Fw: Re: Fw: Re: Modern myths

From: Todd S Elliott (eyethian_at_juno.com)
Date: 1999-04-23 01:49:25

Another response by Jim Butterfield. I will post the earlier response
very soon.

-Todd Elliott

--------- Forwarded message ----------
From: Jim Butterfield <fjb@freenet.toronto.on.ca>
To: Todd S Elliott <eyethian@juno.com>
Date: Thu, 22 Apr 1999 11:34:29 -0400 (EDT)
Subject: Re: Fw: Re: Modern myths
Message-ID: <Pine.3.89.9904221104.B7311-0100000@queen>

 Good for Andre ...

OK, have looked into stuff a little more.  My original response can be 
set loose, but I'll offer the following revision...


Looking further at the original PET interfaces:

Levak's confusion regarding the PIA chip may stem from this:  That 
although the EOI-input bit he identifies (bit 6 at 59408) has NOTHING to 
do with the screen ...

  ... the EOI-output bit (controlled by bits 3 to 5 at 59409) was also an

OUTPUT control to blank the screen.  Early users of disk drives may 
recall the screen "blinking" when they wrote to the drive.

  On the same chip (at 59411, bit 7), the retrace-interrupt flag is 
found; I don't recall this flag being used for anything.

  Yes:  in early 1979, Commodore changed the cicuitry in this area.  To 
quote from an article by Jim Russo in The Transactor, V2, N1, May 1979 
(at that time, The Transactor was published by Commodore) dealing with 
changes between the original 8K PET and the newly released 16/32K PET:

  " - The signal which blanks the video on the 8K is not connected on the

16/32, so POKE 59409,52 no longer works.  The ROM routines still 
reference this address but the required hardware seems to have been 

  Note that we're talking about an output bit here, which has nothing to 
do with detecting whether we're in retrace mode or not.  As I previosly 
noted, that's on the VIA at 59406.  I would have thought that a 
self-styled qualified engineer would have noticed the difference between 
input and output circuitry.

  The video-blanking circuitry was popular with game-writers .. you could

get a good explosion effect by blasting the screen on and off.  It was in

no way related to the later C64 screen-blanking feature.


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