[atari7800] Re: Questions about character/indirect mode

----- Original Message -----
From: "Eric Ball" <ek-ball@xxxxxxxxxx>
To: atari7800@xxxxxxxxxxxxx
Subject: [atari7800] Re: Questions about character/indirect mode
Date: Fri, 29 Apr 2005 22:56:09 -0400

> 
> From: "Dennis Debro"
> > I understand that for direct mode your graphics must be stored upside down
> > and separated by a page. Does character mode act the same way? I did a
> > disassembly on Ms. Pac-man and the characters in this game are not stored
> > upside down and they are stored similarly to the 2600 or A8.
> 
> Weird.  No, the graphic data for indirect mode is handled the exact same way
> as for direct mode (one line per page, bottom line first).  See my paddle
> demo (which uses indirect mode for the digits in a very, uh, interesting
> manner).  The address in the display list entry points to the first
> tile/character.  The value of each tile is used as the LSB part of the
> sprite address, and the CHARBASE register is the MSB.  It's then handled
> exactly the same as a normal direct mode sprite (including Holey DMA).
> 
...
> 
> As suggested, they must copy the graphics data to RAM for use.  (This also
> explains why there isn't any recognizable graphics when I look at the ROM
> with my a78sprite tool.)  Note: according to the docs Holey DMA doesn't
> apply for addresses < $8000, so they must adjust the graphics data for
> vertical motion - ick.
> 
> One possible reason they did it this way is to squeeze the game onto a 16K
> ROM; so they don't have to waste 4K of ROM space for less than 256 sprites
> of graphics data.

That's a good explanation.  It's also possible that Holey-DMA wasn't working 
yet when this game was being programmed.  The "Maria-2 Handy Hints" from 
October 1983 suggests that Holey-DMA was a new feature in the 1702 version of 
the chip.  I wouldn't be surprised if Ms. Pac-Man was programmed early on, 
given GCC's relationship with the arcade game.  It could have even served as a 
testbed while the chip was being developed and revised.
-- 
_______________________________________________
NEW! Lycos Dating Search. The only place to search multiple dating sites at 
once.
http://datingsearch.lycos.com

Other related posts: