Re: Unknown holes in the motherboard of the CBM610

Den Wed, 2 May 2018 10:13:53 +0000 skrev "Baltissen, GJPAA (Ruud)"
<ruud.baltissen@apg.nl>:
> Hallo Mia,
> 
> 
> > The 18MHz clock is needed for the pixel clock anyways
> 
> I completely forgot that. But OTOH, there are a lot of systems that
> use two different clocks; one for the video and one for the system. 

Yes, but more than one clock just makes things more complicated.

> > but I see no reason for throwing away that possibility to control
> > timing exactly.
> 
> We'll see what can be done. But I know one possible reason to keep
> it:  what did come to my mind is the so called "snow effect" on video
> cards when writing directly to the video memory. A good example is
> the CGA card. But for one or another reason the MDA card doesn't
> suffer this problem. One possibility to solve the problem is to halt
> the CPU the moment it wants to write to the memory when it is read by
> the video system at the same time. But I'm quite sure the MDA card
> doesn't do this. If anyone knows how snow can be avoided in other
> ways and want to share this knowledge, please be welcome!

The way to avoid snow is to have fast enough memory and split the
access "time slots" between CPU and video so video never gets "locked
out" of it's own cycles. The B series does this by letting the CPU
access memory while the 2MHz clock is in one state, and video accesses
memory when the clock in the other state. 8296 runs memory at the same
speed and lets video do two accesses in the same time that the CPU does
one access. The older 8032/8096 uses 16 bit wide video memory for video
to be able to read at 2MHz rate for 80 columns. The older PET's and
VIC-20 is just like the B series but at 1MHz. C64 & co is a special
case as it has an internal 40 word cache (where a word is 12 bits, 8
bit char and 4 bit color ram, or in hires 8 bits color and 4 bits
discarded, or in multicolor 12 bits of color) which is read every 8th
line by locking out the CPU from the bus during 40 cycles.

Btw there were CGA cards without show. They just use fast enough RAM.
Also there were CGA cards with "less visible snow" where the snow is
always black.

But I thing that if this is changed significally then it's not really a
B machine any more :)

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