800 RAM & Graphics Modes Questions

[+therealbountybob]

A couple of my atari basic games use Graphics mode 28 and pretty much all the free bytes or RAM available on my XE.

 

Booting up atari800win+ as an OS/A 800 with 48K and doing a ? fre(0) gives the same 37902 bytes as I get on the XE is this correct

 

Were there some 800's (with the GTIA chip?) that supported GR.28 and if so how would I access it - I think there was a RAM address to set the graphics mode

 

How would someone check their 800 to see if they had GTIA - just run the poke to get GR.12 etc

 

Thanks

[Bryan]

Even 64K Atari's are set up pretty much like a 48K 800. The difference is you can disable the OS ROM and have more RAM instead, but that would only be an option for software that completely takes over the machine.

[+Stephen]

A couple of my atari basic games use Graphics mode 28 and pretty much all the free bytes or RAM available on my XE.

 

Booting up atari800win+ as an OS/A 800 with 48K and doing a ? fre(0) gives the same 37902 bytes as I get on the XE is this correct

 

Were there some 800's (with the GTIA chip?) that supported GR.28 and if so how would I access it - I think there was a RAM address to set the graphics mode

 

How would someone check their 800 to see if they had GTIA - just run the poke to get GR.12 etc

 

Thanks

That is BASIC mode 12 with no text window. To use that on the old 400/800 machines, you would have to manually set up the Display List, screen RAM, etc. If I am not mistaken, you want ANTIC mode 4 and need to allocate 40 bytes per line.

[Rybags]

The Atari-supplied 400/800 OS versions don't allow Graphics modes between 12-15. As stated, you need to setup your DList manually.

 

The Supermon OS, and possibly some other 3rd party OSes do support the extra modes though.

 

Best bet is to not use those modes, setup your DList manually otherwise you'll end up with something that will only work on XL/XE.

 

Gr. 12 addressing is exactly the same as for Gr. 0 anyway, so you're not really losing out. Although having the distinctly seperate text window not affecting the scrolling can be handy.

[Synthpopalooza]

Best way to test for GTIA:

 

Set up an ANTIC 4 display list with text on it, do POKE 623, 64, then place a player over the text. Check the collision register for that player. If it registers a collision you haven't got GTIA. If it doesn't register a collision, you do have GTIA. That's because the collision register won't work in GTIA graphics.

 

GTIA really has nothing to do with Graphics modes 12-15 anyway (only GRAPHICS 9-11). Those modes were just included in the XL/XE OS to make it easier to access them, while you had to manually alter the display list in the old OS in order to access them.

Edited November 10, 2010 by Synthpopalooza

[Rybags]

I think it's more to do with Atari's early witholding technical information policy.

 

The "good stuff" happens in 12/15, and maybe 13 to a lesser degree.

 

The only real difference from the OS POV is a patch or two in the code and maybe a couple dozen or so bytes worth of table information.