Posted By
IstvanV
on 2011-05-07
17:46:04
|  Re: Bitmap colors'reversed nibbles
With the default vertical scroll, the DMA lines are 3, 4, 11, 12, 19, 20, and so on. Line 4 is the first raster line of the first character.
Here is a simplified description of how the DMA logic works:
- if the lower 3 bits of $FF06 and (latched) $FF1D are equal anywhere between a few characters before the beginning of the horizontal window for fetching video data, and the end of it, then the first (luminance) DMA will occur in the current line (it may be started mid-line, in that case not all characters will be updated, and the first 3 bytes read will be "garbage" from the data bus). The new attributes will only have a visible effect in the next line
- if the same $FF06 / $FF1D equality happens just before the end of a line, then the second (color) DMA will be done in the next line
- it is possible to enable both DMAs at the same line. In this case, attribute data is read from the "luminance" memory, and is stored/interpreted as both types of data
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