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Re: PCI bus speed on OldWorld PCI macs

On   1 Aug, this message from Chris echoed through cyberspace:
> Pardon my ignorance but in a conversation a while ago someone asked this
> listserve if upgrading to 100mbs ethernet on an older PCI powermac lab would be
> worth it to improve terminal performance.
> Most people seemed to say 'NO' because of the limits of the PCI bus.

Well, it doesn get you better performance, but from experience, I can
also say that with some cards, you _can_ reach the PCI bus limit.

> I know on my older x86 stuff the PCI bus is 50% of the system bus (I'm talking
> about 66, 76 and 83 mhz Socket 7 boards) but I never really played w/ my Mac
> hardware, until I started playing w/ linux.

Not on Macs. Early x86 PCI-based systems had the system and PCI buses
in sync with a fixed multiplier, i.e changing system bus speed changed
the PCI bus as well. Not sure about recent systems, by I think they
still run the buses from a common clock source, albeit with more
flexible multipliers between them.

This was never the case on Macs: the host bridges drive the PCI bus at a
fixed 33 MHz (or 66 MHz for some slots on some more recent systems),
async re. the system bus. So changing the system bus frequency (which is
possible on first-generation PCI systems) does not affect the PCI bus.

> -what is the speed of the PCI bus on an older powermac?  (is is 50% bus speed?)

I think all oldworls have a fixed 33 MHz PCI bus. Some more recent
systems (B&W G3's?) have a single 66 MHz slot.

> -If I clocked up my PowerMac 7300/180 (45mhz bus) to 175 to 200 mhz on a 50mhz+
> bus would I be getting a faster PCI bus as well?

Basically no; the faster host bus _might_ give a small improvment. Also,
you will have a hard time going above 50 MHz, and you _may_ have a hard
time already getting close to 50.

> -What about the onboard video and ethernet?  Is that effected performance wise
> by the increased bus
> speed?

On your 7300, the ethernet will not benefit (it's on the PCI bus); the
on-board video ('control') will, since it is on a separate bus clocked
in sync with the CPU bus. However 'chaos' and 'control', the video bus
bridge and the video chip, are also limitting the possible bus

> -Should I worry about old PCI cards not working at "overclocked" PCI bus
> settings on the PPC-MAC side?

No, since the bus stays at 33 MHz.



Michel Lanners                 |  " Read Philosophy.  Study Art.
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