Color Look-Up Table; Drcs Attributes - NCR PC4I Technical Reference

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C
VIDEOTEX
Color Look-up Table
As already mentioned, you can select 32 colors from a
total number of 4096. This means that the 5-bit value
cannot directly control the RGB outputs to the CRT
circuitry. Instead, the 5-bit value refers to a
palette, also called "color look-up table" (CLUT), in
which amplification values in four bits for each of
the three color guns are stored. 4096 (2 to the power
of 12) colors are available, any one of which can be
selected by each of the 32 table pointers. Figure
10.27 describes these bits as Palette, Intensity,
Blue, Green, and Red simply because of the CEPT
standard. This standard specifies that the first
group of eight colors should be black, red, green,
yellow, blue, magenta, cyan, and white, while the
second group yields the same colors in half-
intensity.
These 32 12-bit items of color information are stored
in 96 bytes near the top of the 4 KB RAM area at the
disposal of the Videotex Controller (see "Videotex
Memory Organization"): the lowest byte contains in
its four MSBs the red color value for CLUT color O,
the next two bytes contain corresponding values for
green and blue; the next three bytes define CLUT
color
1,
and so on. You can read this 96-byte area in
order to ascertain color set tings. However, the
conventional way of writing the color items is by
means of Videotex Controller Registers (see section
below).
DRCS Attributes
DRCS characters, like the standard characters, derive
their attributes from the dynamically allocatable
attribute stack. Where more than one bit per pixel
applies,
the entire character is regarded as foregound
color
the attributes Conceal, Blink and Invert make
use of the background color
the Underline attribute has no effect
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