Hardware Reference
In-Depth Information
1080 values, one for each pixel. In
fact, it might contain many such bit maps, to allow rapid switching from one
screen image to another.
On a garden-variety display, each pixel would be represented as a 3-byte RGB
value, one each for the intensity of the red, green, and blue components of the
pixel's color (high-end displays use 10 or more bits per color). From the laws of
physics, it is known that any color can be constructed from a linear superposition
of red, green, and blue light.
A video RAM with 1920
pixels, the video RAM would contain 1920
×
1080 pixels at 3 bytes/pixel requires over 6.2 MB
to store the image and a fair amount of CPU time to do anything with it. For this
reason, some computers compromise by using an 8-bit number to indicate the color
desired. This number is then used as an index into a hardware table called the
color palette
that contains 256 entries, each holding a 24-bit RGB value. Such a
design, called
indexed color
, reduces the video RAM memory requirements by
2/3, but allows only 256 colors on the screen at once. Usually, each window on the
screen has its own mapping, but with only one hardware color palette, often when
multiple windows are present on the screen, only the current one has its colors ren-
dered correctly. Color palettes with 2
16
entries are also used, but the gain here is
only 1/3.
Bit-mapped video displays require a lot of bandwidth. To display full-screen,
full-color multimedia on a 1920
×
1080 display requires copying 6.2 MB of data to
the video RAM for every frame. For full-motion video, a rate of at least 25
frame/sec is needed, for a total data rate of 155 MB/sec. This load is more than the
original PCI bus could handle (132 MB/sec) but PCIe can handle it with ease.
×
As time goes on, computers are being used by people with less expertise in
how computers work. Computers of the ENIAC generation were used only by the
people who built them. In the 1950s, computers were only used by highly skilled
professional programmers. Now, computers are widely used by people who need
to get some job done and do not know (or even want to know) much about how
computers work or how they are programmed.
In the old days, most computers had command line interfaces, to which users
typed commands. Since people who are not computer specialists often perceived
command line interfaces as user-unfriendly, if not downright hostile, many com-
puter vendors developed point-and-click interfaces, such as the Macintosh and
Windows. Using this model requires having a way to point at the screen. The
most common way of allowing users to point at the screen is with a mouse.
A
mouse
is a small plastic box that sits on the table next to the keyboard.
When it is moved around on the table, a little pointer on the screen moves too, al-
lowing users to point at screen items. The mouse has one, two, or three buttons on
top, to allow users to select items from menus. Much blood has been spilled as a
