Re: PET 4032 video RAM size/mirrors

From: Andre Fachat (a.fachat_at_physik.tu-chemnitz.de)
Date: 1998-03-28 14:05:41

Larry Anderson wrote:
> Programs I have that test for 80 and 40 column displays poke a character at
> 32768 and check to see if the same value is at 33792 thich it is on 
> a 40 column machine (with 1k?) so the video RAM is mirrored, but only
> once, the next 1k block ($8800) did not mirror. (I just checked this on
> a 2000 series PET)
> > 
> > What about the 8032? The reengineered schematics suggest it
> > being 2k, mirrored 2 times ($8000 and $8800).
> 
> My maps show a gap until $9000, so there is that possibility (sure wish I
> could test it, but that machine is in storage.)  I kinda doubt it though.

Ok, so if noone says something else, I will assume the video memory address
range 2k, with the 1k models mirroring it once (at $8400).

Someone on the list (MagerValp?) said he had aquired a 8032, probably he can 
check that? If it's not mirrored then it most probably isn't in the
3000 and 4000 at $8800 also.

> > Another thing: the PET is supposed to be able to blank it's screen
> > with PIA1 CA2 (also used as IEEE EOI line). I assume that this only
> > works for non-CRTC machines, is this correct?
> 
> You remember correctly (took a little searching to verify it).  And
> as I had found out years ago it only works on the original PETs
> (first motherboard, not 3000, 4000) doing on the later systems results
> in no change.  Doing it on an original PET does blank the screen.
> The screen blanking was used to reduce the visible 'snowing' of the screen
> during scrolling which was quite a problem on the original model.

So only the first board used the video RAM with 1MHz clock frequency, the
second motherboard used 2MHz RAM (video access during Ph2 low) already?

Then I assume the 3000 models (i.e. the upgrade ROMs) showed a bug in the 
scroll routine, because they copy a line from memory behind the screen
to the last screen line before clearing the line. On the first ROMs
you didn't see anything because the screen was blanked anyway, but 
with the upgrade ROMs on a machine where this doesn't work it would show.
(A VICE user pointed me to that one :-)
What do you think?

Andre

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