2 Jun
2008
2 Jun
'08
2:16 p.m.
Jim wrote:
Hm... I use the same bit-pattern in composite color on
an IBM CGA, and I
see that same behavior, so does that mean IBM CGA is doing the same
thing, or is the card reversing the supplied bit pattern on every
alternate scanline?
The CGA does color generation in an entirely different manner. Three
flip-flops driven by the 3.58 and 14.32 MHz clocks generate six different
color signals with continuous phase, and the color bits from the display
RAM drive a multiplexer to select between those, black, and white. (An
additional bit selects intensity, but that's independent of the actual
color.)
I haven't verified that they make the scan lines exactly 227.5 color
carrier cycles, but they probably do, as there's no good reason for
them not to. That's mostly determined by how they program the MC6845
registers.
Eric