Posted By
IstvanV
on 2011-05-05
17:28:07
|  Re: Bitmap colors'reversed nibbles
The slower speed of 1541 on C64/Plus/4 is not really a bug, it is more of a result of limitations of the CBM serial bus protocol and the C64 hardware. To read the character codes and attributes from memory, the VIC-II needs to halt the CPU for 43 cycles at the first raster line of each row of characters. With the original, faster mode of the 1541, the kernal would not be able to read the data sent by the drive reliably, due to the slowdown resulting from the CPU being halted by the VIC-II. On the Plus/4, there are actually two DMA lines per character, since there is no separate color RAM which could be read by the video chip at the same time as the main memory, using a wider data bus. However, if you blank the screen during disk I/O, the slightly faster "VIC-20" mode of the drive will also work on the Plus/4.
The 8 shades of black is probably just a result of trying to keep the video hardware simple, i.e. 3 bits for luminance and 4 bits for color, rather than interpreting the redundant black colors in a special way.
There are two "fixed" colors in multicolor bitmap mode because the limited memory bandwidth (2 bytes per character line) of the machine does not allow for more separate colors per character than what is already available in hires mode. It could have been solved e.g. by having four DMA lines per character, but that would have made the TED too complex, in addition to more CPU slowdown.
There is one graphics limitation, however, that I think could have been avoided easily: in multicolor text mode, it would have been a better idea to use the flash (7) bit for selecting the use of high resolution or multicolor per character, since the flashing does not work in multicolor anyway. With the use of bit 3 (which made sense on the C64, where the color RAM is 4 bits wide only), half of the palette is lost.
Of course, since the flashing is not very useful, it might have been even better to have more colors instead of it, for example one bit to select low/high saturation.
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