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Routes to upgrade a B/W Powermac
- Thread starter haemogoblin
- Start date
My memory's fuzzy but I think the B&W is slightly more (or less?) complicated than that. As I recall the 66Mhz slot is the on the "native" 32 bit-wide PCI bus provided by the MPC 106 ("Grackle") memory/PCI controller, while the 33mhz/64bit slots are all hanging on a PCI->PCI bridge that's essentially mapped into one of the native PCI controller's virtual card slots.
This is basically a bummer, because it means that aggregately the entire bus->memory/CPU bandwidth for all the slots is limited to essentially the best that a single 66mhz card can do, and it's shared between video and other functions. (A typical Pentium II/III PC of the era would have the AGP slot hanging directly off the "Northbridge", while the PCI controller would be in a "Southbridge" communicating with the NorthBridge over a proprietary bus. This, among other things, reduces contention between a video card that's fetching textures from RAM and other functions like the hard disk controller and whatever slots that are hanging off the PCI bus, even in cases where the Northbridge->Southbridge bus isn't any fatter than the B&W's PCI link. It's one of the reasons why it's of "questionable wisdom" to, for instance, use those hacks that enable Quartz Extreme over PCI on a B&W.)
Given that's a flaw in the machine you can't exactly fix it's not really worth losing sleep over or anything, but it does put real limits on how much improvement you can reasonably expect in upgrading one. The B&W basically took a 1996-era system design to its max out of the box; Sawtooth and later G4s have *substantially* better system architectures.
The Radeon 7000s I have work okay in the 66mhz slot; granted I have no idea if that applies to other models.
(Interestingly I did find something that says the original 128 *doesn't* run in one of the 33mhz slot, because those are keyed for 5v cards while the R128 supplied was 3.3v only. Part of me swears I put it one of the other slots, but I could be wrong thinking about it, as I don't think I ever put more than two heads on it and I had a pair of R7000s to do it with, both of which were dual-voltage tolerant.)
Pretty sure the 66mhz slot in the B&W has no capability to slow to 33mhz; a card stuck in it either has to be able to deal or it won't work. (I vaguely recall reading a tech note that specifically said the slot was PCI 2.1 compliant other than not supporting the speed selection mechanism.) Note that support for 66mhz PCI, at least for video cards, isn't that rare. Pretty sure the justification is so the cards can work in PCI-X systems without slowing them down excessively.
Edit: yes, the fixed speed thing is in the Apple service source document.
Edit: yes, the fixed speed thing is in the Apple service source document.
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... a further note on this subject: it makes perfect sense that the 66mhz slot isn't capable of slowing down. Remember, it's not two independent PCI busses, the 64bit/33mhz slots are slaved off the 32bit/66mhz bus coming off the MPC 106 with a synchronous PCI bridge. (Do the math, of course, and you'll see that the theoretical total bandwidth of each type of slot is the same.) IF you were to force the 66mhz slot to slow to 33mhz you'd also halve the bandwidth available to the PCI bridge, so... what would happen in that case? Said bridge would either have to slow the slots down to 16mhz, which is out of spec, or disable the 64 bit extension.
Nice, would love to find a faster CPU for it. But dont processors above 700mhz cause the system bus to go slower? I'm sure I read that somewhere.
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Yes, Sonnet upgrades 800MHz/1GHz:
http://www.xlr8yourmac.com/g3CARDS/G3_1.1GHz_vs_G4_1GHz_review/index.html
http://www.xlr8yourmac.com/g3CARDS/G3_1.1GHz_vs_G4_1GHz_review/index.html
