Hardware Reference
In-Depth Information
communication and can be established between two or more computer elements.
The PC has a hierarchy of different buses. Most modern PCs have at least three buses; some have four
or more. They are hierarchical because each slower bus is connected to the faster one above it. Each
device in the system is connected to one of the buses, and some devices (primarily the chipset) act as
bridges between the various buses.
The main buses in a modern system are as follows:
Processor bus —Also called the FSB, this is the highest-speed bus in the system and is at the
core of the chipset and motherboard. This bus is used primarily by the processor to pass
information to and from cache or main memory and the North Bridge of the chipset. This is
generally the fastest bus in the system, and the speed and width depend on the specific
processor and chipset combination.
PCI Express —The PCI Express (PCIe) bus is a third-generation development of the PCI bus
that began to appear in mid-2004. PCI Express is a differential signaling bus that can be
generated by either the North Bridge or the South Bridge. The speed of PCI Express is
described in terms of lanes. Each bidirectional dual-simplex lane provides a 2.5Gbps (PCIe
version 1), 5Gbps (PCIe version 2), or 10Gbps (PCIe version 3) transfer rate in each direction
(250MBps, 500MBps, or 1GBps effective speed). PCI Express video cards generally use the
x16 slot, which provides 4,000MBps, 8,000MBps, or 16,000MBps in each direction. Most
chipsets released after 2008 include PCIe version 2.0, and PCIe version 3.0 is now supported
by some of the latest processors (which now perform the former North Bridge functionality of
controlling high-speed expansion slots).
PCI bus —This is a 33MHz 32-bit bus found in virtually all systems since the days of the Intel
486 CPU. Some newer systems include an optional 66MHz 64-bit version (mostly workstations
or server-class systems). This bus is generated by either the chipset North Bridge in
North/South Bridge chipsets or the I/O Controller Hub in chipsets using hub architecture. This
bus is manifested in the system as a collection of 32-bit slots, normally white in color and
numbering from one to three on most motherboards. Peripherals, such as SCSI adapters,
network cards, video cards, and more, can be plugged into PCI bus slots. PCI Express is a
faster development of the PCI bus.
Some older motherboards feature a special connector called an Audio Modem Riser (AMR) or a
Communications and Networking Riser (CNR) . These are dedicated connectors for cards that are
specific to the motherboard design to offer communications and networking options. They are not
designed to be general-purpose bus interfaces, and few cards for these connectors are offered on the
open market. Usually, they're offered only as an option with a given motherboard. They are designed
such that a motherboard manufacturer can easily offer its boards in versions with and without
communications options, without having to reserve space on the board for optional chips. Normal
network and modem options offered publicly, for the most part, are still PCI based because the
AMR/CNR connection is somewhat motherboard specific. Figure 4.41 compares these connectors.
Current motherboards have abandoned AMR and CNR slots.
 
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