Chapter 11: Expansion Cards^239
Most video cards are standard with at least 16MB of memory; some go all the way up
to 64MB of VRAM. Much like the standard memory in most PCs, the type of memory on
video cards is usually SDRAM. VRAM is usually either SDR (single data rate), which is
less expensive, or DDR (double data rate). DDR memory increases the bandwidth and
speed (and cost) of your video card. Video memory is usually dual-ported, which means
it can be written to at the same time it is being read. This allows the CPU to write to
VRAM while the monitor is reading it. A new type of video RAM that is becoming very
popular on high-end graphics packages is RAMBUS memory, which operates much
faster than other forms of VRAM.A/V Outputs
Beyond the standard output port for the monitor, some video cards may also include
additional output ports that can be used to connect the video card to a TV, VCR, or projector.
Generally, these extra video output ports are either composite, which is the most common
type of video output, or S-Video. Composite video supports good image quality and will
interface directly to virtually all TVs and VCRs. S-Video is a high-quality display interface
that produces better color and resolution than composite video.Other Video Outputs
Some miscellaneous output ports and interfaces are included on some video cards. Here
are a few of the most common: VR (virtual reality) goggles This port supports video formatted for VR
goggles or can be used to enhance the depth perception of a standard monitor.
DVD DVD (digital versatile disc) drives need special video interfaces; many
of the newer high-end video cards come with ports to support DVD drive or
MPEG-2 decoder card interfaces.
TV tuner This port allows the computer to receive video streams from a TV,
VCR, laserdisc, or a TV cable feed or antenna.
SLI (scan line interleaving) Through this interface, two 3D acceleration cards
can divide the monitor’s display and share the load of generating the displayed
image between the two cards.Expansion Card Operation
The CPU communicates with the expansion bus and the cards inserted into it to request
data, give commands, or write data. This communication is conducted through the system
resources of a PC, which consist of the IRQs (interrupt requests), I/O (input/output)
addresses, and DMA channels of the PC. Chapter 13 discusses the elements of the system
resources in detail.