_m_a_s_k Specifies a bit mask to enable and disable the writing of
individual bits in the color index buffers. Initially, the mask is
all 1's.
DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
ggggllllIIIInnnnddddeeeexxxxMMMMaaaasssskkkk controls the writing of individual bits in the color index
buffers. The least significant n bits of _m_a_s_k, where n is the number of
bits in a color index buffer, specify a mask. Where a 1 (one) appears in
the mask, it's possible to write to the corresponding bit in the color
index buffer (or buffers). Where a 0 (zero) appears, the corresponding
bit is write-protected.
This mask is used only in color index mode, and it affects only the
buffers currently selected for writing (see ggggllllDDDDrrrraaaawwwwBBBBuuuuffffffffeeeerrrr). Initially,
all bits are enabled for writing.
EEEERRRRRRRROOOORRRRSSSS
GGGGLLLL____IIIINNNNVVVVAAAALLLLIIIIDDDD____OOOOPPPPEEEERRRRAAAATTTTIIIIOOOONNNN is generated if ggggllllIIIInnnnddddeeeexxxxMMMMaaaasssskkkk is executed between the
execution of ggggllllBBBBeeeeggggiiiinnnn and the corresponding execution of ggggllllEEEEnnnndddd.