Reply 40 of 42, by DistWave

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I found this thread today. It seems that I got the same kingston sx/now upgrade to make similar experiments. I replaced the 386sx with a TI486SXLC40 (5v part, it can run in a 40 MHz or 20 MHz system ,with 2x internal clock multiplier)


This cpu runs really hot a lot more than the original 386sx... I'd put a heatsink. It has the metal pad below, but it is not making contact with the PCB.
The main problem is that when I enable the internal 8kb cache I get an "A20 hardware error" and the system hangs (tested in a generic 286 12 MHz PC motherboard with SUNTAC chipset). The "external" 16 kb cache present in the upgrade can be enabled without problems.

Reply 41 of 42, by Paralel

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It's because it doesn't have support for Cyrix/TI's implementation of A20M#, Address Bit 20 Mask. It's not part of the typical pin out of a 386SX, as such, the pin on the SXLC-40 that would assert A20M# is likely not connected (N/C). If you have a scope, you can easily check it, but even a simple multimeter should work, since it has an internal pull-up resistor.

A20M# is asserted when internal processor cache is accessed. Without it, you would get an A20 error, so you can't use internal cache.

You may be able to connect the A20M# pin using a patch wire, but, that goes beyond my abilities.

Reply 42 of 42, by Deunan

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Connecting A20M is the best solution but might not be possible if that function is internal to one of the 286 mobo ASICs. But there is a software workround, the control registers on Cyrix/Ti have a bit to disable caching of the first 64k of every 1MB segment - that should fix this issue but obviously at a performance cost.