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382 AMD Geode™ LX Processors Data Book
Display Controller Register Descriptions
33234H
6.6.21.7 Color Select
6.6.22 Video DAC Registers
Video DAC palette registers are accessed by writing the Palette Address register at the read or write address, then perform-
ing three reads or writes, one for each of the red, green, and blue color values. The video DAC provides an address incre-
ment feature that allows multiple sets of color triplets to be read or written without writing the Palette Address register again.
To invoke this feature, simply follow the first triplet read/write with the next triplet read/write.
The original IBM video DAC behavior for read operations is:
1) CPU initiates a palette read by writing INDEX to I/O address 3C7h.
2) Video DAC loads a temporary register with the value stored at palette[INDEX].
3) Video DAC increments INDEX (INDEX = INDEX + 1).
4) CPU reads red, green, blue color values from temporary register at I/O address 3C9h.
5) Loop to step 2.
The original IBM video DAC behavior for write operations is:
1) CPU initiates a palette write by writing INDEX to I/O address 3C8h.
2) CPU writes red, green, blue color values to temporary DAC registers at I/O address 3C9h.
3) Video DAC stores the temporary register contents in palette[INDEX].
4) Video DAC increments INDEX (INDEX = INDEX + 1).
5) Loop to step 2.
Index 14h
Typ e R /W
Reset Value xxh
Color Select Register Bit Descriptions
Bit Name Description
7:4 RSVD Reserved.
3:2 P[7:6] P7 and P6. These bits are used to provide the upper two bits of the 8-bit pixel value sent
to the video DAC in all modes except the 256 color mode (mode 13h).
1:0 P[5:4] P5 and P4. These bits are used to provide bits 5 and 4 of the 8-bit pixel value sent to the
video DAC when the P5:4 Select bit is set in the Attribute Mode Control register (Index
10h[7]). In this case, they replace bits [5:4] coming from the EGA palette.
Table 6-58. Video DAC Registers Summary
I/O
Address Type Register Reset Value Reference
3C8h RO Palette Address (Write Mode) 00h Page 383
3C7h RO Palette Address (Read Mode) 00h Page 383
3C7h RO DAC State 00h Page 383
3C9h R/W Palette Data 00h Page 380
3C6h R/W Pel Mask 00h Page 380