[atari7800] Re: [personal] Holey DMA question

These are some good questions. I wish I had a 7800 dev system readily
available so I could try these things out. 

Dan

-----Original Message-----
From: atari7800-bounce@xxxxxxxxxxxxx [mailto:atari7800-bounce@xxxxxxxxxxxxx]
On Behalf Of bob.montgomery@xxxxxxxxxxx
Sent: Monday, March 17, 2008 12:23 PM
To: atari7800@xxxxxxxxxxxxx
Subject: [personal] [atari7800] Holey DMA question


Thanks Dan.

And another, similar question:  What happens if you turn on 16-line holey
DAM and 8-line holey DMA at the same time?  Here's the Maria guide
again:
      H16           -    16 high zone Holey DMA enable.
                         0 => Not enabled.
                         1 => Enabled. DMA interprets odd 4K
                         blocks as zeros. (A12 high => data=0)

      H8            -    8 high zone Holey DMA enable.
                         0 => Not enabled.
                         1 => Enabled. DMA interprets odd 2K
                         blocks as zeros. (A11 high => data=0)

So would it mean that if A12 or A11 was high the data would be read as zero?
I.e., you would have 2K of data followed by 6K of zeroes, and so on.  That
could be useful if true.  

And sort of relatedly, if you are using holey DMA, you can still use ROM
below $8000 to store graphics data that is taller than 16 lines, right?

-Bob

PS Is anyone else still active on this list?  


-----Original Message-----
From: atari7800-bounce@xxxxxxxxxxxxx [mailto:atari7800-bounce@xxxxxxxxxxxxx]
On Behalf Of Dan Boris
Sent: Saturday, March 15, 2008 5:22 PM
To: atari7800@xxxxxxxxxxxxx
Subject: [atari7800] Re: [personal] CTRL register RM bits

That's a good question, haven't heard of anyone trying that. If you look at
the GCC1702B MARIA Chip specs document it lists the 160 modes like this:

MODE            WM      RM1     RM0     CRA4    CRA3    CRA2    CRA1
CRA0

160(A)  0       0       X       P2      P1      P0      D7      D6

160(B)  1       0       X       P2      D3      D2      D7      D6


From this we can assume that setting RM to 1 is the same as setting it to 0.

Dan

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