According to sources, Apple plans to use a slot labeled "Mezzanine" on the underside of the iMac's motherboard to expand the functionality of future iMacs. The 160-pin slot--called a Perch slot internally at Apple--is located on the underside of the iMac's motherboard, and sources say the slot is electrically identical to the personality slot found in desktop G3 Power Macintoshes. The personality slot is essentially a modified PCI slot and is suitable for high-bandwidth applications; Apple's latest use for the Power Mac personality slot is an MPEG-2 decoder card for watching DVD Video.
Buried as it is in the guts of the iMac, it's unlikely that the average consumer would accidentally discover the Perch slot. The only clue to the slot's existence is a blank space on the iMac's port panel where a hole has been cut in the case and then covered with a metal panel.
There is no mention of the slot in the iMac's developer tech notes; sources said that Apple has refused to disclose any information to curious developers. Furthermore, the company has reportedly warned expansion companies not to create products that used the Perch slot. When MacWEEK contacted Apple, a spokeswoman said that the slot was "intended for Apple's internal use," and that the port is not open for third-party development.
The Mezzanine label on the motherboard and the physical appearance of the slot has led some to speculate that the slot follows the PCI Mezzanine Card (PMC) specification. While the Perch slot uses the PCI electrical connections like PMC, the PMC standard specifies up to four separate 64-pin connectors.
The question of what Apple will use the Perch slot for remains unanswered, although industry sources have indicated that future iMac purchasers could look forward to FireWire or DVD Video expansion hardware.
DVD seems an especially likely candidate, observers speculated. Apple's DVD kit for the G3 PowerBooks carries the MPEG-2 video decoder hardware on a CardBus card, and CardBus is another variant of the PCI bus that has been modified for the PowerBook form factor.