Tandy 1000 HX Technical Reference Manual page 281

. TANDY COMPUTER PRODUCTS •
ARRAY PALETTE REGISTERS
There are sixteen 4 bit wide palette registers implemented
by a 16x4 bit RAM. These registers are 'write' only; they
cannot be 'read
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. Their address in the Video Array are from
hex 10 to IF. They can be used to redefine any color.
To load the palette, write the hex address to the Video
Array register at 3DA. Then, the new palette color is
written to 3DE.
Palette address hex 10 is accessed whenever the color code
from memory is a hex 0, address hex 11 is accessed whenever
the color code from memory is a hex 1, and so forth. A
description of the color codes is in Table 1 "Available
Colors Table" at the beginning of this section.
Note: The palette address can be 'masked' by using the
palette mask register.
The following is a description of the register's bit
functions:
Bit Number Function
0 Blue
1 Green
2 Red
3 Intensity
When loading the palette, the video is 'disabled' and the
color viewed on the screen is the data contained in the
register being addressed by the processor.
When the program has completed loading the palette, it must
change the hex address to some address less than hex 10 for
video to be 'enabled' again.
If a programmer does not wish a user to see the adverse
effects of loading the palette, the palette should be loaded
during the vertical retrace time. The program must modify
the palette and change the address to less than hex 10
within the vertical retrace time. A vertical retrace
interrupt and a status bit are provided to facilitate this
procedure.
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