Datasheet

14
Chapter 1
N
Personal Computer System Components
FIGURE 1.7 An AGP slot compared to a PCI slot
PCI slot
AGP slot
AGP performance is based on the original specification, known as AGP 1x. It uses a
32-bit (4-byte) channel and a 66MHz clock, resulting in a data rate of 266.67MBps. AGP
2x, 4x, and 8x specifications multiply the 66MHz clock they receive to increase throughput
linearly. For instance, AGP 8x uses the 66MHz clock to produce an effective clock frequency
of 533MHz, resulting in throughput of 2133.33MBps over the 4-byte channel.
PCIe Expansion Slots
A newer expansion slot architecture that is being used by motherboards is PCI Express
(PCIe). It was designed to be a replacement for AGP and PCI. It has the capability of being
faster than AGP while maintaining the flexibility of PCI. And motherboards with PCIe
might have regular PCI slots for backward compatibility with PCI.
PCIe is casually referred to as a bus architecture to simplify its comparison with other
bus technologies. In fact, unlike true I/O buses, which share total bandwidth among all
slots in a hub-like interconnectivity, PCIe uses a switching component with point-to-point
connections to slots, giving each component full use of the corresponding bandwidth. Fur-
thermore, true bus architectures are parallel in nature, while PCIe is a serial technology,
striping data packets across multiple serial paths to achieve higher data rates.
PCIe uses the concept of lanes, which are the switched point-to-point signal paths
between any two PCIe components. Each lane that the switch interconnects between any
two intercommunicating devices comprises a separate pair of wires for both directions of
traffic. Each PCIe pairing between cards requires a negotiation for the highest mutually
supported number of lanes. The single lane or combined collection of lanes that the switch
interconnects between devices is referred to as a link.
There are seven different link widths supported by PCIe, designated x1 (pronounced
“by 1), x2, x4, x8, x12, x16, and x32, with x1, x4, and x16 the most common. The x8
link width is less common than these but more common than the others. A slot that sup-
ports a particular link width is of a size related to that width because the width is based
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