Slots

Some desktop Macintosh computers have expansion slots for adding capabilities. For example, to connect an external SCSI drive to a Power Mac G5, you can install a SCSI card in an expansion slot and connect the drive to that card. Or to attach an additional monitor to a Macintosh, you could add a second video card. Macintosh desktop computers have two kinds of expansion slots: Peripheral Component Interface (PCI) and Accelerated Graphics Port (AGP). In contrast, PowerBooks use Personal Computer Memory Card International Association (PCMCIA) slots for expansion.

PCI

PCI expansion slots aren't just connectors tacked on to the logic board as an afterthought. They're actually an integral part of the logic board—in fact, PCI is the computer's main bus, which it uses to communicate between the processor, RAM, and expansion slots. Even iMac and PowerBook models, which don't have expansion slots, have PCI buses.

When a Macintosh starts up, Open Firmware software in the PCI card's ROM contacts software in the Name Registry on the Macintosh. The Macintosh stores the configuration and location of PCI cards in RAM. Meanwhile, Open Firmware checks to see if the card is in a Macintosh or PC, and then loads the necessary drivers, if any, either immediately from the PCI card's ROM or from the hard disk after the operating system takes control.

AGP

The AGP slot found in some Macintosh computers is essentially a souped-up PCI slot. It's built on the same protocol, but instead of transferring data at a maximum rate of 132 MB/s, AGP can move data at up to 528 MB/s. The AGP slot also has a faster bus to the Macintosh computer's main memory.

The primary function of such a high-speed slot is to accelerate graphics, whether the latest games, 3D rendering, or video editing. AGP does this in several ways. As mentioned, AGP is a subset of PCI, but with a few twists.

An integrated circuit (which you might know as a “chip”) called the UniNorth acts as the AGP controller. One of its functions is to remap data in the Macintosh computer's main memory to make it more readily accessible to the AGP bus. This speeds up access to large chunks of data, such as the kind of data that video and 3D images use.

If you ever need to equip a Macintosh with multiple displays, keep in mind the performance advantage of the AGP slot over ordinary PCI slots. In general, it makes more sense to find an AGP card that can drive two monitors than to put a second video card in a PCI slot. The Macintosh works in either case, but the all-AGP solution produces better video performance.

PCMCIA

We've already mentioned that PowerBook computers don't have expansion slots—after all, they are built for maximum portability—but even so, PCMCIA devices provide several upgrade opportunities. PC Card devices (as they are more commonly known) give faster Ethernet access, wireless networking capabilities, FireWire and USB ports, enhanced video support, and other features. PC Cards come in 16-bit and 32-bit versions. They also have a size range (Type I to Type III) corresponding to various thicknesses.

Now that we have a grasp of the components that make up a Macintosh computer's hardware and an understanding of how they communicate, let's take a look at the software that runs on the Macintosh.