mpe wrote on 2020-06-26, 12:04:
For the 233MMX you just need a higher multipler.
If your board supports a 200MMX I would be surprised if the 233 wouldn't work.
Actually you don't need a higher multiplier for 233MHz, as Intel recycled the basic 1.5x setting (that every So5/7 board has) for 3.5x in the P55C. So if your board supports 200MMX it's always possible to run 233MMX. The only limitation is current - if the board has an old linear voltage regulator, it might be at its thermal limits at 200MHz. Then again, the difference between 200MMX and 233MMX is a measly 1.3W, so it's not likely to be a show-stopper.
More generally, for CPU support you need the following:
1) Core voltage. You can get away with minor overvolting (0.1 or 0.2V, with added cooling), but if your board can only do 3.3V and your CPU does 2.0V, forget it.
2) BIOS support. This has various shades of grey. Biggest issue is OEM BIOS refusing to boot with unknown CPU, but sometimes there are cache issues (Tillamook mobile MMX...) or other features not enabled if unsupported. If contemplating K6plus, take a look here for official and unofficial BIOS.
3) FSB+multipliers support. Least relevant, as the CPU will run of 1+2 are OK, but if you want to run a CPU designed for 4x100MHz on a board that can't support more than 2x66MHz, you're not going to get as much out of it as hoped.
Note that there is wiggle-room here:
- 1.5x multiplier is remapped to 3.5x in Pentium MMX and K6 and faster CPUs.
- 2x multiplier is remapped to 6x in later K6-2 and all K6-3 and K6plus CPUs.
- if your board lacks multiplier settings, you can hack them in: look at So7 pinout. Find BF2 (and BF1 if needed). Connect pin in socket (or on CPU) to a Vss=ground pin. Congratulations, you just unlocked the higher settings 😉
- FSB options depend on the PLL and on jumpers/switches. n jumpers give you 2^n options. So two jumpers give 4 options, three 8 etc. Frequently boards have undocumented settings. Set multiplier to lowest non-remapped setting and play around with the undocumented FSB options. With three jumpers you'll usually have 75MHz and 83MHz options. They overclock the PCI bus and connected devices (IDE!), but can be stable and if so help get a faster CPU. Take a K6-2 500 on an i430HX chipset board. At 66MHz FSB, it does max 6x66=400MHz. At 6x83 it will run at 500MHz. If you don't have the jumpers/switches, find the PLL chip and look up its data sheet. If it's capable of more, you might be able to hack an extra switch onto unused legs for those missing speeds. Or for real pro-level hacking, consider replacing the PLL. Take the Asus XP55T2P4. It's the ATX version of the legendary P55T2P4. Great board, particularly as last revisions have VRMs that can deliver 2.0V for K6plus CPUs. Unfortunately the XP55T2P4 PLL only goes up to 66MHz, where the P55T2P4 can do 83MHz. Swap PLLs and you can run those K6Plus CPUs at 500MHz 😀