How Apple Messed Up the PCI Bus

1998 – There is no functional difference between the Power Mac 7100 and the Quadra 650 with the PowerPC upgrade card: They use the same ROM. As for the PPC Toolbox, that appears on all the 68040 Macs I’ve worked with, simply because the PPC Toolbox is 68k code.

The PPC Toolbox

Here’s why – the 68k emulator for PPC must be allowed to make native calls through the emulator at times. Therefore, the PPC Toolbox is in 68k code, so that it can receive 68k commands. This slows things down in certain ways on PPC machines, but it is never used on 68k machines, simply because the expected PPC calls are never made. If you jump into ResEdit and send a native call, your machine will crash (because there is no PPC).

Just because the toolbox is named PPC doesn’t mean it’s PPC native code – only that it’s an interpreter.

You haven’t mentioned that you have a 650 with the PPC upgrade card, so I won’t assume it, though that could answer easily the 7100/xx detection the 650 ROM is a different version, but the PPC upgrade card for the 650 and the 7100’s ROM are the identical version number.

This brings out a few choices, as well, because there are few functional differences. Apple changed very little of the 650 motherboard to create the 7100 – so dependent on the existing motherboard designs and terribly concerned about the fact that there wasn’t native code at the time. The PPC changeover was a considerable task, not one easily performed under the management at the time. The 7100 did have GeoPorts, but that was a matter of changing a controller chip. Bus speeds were identical, the 32-bit memory bus was rewired for 64-bit memory without changing the chipset and with minor changes to the motherboard itself.

PCI on Power Macs: Not Ideal

As for swapping out for PCI, the PCI motherboards represent a real diversity in Apple’s engineering steps – actually creating a motherboard strictly designed for the PPC chip with no real attachment to previous technologies. The 68k specific pieces were lifted off the board and directly supported by the 68k emulator. This prevents a considerable amount of PCI compatibility – so many components changed, well, it a leap that I really don’t think can be taken.

Beware, this is where Apple suffers over the clones. Several manufacturers realized that Apple wasn’t preparing PCI bridges properly – the PCI bus on the new Power Mac G3 has a maximum total bus speed between the PCI slots of the motherboard bus….

*Sigh*, I better say this a different way.

Take a 50 MHz motherboard, throw five PCI slots on it.* Put two Ultra SCSI-3 cards in, a PCI video accelerator, a 100Base-T or Gigaswitch ethernet card, and something else – perhaps a 3D accelerator as well. On the Apple box, with all devices running at maximum load through several programs, the fastest that these five PCI devices can talk to the rest of the motherboard is at 10 MHz each, for a total through the PCI gateway of 50 MHz.

Specific clone manufacturers realized this limitation and decided to invest more into their motherboards by creating separate PCI bridging, giving each slot total independent bus access to the rest of the motherboard. This is similar to having several PDS slots in a 68k Macintosh.

Go begging for scraps. When Apple decides that building a solid upgradable hardware platform requires less capital and greater potential profit through less support and less engineering, and discovers that software distribution and creation is more cost effective, then maybe Apple will release a machine worth purchasing.

Even now, none of the G3 models come close to the Power Computing PowerCenters and PowerTowers on the market, and the cheap G3 machines are built on the LC model – as many features as possible on the spec sheet, barely enough to get by – and conglomerate these features on the same stop on the bus.

Scott L. Barber <serker@earthling.net>
Pres/CEO, SERKER Worldwide, Inc.
Providing Hardware/Networking/Telecomm for 13 years

Scott L. Barber first posted this to Quadlist, the listserv for users of 68040-based Macs. It is reprinted with his permission.

 * The crux is that the more PCI cards you have installed, less bandwidth you have available when the cards are in use. No Mac had five PCI slots, although some had three, some four, and some six. The Power Mac 9500 and 9600 have six PCI slots and use two bridges, each supporting three PCI slots. The bridge allows three slots to share the full range of PCI bandwidth instead of dividing it among 4-6 PCI cards. These were the last 6-slot Macs and the only ones to have more than one PCI bridge. (In the PC world, PCI slots and I/O are handled by southbridge, while memory, graphics, and PCIe are handled by northbridge. The 9500 and 9600 work as though they have two southbridge controllers, not just one.)

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