386, to be able to be used in the now obsolete computers. This was the Intel InBoard 386 series of upgrade cards.


Quite a shock to see a fully populated ISA card with an umbilical to the CPU socket - although it surely makes sense I was thinking of some small daughterboard sitting in the CPU socket.

There are a variety of in-CPU-socket upgrades in the land of 8 bit. I may dig out some Acorn/6502 examples… but the one which springs to mind is the beeb816, a joint project by @Revaldinho and myself, a few years back now. The idea was to put a fast 65816 and some fast RAM into the CPU socket of a BBC Micro. And it pretty much worked, at least as a proof of concept.

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I was also pretty surprised to see that! It does indeed make sense given that the system would want to have 32-bit RAM on the CPU, though, particularly since it’s populated with individual RAM chips and not SIPPs or SIMMs. I was also surprised that the attachment for the umbilical cord seems to be a 286 CPU socket; why not use a pin header there? Perhaps the electrical properties of the square socket are better for some (unclear to me) reason?

I also find it interesting that a physical faux 80287 was required on the motherboard; I assume this is because the motherboard (or at least some motherboards) determined the presence of the FPU by some mechanism other than querying the processor bus?

Very interesting, all in all.


I always found it odd that CPU upgrades didn’t really catch on in the x86 world, although I suppose constantly dropping prices left a narrow window where they made sense. I do actually remember seeing an InBoard ‘in the flesh’ in the 90s when I opened up a retired PC, by that point it wasn’t worth anything and it went in the bin.

It was quite interesting that Intel decided to go with such a huge and expensive card rather than creating a specific 386-based upgrade CPU, something like the later Pentium Overdrive.

The 286 memory bus is only 16 bits wide, and would have been limited to the ISA clock speed. Later overdrive CPUs were also limited, but not as badly percentage wise compared to the newer platform.

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