First post, by uscleo
Hi Guys! I’ve been reading through the forums and new to tinkering with the Cyrix 5x86. I just wanted to ask a few questions regarding using my Cyrix 5x86 CPU in a very “early” 486 board that does not have clock multiplier jumpers. The computer I’m doing this on is an IBM P75 - a 486 based portable. I’ve modded it so I can swap out the CPU clock crystal and I have had the original 486 DX swapped in with a 486 socket so I can swap out the CPU and change the clock speed (IBM purists, please don’t hate me!).
Right now I have a blue heatsink Cyrix / Gainberry / IBM 5x86c (says IBM26 5x86-3v3100GF under the heatsink) plugged in and running normally with the default 66.6 Mhz crystal . I can boot to DOS and run Windows 95 no problems, very stable. The clock speed however is low (I think CHKCPU mentioned it was measuring 5x.x Mhz or so). Incidentally, does anyone know if the stamp on my CPU means it can take overclocking well? 😀
How do I go about enabling the clock multiplier ? I have read several methods in several threads so I want to make sure I do this right:
1) Download CLKMUL utility and set it through the software (seems like there are limitations on what multipliers I can set through this method)
2) Route the CLKMUL pin from the CPU / socket to a ground pin? (according to this thread - to enable / disable multiplier of 2x or 3x on boot?) https://www.vogons.org/viewtopic.php?f= ... &start=160 post by gerwin
3) Download the utilities mentioned here: https://www.vogons.org/viewtopic.php?f= ... 86#p250032 - do these activate the clock multiplier too, or do I have to use CLKMUL? If they have to be used separately, which utility do I use first? Which utility is better? The thread mentions the IBM utility as the one to use out of the 5 included, but there is no documentation for it in the download.
Does anyone have the typical settings that should be tried first with the IBM utility? Any tips on how to configure it to enable the most stable options?
Does anyone know what clock crystals would work well over 66.6? I tried 80Mhz but that threw memory errors on POST, so I have ordered 70, 72, 75 Mhz, which are in the mail)
This forum has been an amazing resource, thanks in advance for all the help guys!! 😀