>Date: Thu, 29 Aug 2002 23:24:21 -0700 >From: Charles Baker <[EMAIL PROTECTED]>
>As I understand it, there's one overall system PCI bus. There are two >controllers bridged together both of which dump bits onto the overall >PCI bus. Each chip controls 3 slots, and each controller has a >preferable bus-mastering slot. Actually, there are two completely separate PCI busses on the 9x00 machines. The place where they meet is that both Bandit chips' upstream sides reside on the CPU/Memory bus. Download a copy of PPC_9500.pdf from Apple's web site and on page 9 of the document (page 22 in PDF) you'll find a block diagram that makes it clear. All the slots in PCI Macs are bus-mastering. For a long while there folks as well versed as Mike at XLR8yourmac.com and the MacGurus were claiming that the top slot was bus mastering and the others weren't, but that was just wrong. Also, there is no slot that is given preferential bus-mastering treatment. All the slots have equal priority from the PCI bus arbiter. But this has been repeated so many times that folks naturally repeat it. And it is a reasonable explanation for the fact that the top slot does deliver better performance than the other slots under some circumstances. I don't know for sure why the top slot delivers better performance, but Apple's documentation makes it clear that all the slots are created equal as far as bus mastering goes. Of course, Apple's documentation could be wrong... There is a note in one of the documents in Apple's PCI Developer's Kit which mentions that only one slot using a certain range of memory addresses can have block transfers turned on. The first slot to enable this ability gets it, and because the PCI slots are initialized from top to bottom this would cause the top slot to have the best performance if there was more than one card contending for this ability. There's more detail to the problem than that which I always thought presented an interesting opportunity for a PCI card driver developer but I won't bore you with the details. However, the above note claimed that the block transfer issue would be solved in a later release of the OS. I wouldn't be surprised to find that Apple never bothered though. >It couldn't hurt though. I've read it's s900 taboo to put the video >card in anything but the 2nd slot. It depends on the video card. Some, such as the Twin Turbo play fine in the lower slots, but I found that Copy Bits performance is five times lower in the lower slots than the upper slots. But that's just for the S900 and J700 and should not apply to the dual bandit machines. Jeff Walther -- Unsupported OS X is sponsored by <http://lowendmac.com/> Support Low End Mac <http://lowendmac.com/lists/support.html> Unsupported OS X list info <http://lowendmac.com/lists/unsupported.html> Send list messages to: <mailto:[EMAIL PROTECTED]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive <http://www.mail-archive.com/unsupportedosx%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
