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Before PCI it was rare to see more than one type of expansion slot on a motherboard Today this is not only common it s expected!

Shortly after Intel invented PCI, the company presented a specialized, videoonly version of PCI called the accelerated graphics port (AGP) An AGP slot is a

PCI slot, but one with a direct connection to the Northbridge AGP slots are only for video cards don t try to snap a sound card or modem into one Figure 1013 shows a typical AGP slot An AGP slot is almost universally brown in color, making it easy to spot

PCI-X, already available in systems such as the Macintosh G5, is a huge enhancement to the current PCI technology that is also fully backwardcompatible, in terms of both hardware and software PCI-X is a 64-bit-wide bus (see Figure 1014) Its slots will accept regular PCI cards The real bonus of PCI-X is its much enhanced speed The PCI-X 20 standard features four speed grades (measured in MHz): PCI-X 66, PCI-X 133, PCI-X 266, and PCI-X 533

PCI has even made it into laptops in the specialty Mini-PCI format (Figure 1015) You ll find Mini-PCI in just about every laptop these days Mini-PCI is designed to use low power and to lie flat both good features for a laptop expansion slot Mini-PCI returns in 12

PCI Express (PCIe) is the latest, fastest, and most popular expansion bus in use today As its name implies, PCI Express is still PCI, but it uses a point-to-point serial connection instead of PCI s shared parallel communication Consider a single 32-bit chunk of data moving from a device to the CPU In PCI parallel communication, 32 wires each carry 1 bit of that chunk of data In serial communication, only one wire carries those 32 bits You d think that 32 are better than 1, correct Well, first of all, PCIe doesn t share the bus A PCIe device has its own direct connection (a point-to-point connection) to the Northbridge, so it does not wait for other devices Plus, when things start going really fast (think gigabits per second), it s difficult to get all 32 bits of data to go from one device to another at the same time because some bits get there slightly faster than

others That means you need some serious, high-speed checking of the data when it arrives to verify that it s all there and in good shape Serial data doesn t have this problem, as all the bits arrive one after the other in a single stream When data is really moving fast, a single point-to-point serial connection is faster than a shared, 32-wire parallel connection And is PCIe fast! A PCIe connection uses one wire for sending and one for receiving Each of these pairs of wires between a PCIe controller and a device is called a lane Each lane runs at 25 Gbps Better yet, each point-to-point connection can use 1, 2, 4, 8, 12, 16, or 32 lanes to achieve a maximum bandwidth of 160 Gbps The effective data rate drops a little bit because of the encoding

scheme the way the data is broken down and reassembled but full duplex data throughput can go up to a whopping 128 GBps on a 32 connection The most common PCIe slot is the 16-lane ( 16) version most commonly used for video cards, as shown in Figure 1016 The first versions of PCIe motherboards used a combination of a single PCIe 16 slot and a number of standard PCI slots (Remember that PCI is designed to work with other expansion slots, even other types of PCI)