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RE: 66MHz or 33MHz PCI Bus

> How does PCI devices sitting on a PCI bus knows what
> are the speed of the PCI bus that it is sitting on,
> since the M66ENA pin is not a required pin for PCI
> devices. Thanks.
Simple.  If pin M66EN (pin 49B) is low, then it is a 33MHz bus, or a 66MHz
bus running at or below 33MHz.  If M66EN is high, then it is a 66MHz bus
with only 66MHz cards present, running in the 33-66MHz range.

66MHz PCI devices (ICs) can choose to not sense the level of M66EN (pin
49B), but by doing so, they give up the ability to tell whether it's 0-33MHz
or 33-66MHz.  Only devices that don't need to know, would do that.
Regardless, M66EN is required to carry the correct indication of bus speed.