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How do 286/386 accelerators for XT class PCs work?

dhau

Experienced Member
Joined
Feb 13, 2018
Messages
323
Location
Toronto, ON, Canada
I'd like to understand it a bit better from someone how knows. Are those accelerators essentially 286/386 single board computers that use host system only as ISA backplane? Do they disable host motherboard through some signal in 8088 socket? Thanks for your consideration.
 
It's not really an "ISA backplane" so much as hijacking the CPU socket. Notice most of them have a cable that ends in a 40-wire plug that replaces the 8088 CPU.

I suspect most of the logic in the cards is devoted to a few things:
* Buffering and repackaging 16 or 32-bit memory and I/O requests into multiple 8-bit ones.
* Chip select/routing to send supported features to "on card" hardware. Typically this is going to be for cards where they might have some wider, faster memory on the accelerator card.

The resultant machine ends up as a "mutant XT" -- faster CPU capable of 286/386 modes, but coupled to a single interrupt controller, XT-style keyboard controller that doesn't know anything about Gate A20, primitive BIOS with no native high-density disks/hard drive support/CMOS configuration.

I wonder if there were any dedicated accelerator products for more "passive backplane" 8088 systems like the Computerland BC88 or some of the Zenith machines-- those could give you much more of the AT or 386 on the card, although you'd still be hobbled by 8-bit slots only unless you replaced the entire backplane, and at that point, why not just buy a "conventional" 286/386 mainboard?
 
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