Re: [stella] Grid demo

Subject: Re: [stella] Grid demo
From: John Saeger <john@xxxxxxxxxxx>
Date: Tue, 27 Oct 1998 17:59:07 -0800

Erik Mooney wrote:
> 
> >VCS, since the emulators can't display it correctly. I hope,
> >that someone finds out how the multi sprite thing works, so
> >that Bradford can update his emulator.
> 
> How it works?  I thought we knew that.  If NUSIZ0 is set to two or more
> copies of a sprite, and RESP0 is hit during a scanline, the second and
> later copies of the sprite will display on that line.

Generally speaking, this is true for the second and later hits of RESPx
on a line.  The first hit on a line generally gives you all of the
copies including the first.  But there are some subtleties having to do
with the so-called object wrapping.  Like what if it's already
outputting a multiple copy from a late RESPx on a prior line.  If so,
sometimes the first RESPx on the line acts like a second hit, and
sometimes like a first hit, depending on the timing.  If you do it just
right, TIA has trouble making up its mind!  You get an unstable sprite. 
On my 2600 at least, the unstable sprite is initially invisible, but
fading into visibility over time.

Both of the attached .bins display unstable sprites on my 2600.  On my
2600, when you start unstable.bin, columns 10-17 are blank.  But after
running for a while, letters begin to appear, but not completely.  You
can see portions of the letter that are between columns 11 and 16 (start
counting at 1).  Not the dots at the corners.  Funny, the S letters show
up completely, but the E letters don't.  At least this time.  In the
past, I think I've seen the E letters completely too.  Maybe because
it's winter here?  Cooler?  When I start unstable2.bin, there is also
blankness, this time between columns 13 and 20.  After a while, you see
the dots in the corners of the letters at columns 13 and 20, but not the
interior.

My guess is that these programs demonstrate the ragged edge of where the
TIA decides whether to treat a RESPx as a first hit or a second hit. 
Any first hit that comes later than what this program does will probably
always be treated as a first hit.

John

P.S. I've been running unstable.bin for around 10-15 minutes, and the E
letters are beginning to fade to completion!  So they DO show up.

Attachment: unstable.bin
Description: Binary data

Attachment: unstable2.bin
Description: Binary data

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