mcguli wrote on 2024-01-27, 13:05:

Hello friends, I have an Intel 486 DX4 75Mhz microprocessor. This is mounted on a DataExpert EXP4045 motherboard with turbo button.

My question is, what speed does the microprocessor go when the turbo is not activated? It is not clear to me if it will be the multiplier by 1 and it will be 25Mhz or 8Mhz mode.

You can use one of the many DOS based CPU bench-marking tools to find the speed when toggling the turbo button, my guess itll be dropped back to 1x @ 25Mhz FSB, 8Mhz would be very odd for that CPU even with turbo disabled.

On a 486 it most likely will remain on 75 MHz. But L2 cache will be disabled and your motherboard may add a lot of wait states.

It's more likely the speed is maintained internally or just dividers by 2 or so, but the systems also adds a bunch of wait states and disabled some cache to mimic the slower speed. I expect you'd get a 386-16 equivalent speed. But you know only when you test it.

For convenience I'd recommend Philscomputerlab, Phil has distributed a dos benchmark pack which contains lots of benchmark and testing stuff. All free to use 😀

I was messing around with an Asus motherboard from 1993 with AMI bios and a 486-66 yesterday. When the turbo was in the slow setting, the l1 & l2 cache were still enabled, but something was definitely odd about the memory access and the caches were not as effective as they should have been. The speedsys memory stairs had some odd divots that you wouldn’t expect to see if everything was working correctly. Even though there is no CPUid, most info tools still reported it as a 66mhz cpu on a 33mhz fsb.

With the l2 enabled and the turbo on slow, the speedsys benchmark was ~13.5, which would be like a 25mhz 486 with a good cache.

With the l2 enabled and the turbo on slow, the speedsys benchmark was ~15.5, which would be like a 30mhz 486 with a good cache.

With the l2 enabled and the turbo on fast, the speedsys benchmark was ~24.5 whch is pretty close to what I’d expect for a 486dx2-66 in an old motherboard that doesn’t have a write back L2 cache.

Some people wrote more about it here: 486DX2-66 Write-Back and Turbo mode

Here are some posts other people have done with some graphs:
486DX2-66 Write-Back and Turbo mode

Thanks for your answers, I'll try a test program to see what speed it says.