Posted By
IstvanV
on 2011-05-07
06:28:39
|  Re: Bitmap colors'reversed nibbles
It is not necessary to read the memory 4 times for the two colors. The TED reads both (luminance and color) bytes only once, and then buffers them internally for the whole row of characters. Basically, for reading video data, these are the memory operations that occur (there can be only two per character in a single line, and to have two, DMA/halting the CPU is needed):
- bitmap bytes are read in every line from 0 to 203
- luminance (or color in character modes) is read in the last raster line of the previous character (first DMA)
- color (character code in character modes) is read in the first line of a character (second DMA)
I assumed the 1551 is slow simply because of slow ROM code - especially in the drive, as making the Plus/4 CPU 10 times faster does not seem to accelerate the loading, while it does for IEC level emulation - but there might be some other reason as well. With disk turbos, it can be much faster.
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