Specifications

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Chapter 15 Video Hardware
video cards support AGP 8x; however, differences in GPU design, memory bus design, and core and
memory clock speed mean (as always) that AGP 8x cards with faster and wider memory and faster GPU
speeds provide faster performance than AGP 8x cards with slower and narrower components.
Although some systems with AGP 4x or 8x slots use a universal slot design that can handle 3.3V or
1.5V AGP cards, others do not. If a card designed for 3.3V (2x mode) is plugged in to a motherboard
that supports only 1.5V (4x mode) signaling, the motherboard will be damaged.
√√ See “Accelerated Graphics Port,” p. 364.
Caution
Be sure to check AGP compatibility before you insert an older (AGP 1x/2x) card into a recent or current system. Even if you
can physically insert the card, a mismatch between the card’s required voltage and the AGP slot’s voltage output can damage
the motherboard. Check the motherboard manual for the card types and voltage levels supported by the AGP slot.
Some AGP cards can use either 3.3V or 1.5V voltage levels by adjusting an onboard jumper. These cards typically use
an AGP connector that is notched for use with either AGP 2x or AGP 4x slots, as pictured in Chapter 4, “Motherboards
and Buses.” Be sure to set these cards to use 1.5V before using them in motherboards that support only 1.5V signaling,
such as motherboards based on the Intel 845 or 850 chipsets.
PCI Express, which will eventually succeed both AGP and PCI, began to show up in high-performance
systems in mid-2004. Despite the name, PCI Express uses a high-speed bidirectional serial data transfer
method, and PCI Express channels (also known as lanes) can be combined to create wider and faster
expansion slots (each lane provides 250MBps data rate in each direction). Unlike PCI, PCI Express
slots do not compete with each other for bandwidth. PCI Express graphics cards use 16 lanes (x16) to
enable speeds of 4GBps in each direction; when PCI Express is used for other types of cards, fewer
lanes are used.
PCI, AGP, and x16 PCI Express have some important differences, as Table 15.15 shows.
Table 15.15 High-Speed Video Card Bus Specifications
Feature PCI AGP PCI Express
Theoretical 133MBps
1
533MBps throughput (2X) 250MBps/lane
3
maximum 1066GBps throughput (4X)
2
4GBps (x16)
3
2133GBps throughput (8X)
2
Slots
2
4/5 (typical) 1 1/more
4
1. At 33MHz bus speed and 32 bits.
2. Most current systems support AGP 4X/8X only.
3. In each direction; multiply by 2 for bidirectional throughput.
4. Only one PCI Express slot (x16 – video) per system in early implementations; motherboards with multiple PCI
Express slots with fewer lanes for use by other card types will be available by 2005.
The Video Driver
The software driver is an essential, and often problematic, element of a video display subsystem. The
driver enables your software to communicate with the video adapter. You can have a video adapter
with the fastest processor and the most efficient memory on the market but still have poor video per-
formance because of a badly written driver.
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