Hardware Reference
In-Depth Information
faces such as AGP and act as a mezzanine bus to attach other interfaces, such as Serial
ATA, USB 2.0, 1394b (FireWire or i.LINK), Gigabit Ethernet, and more.
Because PCI Express can be implemented over cables as well as onboard, it can be used
to create systems constructed with remote “bricks” containing the bulk of the computing
power. Imagine the motherboard, processor, and RAM in one small box hidden under a
table, with the video, disk drives, and I/O ports in another box sitting on a table within
easy reach. This will enable a variety of flexible PC form factors to be developed in the
future without compromising performance.
Although PCI Express has not and will not replace PCI or other interfaces overnight, over
time, PCI Express will eventually become the preferred general-purpose I/O interconnect
over PCI. I expect the move to PCI Express to be similar to the transition from ISA/AT-
BustoPCIinthe1990s.Currentfull-sizeATXmotherboardshaveaboutone-thirdoftheir
slots as PCI and about two-thirds as PCI Express. And, PCI Express x16 has already re-
placed AGP 8x.
Most recent desktop motherboard designs feature a mix of PCI, PCI Express x1, and x16
slots; workstation and server motherboards are also adding PCI Express slots to their typ-
ical PCI-X and PCI slots.
For more information on PCI Express, I recommend consulting the PCI-SIG website
( www.pcisig.org ) .
Accelerated Graphics Port
In the mid 1990s, Intel created AGP as a new bus specifically designed for high-perform-
ance graphics and video support. AGP is based on PCI, but it contains several additions
and enhancements and is physically, electrically, and logically independent of PCI. For
example, the AGP connector is similar to PCI, although it has additional signals and is
positioned differently in the system. Unlike PCI, which is a true bus with multiple con-
nectors (slots), AGP is more of a point-to-point high-performance connection designed
specifically foravideocardinasystembecauseonlyoneAGPslotisallowedforasingle
video card. Intel originally released the AGP specification 1.0 in July 1996 and defined a
66MHz clock rate with 1x or 2x signaling using 3.3V. AGP version 2.0 was released in
May 1998 and added 4x signaling as well as a lower 1.5V operating capability.
ThefinalrevisionfortheAGPspecificationforPCsisAGP8x,alsocalledAGP3.0.AGP
8x defines a transfer speed of 2,133MBps, which is twice that of AGP 4x. The AGP 8x
specification was publicly announced in November 2000.
Although AGP 8x (2,133MBps) is 16 times faster than 32-bit 33MHz PCI (133MBps),
AGP8xisonlyabout half asfast asPCIExpress x16(4,000MBps).Starting inmid-2004,
motherboard and system vendors began to replace AGP 8x with PCI Express x16 expan-
 
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