[stella] Overscan

Subject: [stella] Overscan
From: Glenn Saunders <cybpunks@xxxxxxxxxxxxx>
Date: Tue, 20 Jun 2000 21:50:36 -0700
At 11:04 PM 6/20/2000 -0500, you wrote:
The 128 bytes of memory is in the PIA.  The PIA contains the 128 byes of
memory, the timer, 2  8-bit input/output ports(all configureable in/out,
but the 2600 circuit board requires some to be input to prevent
contention), and interrupt logic.(which is not used)

So the PIA stores the TIA hardware registers as well as the 128 bytes of regular RAM?

The IEEE diagram shows all the registers inside the TIA (playfield shape, horizontal motion, and so on.

Physical location of components may not be important for programmers, but this is important for me in order to present proper visual diagrams...

I figured out the entire memory map a while ago, and could do it all up
again if you need it.  There are several 'images' of things in the 2600

Is this in the archives?

I'm fuzzy on this, but I think that the overscan does just that, keep
going right(if looked at from the front).  Then it turns around, and goes
left.  The stella manual doesn't mention this(or the hsync, color
burst) because it is not needed for programming, and it may unessecarily
confuse a programmer with little hardware background.

But I'd like to show a scanline draw the image visually. I can pause the animation to indicate various milestones (like WSYNC) and then continue the beam if it is still being drawn just to show people what happens at each point.

huh?  Sorry, I don't follow the question.  I think the answer may involve
the exact decodeing of the horizontal position counters, which we don't
know because we don't know what kinda of counter used(normal, LFSR, and if
LFSR, which version), nor do we know what the decoding was.

I might be able to figure it out by looking at the VCS output on the vectorscope/waveform monitor and comparing it against regular blackburst.

I don't think the beam starts to retrace at 76.  That is where the TIA
lets the processor out of halt(end of WSYNC).  I don't know if the
backgroud color is drawn past this point, but if it is I would be
surprised because it would make sense for the hardware to start the hblank
at that time.  Remember that the blank includes the overscan, sync, and

Well, from my video background, I do know that overscan (from my conception of the word) indicates active picture which may or may not fall underneath the monitor cowl from TV to TV. Beyond a certain maximum point it HAS to go retrace, though, which is almost assuredly beyond the visible portion on ALL TVs, but that is NOT at TIA pixel 160.

If you play your 2600 and then go back to watching TV you'll see that the right hand edge of the picture which was solid black or background color while playing the 2600 is full of regular picture information.

That's where the whole TV-safe area thing comes from. When I create framestores for the Toaster they are at a 752x480 resolution. The horizontal resolution, of course, is artificial for analog but necessary for digital video. This translates into a lot of unseen picture space on my Hitachi TV, so I need to constrain the visual area to a 4-sided letterbox, just as the 2600 does.

As has been previously discussed, on most TVs you can safely expand the 2600's vertical resolution a little taller on the top and bottom without screwing up the NTSC signal. NTSC is 525 lines, of which I believe 482 can have visual information (this translates into 241 2600 "noninterlaced" scanlines). Beyond that I think it requires sync information. I believe the same is true with the horizontal. I think the 160 pixels of the 2600 translate into 640 pixels in a Toaster framestore, leaving 112 (digital) pixels/2 for left and right "overscan".

I think there are settings for ANTIC on the Atari 8-bit which will expand the width of the screen to allow drawing further into horizontal overscan, but the 2600 doesn't allow it.

Glenn Saunders - Producer - Cyberpunks Entertainment Personal homepage: http://www.geocities.com/Hollywood/1698 Cyberpunks Entertainment: http://cyberpunks.uni.cc

-- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/

Current Thread