Rastah Bar
Account closed
Registered: Oct 2012
Posts: 336 |
Has ECM been implemented on the VIC chip in the most efficient way?
In ECM mode bit 6 and 7 are used to select a color register, but they are not used for selection of the character image. It seems to me that this may be inefficient.
To explain what I mean: suppose bits 6 and 7 were also used for the character image (I don't know if that was technically possible when the VIC chip was designed, but let's suppose it was). Let's call this ECM2 mode. This hypothetical mode uses bits 6 and 7 twice: for the character code and for selecting a color register.
Suppose also that a character set is used with 256 distinct characters. Now, if two or more of the color registers contain the same color code, then it is possible in ECM that two different character codes, for example code 65 and 129, are indistinguishable from each other on the screen, if $d022 and $d023 contain the same color.
With the hypothetical ECM2 mode, however, these are distinguishable, since they show a different character, although on the same background color, because all bits are used for the character code.
So I seems ECM is wasting bits compared to ECM2 mode. But probably there is a flaw in my logic and I would appreciate it if someone could point that out.
If this reasoning is correct, then a more efficient multicolor mode could have been implemented by displaying an "10" and "01" exactly like that on the screen (in high resolution, that is), and let the color of the 1 be determined as in MC mode as it exist on the C64 now. |
