[Coco] Artifacting (again)

[Mark McDougall]

On 3/09/2010 8:31 AM, jdaggett at gate.net wrote:

> The SAM chip also expands the video modes. S8, S12 and S24 modes  are
> added via the SAM chip. The INV, and S/A pins of the 6847 are also mapped
> to the DD /DD7 pins of the SAM chip. Supposedly this is to allow changes
> on the fly.

Yes, I know, but the SAM still only affects video memory addressing, i.e. 
from where in memory the 6847 fetches video data for each pixel. The SAM 
itself doesn't directly drive any video signals, so it can't affect the 
artifacting.

Mapping INV# and A#S to D6/7 don't allow changes on the fly. The (non-T1) 
6847 only has 64 characters, enough for uppercase only. Connecting D6 to 
INV# causes the 6847 to display inverse characters for codes 0-63. 
Connecting A#S to D7 causes semi-graphics to be displayed for codes 128-255.

The SAM allows changes to the way video memory is addressed in semi-graphics 
mode. In effect, it changes the vertical scaling of the semi-graphics 
blocks, by altering the number of times each character row is scanned in 
video memory.

As an aside, the 6847 would normally do its own video addressing. But since 
the SAM was including in the design anyway, they disconnected the 6847 from 
the address bus and used the SAM to add more video modes, just because they 
could.

Regards,

-- 
|              Mark McDougall                | "Electrical Engineers do it
|  <http://members.iinet.net.au/~msmcdoug>   |   with less resistance!"