What's your actual aim? VLB is already flaky at 40 MHz, let alone 50 MHz. And why do you want to overclock the CPU, too? If you want to build a very fast 486, the UMC CPU is not suitable for that. And if you just want to build an UMC-based 486, go without overclocking. Even if you get it working, the additional wait states you need for a 50 MHz system will eat up any speed gain you get from the higher CPU clock.

And, by the way, while the UMC stays cold during operation, it does not overclock very well. None of my 33 MHz ones run at 40 MHz and even if they would, what't the point anyway? If we want more speed, we can use faster CPUs.

I like the umc cpu because despite being a common SX cpu it was faster (clock to clock) than intel\amd counterparts.
I already have a fast 486 build with amd 5x86 cpu, I just want to try overclock the umc cpu, since the vlb experiment failed I'm looking for some advices about what motherboard to use (I suppose pci would be better) to be able to achieve the 50 mhz.

derSammler wrote on 2020-03-21, 13:44:

What's your actual aim? VLB is already flaky at 40 MHz, let alone 50 MHz. And why do you want to overclock the CPU, too? If you want to build a very fast 486, the UMC CPU is not suitable for that. And if you just want to build an UMC-based 486, go without overclocking. Even if you get it working, the additional wait states you need for a 50 MHz system will eat up any speed gain you get from the higher CPU clock.

And, by the way, while the UMC stays cold during operation, it does not overclock very well. None of my 33 MHz ones run at 40 MHz and even if they would, what't the point anyway? If we want more speed, we can use faster CPUs.

Hi, my UMC 40 CPU working perfectly on 50 mhz. SpeedSys give it 28.3 points. It's more then 486DX4-75 mhz.

On that VLB system, what VGA and - particularly - I/O did you use?

50MHz was above what a lot of I/O cards could handle, it's likely that's your bottleneck, not the CPU. Try adding wait states on VLB, or use an ISA I/O card.