Re: [stella] VDEL

Subject: Re: [stella] VDEL
From: Eckhard Stolberg <Eckhard_Stolberg@xxxxxxxxxxxxxxxxxxxxx>
Date: Tue, 25 Jul 2000 22:33:54 +0200
At 11:35 25.07.2000 -0700, you wrote:

>Could someone write an overview of how VDEL is actually used in a kernel as 
>a vertical positioning system for me in such a way that I can just 
>paraphrase it into a voiceover?  It seems that would be the easiest way to 
>do it.

OK, lets define the situation:

Each player graphics has two registers GRP0-A and GRP0-B, GRP1-A and GRP1-B.

The VDEL register for a player decide whether the A or the B register gets
used for display in a given line.

The graphics from the A register gets copied into the B register when
some data gets written into the A register of the other player.

Therefore in a double scanline kernel you would in every scanline:
   check if the player has to be displayed.
      if so, check which line of graphics has to be displayed ...
          and write that out.
      if not, write out a blank graphics byte.

You alternate between players in every scanline.

Now lets have a look at two double scanlines. All four graphics registers are
set to $00. GRP0 gets handles in the first scanline and GRP1 in the second
scanline of each double scanline set:

If you want to display player0 in line 3, you would turn off it's VDEL
register and set it's verical position counter to the second line-set.

If you want to display player0 in line 4, you would turn on it's VDEL
register and set it's verical position counter to the second line-set.

If you want to display player1 in line 3, you would turn on it's VDEL
register and set it's verical position counter to the first line-set.

If you want to display player1 in line 4, you would turn off it's VDEL
register and set it's verical position counter to the second line-set.


>I'm including a file here which is a shockwave flash that is supposed to 
>illustrate a 4-bit polynomial counter.

That one didn't make it. 

>I would like to expand on this diagram to show how the TIA drives the 
>speaker with this type of counter, and how it performs sprite positioning 
>(via coarse and fine registers) in combination with the binary/decimal -> 
>poly value code.

Sorry, I can't help you here. Did anyone here ever figure out how the
poly counters relate to the TIA register?


Ciao, Eckhard Stolberg



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