I've been building up a 486 system lately after reclaiming all my collection from a storage unit it's been in for years. I have a M919 486VIP V3.4b mainboard, with a 256kb COAST module (part number ending in 5). Also, an AMD 5x85 133MHz (ADW, 3.45v) chip that I'm targeting (I have several other 486 chips to test with too). When I first jumpered everything for the AMD, I could not get the floppy working correctly. Always recognized, and would write/format without error, but all the writes would end up corrupt. I tried toggling every CMOS setting relating to caching, shadowing, timing, and nothing made a difference. I tried other floppy controllers, drives, and cables, and did the M919 "cut the capacitor" mod, but no change.

I put in a 486DX50, and everything worked correctly. I put the AMD back in with the same jumper settings as the DX50, and ... it all still works correctly. I've jumpered it as an AMD DX4, and running it at 4x60MHz @ 5V (with an active cooler) - the critical thing seems to be the write-back internal cache of the AMD. If I switch the jumpers back to AMD 5x86@133, the writeback turns on (according to CHKCPU) and the floppy goes wonky again.

Does this sound familiar to anyone? I've found a happy place with the board and chip now, but just wanted to see if anyone else had thoughts on this, or if there's any way I can use writeback without borking the floppy access.

P.s. the -5 cache module seems to work just fine at 3.3v.