[atari7800] AW: Re: DMA TIMING

  • From: "Matthias" <matthias@xxxxxxxxxxxx>
  • To: <atari7800@xxxxxxxxxxxxx>
  • Date: Tue, 9 Nov 2004 23:40:49 +0100

Ok, if I understand it correctly than you have a max. of 8 colors in 320b
mode, right?
How do I have to handle the color registers than?

 > -----Ursprüngliche Nachricht-----
 > Von: atari7800-bounce@xxxxxxxxxxxxx [mailto:atari7800-
 > bounce@xxxxxxxxxxxxx] Im Auftrag von Eric Ball
 > Gesendet: Dienstag, 9. November 2004 13:29
 > An: atari7800@xxxxxxxxxxxxx
 > Betreff: [atari7800] Re: DMA TIMING
 > > > FWIW, MESS doesn't work according to spec, it reads all three palette
 > bits
 > > in 320B mode not just the high bit.
 > >
 > > Whoops.  Let me verify that with the CC2, then I can fix
 > vidhrdw/a7800.c
 > I just had an AH-HH! moment about this.  MARIA has two 800 bit line RAMs,
 > so
 > each 160H pixel has 5 bits as a color LUT index.  The 320H modes don't
 > change how the line RAM is written to, just how it is read.  Which,
 > unfortunately, means there are only 2.5 bits per 320H pixel, or max 8
 > colors
 > per pixel with one bit shared between the two half pixels.
 > It's unfortunate that the MARIA designers didn't have enough transistors
 > for
 > 320x5 (or even 320x6) bit line RAMs, but there is a little bit of genius
 > in
 > how they did manage to squeeze the 320H modes out of the 160H line RAM.
 > I
 > also wonder whether this means there are a couple more 160H modes hidden
 > in
 > there.

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