So I was testing out a cyrix CPU I got a while ago. I tested it once before, and did some clock per clock comparrisons with other CPUs - but now I wanted to use it in a build.

The motherboard of choice is a Lucky Star 5V-1A (VIA VP3 based) - CR2032, PS/2 header, SD-RAM slots, 4.5x multiplier, USB, working turbo function from keyboard key combination in dos (it seems to disable internal and external cache), all the good stuff. It even has an undocumented jumper setting for 7.33MHz FSB that lets you run a garden variety pentium @ 11MHz (7.81 points in speedsys / 10.2 fps in 3dbench witch all cache enabled, and a fun 1.06 points in speedsys with cache disabled - XT speeds). Great little board. It was a toss-up between this board and a Pine Tech PT-730A (socket 5, 486 style cache, can do FSB 8, 20 and 40MHz for slowdowns). I chose the 5V because of the CR2032 battery and PS/2 port.

The issue I'm having with it is: with the Cyrix 686 CPU, even at 133MHz (it runs at 3x66 = 200MHz normally) I get artifacts and graphical corruption in some dos games, regardless of what video card I use. Issues occur in Quake, where there's some vertical lines and stuttering, Duke 3D in vesa modes, as well as Red Alert for DOS, witch displays artifacts under the mouse cursor witch leave trails. After quitting the game, characters are messed up in dos and norton commander.

The machine works fine in windows - GLQuake works great, Red Alert 95 works great and so on (with the 686 chip) even at 233MHz. The thing will even post at 266 (4x66), but I haven't tested it extensively. If I switch the 686 for a pentium or AMD K6/K6-2, the corruption is gone and all dos games run normally.

Anyone experience this before?

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Here are the undocumented jumper settings, in case someone has this board and wants to mess around with it:

JCK1 JCK2 JCK3
1-2 1-2 2-3 = 7.3MHz !!!
2-3 2-3 2-3 = 50MHz
2-3 2-3 1-2 = 55MHz
1-2 2-3 2-3 = 60MHz
2-3 1-2 2-3 = 66MHz
1-2 2-3 1-2 = 75MHz
2-3 1-2 1-1 = 83mhz
1-2 1-2 1-2 = 100MHz ??? won't post

JBFO JBF1 JBF2
closed open closed = 4X
closed closed closed = 4.5X

Also, I can post some speedsys screenshots with the board running a non-mmx pentium 166 at 10.76MHz, with and without cache, if anyone is interested.