[Coco] Writing a CPU simulator for the 6809

[Torsten Dittel]

Steve Bjork <6809er at ...> writes:

> [...]  Flipping the bits on the SAM and $FF22 is how us video game 
> programmers got our little special effect to work.  As side note, the 
> VDG reads $FF22 mode control on every scan line.  So, you can change the 
> video mode from time and time again on every screen. 

Hmmmm... I remember (at least on a CoCo1/2) several VDG mode changes had 
been possible even during a single scan line. Example here:

http://www.6809.org.uk/tmp/da/pal_coco_probe/pal-dragonfire-1.png

Looks like I had been mixing a SG24 mode with a PMODE4:SCREEN1,1 synced 
within a block of scanlines - starting even on the left border which causes 
the SG24 block to "shift out". Same is true for the 2 thin black lines on 
the top and bottom of the vertically "bouncing" (smooth 50Hz animation...) 
text line ("JUST FOR THE [...]").

PS: I'm still impressed how accurate XROAR emulates the PAL 50Hz timing. 
One day I will dig out the other "emualtor benchmarks" I wrote some 30 
years ago which will allow more stress testing... ;-)