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Re: Mystified by an Atari 2600 demo.

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On 2016-06-08, Chad <cdalten@gmail.com> wrote:
>
> In the demo, the guy enters the coordinates(?) to a sprite. Right
> after that, this sprite then gets displayed on a screen. Right after
> that, he is able to do things like type text and clear the screen.
> This is done without impacting the sprite. 
>
> The question is how can such a primitive system display two things
> independently of each other? To the best of my knowledge, the system
> didn't have threads nor multiple processors. The only clue I got was
> the guy said that hardware based sprite generators were something the
> engineers came up. 

But that's where you are wrong; there are in fact multiple processors.

One processor is the main CPU.

The other processor is the video display hardware which scans video
memory and generates a signal.

This video display hardware takes into account the hardware sprites that
it is configured with.  As it scans through the pixel positions to drive
the monitor, any position corresponds to some piece of video memory, and
possibly to one or more sprites. It determines what color has to be
displayed: that of the topmost sprite, or else the background from the
video RAM.

The text doesn't interact with the sprite because the compositing
of the sprite is a nondestructive function done in the video pipeline,
and not in video memory. The sprites live outside of video memory
somewhere and are just superimposed virtually. The hardware re-draws
them 60 times a second (or whatever) and tracks their movement
as their position is reconfigured. The sprites show no matter what
the computer does to the video memory, like clearing the screen,
drawing characters or lines or whatever.

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Thread

Mystified by an Atari 2600 demo. Chad <cdalten@gmail.com> - 2016-06-08 16:05 -0700
  Re: Mystified by an Atari 2600 demo. Kaz Kylheku <545-066-4921@kylheku.com> - 2016-06-08 23:51 +0000
    Re: Mystified by an Atari 2600 demo. Chad <cdalten@gmail.com> - 2016-06-08 19:10 -0700
      Re: Mystified by an Atari 2600 demo. Kaz Kylheku <545-066-4921@kylheku.com> - 2016-06-09 04:18 +0000
  Re: Mystified by an Atari 2600 demo. Kaz Kylheku <545-066-4921@kylheku.com> - 2016-06-09 00:01 +0000
  Re: Mystified by an Atari 2600 demo. commodorejohn <commodorejohn@gmail.com> - 2016-06-09 09:47 -0700
  Re: Mystified by an Atari 2600 demo. "J. Clarke" <j.clarke.873638@gmail.com> - 2016-06-10 20:41 -0400
    Re: Mystified by an Atari 2600 demo. Daniel Pitts <usenet@coloraura.com> - 2016-06-11 12:47 -0700
      Re: Mystified by an Atari 2600 demo. "J. Clarke" <j.clarke.873638@gmail.com> - 2016-06-12 00:00 -0400
        Re: Mystified by an Atari 2600 demo. Daniel Pitts <usenet@coloraura.com> - 2016-06-12 21:08 -0700
        Re: Mystified by an Atari 2600 demo. Randy Howard <rhoward.mx@EverybodyUsesIt.com> - 2016-06-13 04:12 -0500

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