Page 75 - A Practical Guide from Design Planning to Manufacturing

Computer Components 51

Suffice it to say that one must be very careful in buying DRAM to make
sure to get the appropriate type for your computer. Ideally, the memory bus
standard will support the same maximum bandwidth as the processor bus.
This allows the processor to consume data at its maximum rate without
wasting money on memory that is faster than your processor can use.

Video Adapters (Graphics Cards)
Most output devices consume data at a glacial pace compared with the
processor’s ability to produce it. The most important exception is the
video adapter and display. A single high-resolution color image can con-
tain 7 MB of data and at a typical computer monitor refresh rate of
72 Hz, the display could output data at more than 500 MB/s. If multiple
frames are to be combined or processed into one, even higher data rates
could be needed. Because of the need for high data bandwidth, the video
adapter that drives the computer monitor typically has a dedicated
high-speed connection to the Northbridge of the chipset.
Early video adapters simply translated the digital color images pro-
duced by the computer to the analog voltage signals that control the
monitor. The image to be displayed is assembled in a dedicated region
of memory called the frame buffer. The amount of memory required for
the frame buffer depends on the resolution to be displayed and the
number of bits used to represent the color of each pixel.
Typical resolutions range anywhere from 640 × 480 up to 1600 × 1200,
and color is specified with 16, 24, or 32 bits. A display of 1600 × 1200 with
20
32-bit color requires a 7.3 MB frame buffer (7.3 = 1600 × 1200 × 32/2 ).
The Random Access Memory Digital-to-Analog Converter (RAMDAC)
continuously scans the frame buffer and converts the binary color of each
pixel to three analog voltage signals that drive the red, green, and blue
monitor controls.
Double buffering allocates two frame buffers, so that while one frame
is being displayed, the next is being constructed. The RAMDAC alter-
nates between the two buffers, so that one is always being read and one is
always being written. To help generate 3D effects a z-buffer may also be
used. This is a block of memory containing the effective depth (or z-value)
of each pixel in the frame buffer. The z-buffer is used to determine what
part of each new polygon should be drawn because it is in front of the
other polygons already drawn.
Texture maps are also stored in memory to be used to color surfaces
in 3D images. Rather than trying to draw the coarse surface of a brick
wall, the computer renders a flat surface and then paints the image with
a brick texture map. The sky in a 3D game would typically not be mod-
eled as a vast open space with 3D clouds moving through it; instead it
would be treated as a flat ceiling painted with a “sky” texture map.