Trixter and I figured this out a while ago by looking at the CGA schematics. Interlaced mode works by making an odd number of half-scanlines per field and is theoretically supported by the 6845. However, the IBM CGA hardware manipulates the horizontal and vertical sync pulses that come out of the …

I tried to implement something similar for MESS a while back, but it turns out to be quite difficult. While it's possible to use trickery to get good sound quality in games like Digger (which modulate pulse widths to control the volume of rectangular waves) and also possible to use different …

In 7 years of PC1512 usage, I only came across two pieces of software that used the mode (other than programs written in BASIC 2 and programs I had written myself). Dragonfly is a game that uses the mode for its title screen (the game itself uses a standard CGA mode). Flags supports EGA and VGA as …

I had a conversation with Trixter about this quite recently. Actually we were talking about sampled sound rather than palette effects, but the code is almost identical. We determined that (taking into account interrupt overhead) it is possible to output a byte to port 42h every 44 microseconds. A …

Yes, exactly. When I made this table I realized that it might be possible to do some rather spectacular palette effects in by changing the background color. With the right image, it might be possible to do something like the sunset effects in Dune (which were beautiful but did require VGA). But I …

But what would happen if the programmer chose a color other than white? Mightn't that complicate the composite color model? Yep. In fact, you can get interesting artifact colors in the 320-pixel modes as well. The best way to think of it is that the composite screen has 1280 sub-pixels horizontally …

That's my page. I've gone over the CGA schematics with a fine toothed comb. If anybody has any questions about the CGA, 6845-derived CRTC controllers, NTSC or composite color that aren't covered on that page, I'd be happy to answer them.

For graphics modes in CGA, address_line_total should be 2 and vtotal should be 128 (one more than the programmed value of 127) giving a total number of scanlines of 2*(127+1)+6 = 262. So the fix is again to add a +1, this time to vtotal before the multiplication by address_line_total.

Looks like the totals are wrong: they should be 57 and 262 respectively. The htotal problem is pretty simple - the actual value is one more than the programmed value so it just needs a +1 in there somewhere. I'm not sure where the 260 comes from though.

Sorry to comment on an old thread. I was doing some research recently on CGA, the 6845 CRTC and composite colour in order to improve the PC emulations in MESS - I posted a big page about it at http://www.reenigne.org/crtc.html . JAL pointed me to this conversation and I thought I should mention it …