I found some old BrookTree docs and I notice that a RAMDAC has other
functions.
Hardware cursor: This is basically a small amount of RAM that is
16x16x2 and two registers X,Y indicating the position of one of the
corners and sometimes a 2 bit register indicating which corner (or two 4
bit registers with the "hot spot"). One bit of the RAM chooses the
color which can be hardwired as black or white or can have two 24 bit
registers to choose, and the other bit chooses either the screen image
or the cursor image (transparency).
It occurred to me that this cursor function could be expanded into a
Drag 'n' Drop accelerator if it was larger and had a full 32 bit depth.
This would make it possible to load the icon to be D'n'Ded into the
ram and then it could be moved by changing the X and Y registers. This
would make dragging as smooth as the hardware cursor. Transparency
would take a ones complementer (out = 1 - in), 2 x the 8 MSBs quarter of
an 8 x 8 multiplier, and an adder for each of the three colors. One of
the multiplies (for the cursor) could be realized in software.
Chroma Key: works like the TV weather map. A 24 bit register is loaded
with a key color and the RAMDAC outputs one bit when it matches. What
this does is determined by other hardware.
--
JRT
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