Re: PET 4032 video RAM size/mirrors

From: Larry Anderson (
Date: 1998-03-27 20:37:26

Andre Fachat wrote:
> Hi!
> I am wondering, having found no hard facts for my petindex:
> The 4032 machines (with the upgradable-to-8032 board? are there
> 4032 without CRTC (I think they are mentioned but not confirmed,
> although 4.0 ROMs might have been applied...)), or any 4032
> in general, what video RAM size do they have? 1k or 2k?

ALL the PETs with the 12"CRT have the newer CRTC controller.  
The other 4032s mentioned probably are 9" display units without the
newer CRTC. (note there are some programs that are written for 4.0 ROMs
with the older CRTC, and thus only work on 9" 4.0 PETs.)  Yes there are 9"
display models labeled in the 40xx series.

I would assume the video RAM is 1k on a 4032 unless Commodore was feeling
generous at the time as the 8032 upgrade consists of an extra RAM chip
and an updated ROM with the 80 column screen seed settings.

> Is this memory mirrored 2 times ($8000 and $8800, or even $8000 and
> $8400) or 4 times (if 1k, $8000, $8400, $8800 and $8c00).

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.

> 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?
> (should be $e811,52/60 if I remember correctly...)

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.

Larry Anderson - Sysop of Silicon Realms BBS (300-2400bd) (209) 754-1363
Visit my Commodore 8-Bit web page at:

Archive generated by hypermail 2.1.1.