AMD Geode LX 800@0.9W - Manual (Page 263)

AMD Geode™ LX Processors Data Book 263
Graphics Processor Register Definitions 33234H
6.4.2.8 Source Color Background (GP_SRC_COLOR_BG)
When source data is monochrome, the contents of this register are used for expanding pixels that are clear in the mono-
chrome bitmap, thus replacing the monochrome bit with a color that is appropriately sized for the destination.
When source data is color, this register contains the color key mask for transparency. The value(s) in this register are
inverted and OR’ed with the result of the compare of the source data and the GP_SRC_COLOR_FG register. Thus, a bit
that is clear implies that bit position is a don’t care for transparency, and a bit that is set implies that bit position must match
in both the source data and GP_SRC_COLOR_FG register. If the result of the OR produces all ones for an entire pixel and
transparency is enabled, then the write of that pixel is inhibited and the destination data is unchanged.
This register should only be written after setting the BPP/FMT bits in GP_RASTER_MODE (GP Memory Offset
38h[31:28]), since the value written is replicated as necessary to fill the register. Thus a write to this register in 8-bpp mode
takes the least significant data byte and replicates it in all four bytes of the register. In 16-bpp mode, the least significant two
bytes are replicated in the upper half of the register. A read returns the replicated data.
GP Memory Offset 14h
Type
R/W
Reset Value 00000000h
GP_SRC_COLOR_BG Register Map
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
SRC _BG
GP_SRC_COLOR_BG Bit Descriptions
Bit Name Description
31:0 SRC_BG Source Background.
Mono source mode: Background source color.
Color source mode: Color key mask for transparency.