[Coco] Writing a CPU simulator for the 6809

Simon Jonassen simon at roust-it.dk
Thu Aug 29 14:24:36 EDT 2013

I'm slightly lost....

I didn't follow this whole conversation (even though i would like to).....

@torsten... You can indeed get colour to bleed into the border - actually
any of the 10 colours available.... (requires VERY careful timing

You can also do a form of hardware scrolling using $ff22 (if you twiddle at
the right time)... You can reset the line counter for the chars....

ME thinks $ff22 is hiding alot more secrets... (and that torsten knows some
of them, hehe).....

/Simon :-)

-----Oprindelig meddelelse-----
Fra: coco-bounces at maltedmedia.com [mailto:coco-bounces at maltedmedia.com] På
vegne af Torsten Dittel
Sendt: 29. august 2013 13:38
Til: coco at maltedmedia.com
Emne: Re: [Coco] Writing a CPU simulator for the 6809

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:


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... ;-)

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