16K cartridges on 400/800

[peteym5]

I ran into a snag with getting one of my upcoming cartridge games to run on the 400/800 in emulation, would imagine it will be an issue on real hardware as well. Appears the 400/800 OS checks memory location $9FFC to see if it is 0, if it is, it attempts to run the cartridge at $9FFE, $9FFF. I am not sure is this is something we need to watch out for when making cartridges and like games to run on the 400.800. I had some data stored at that address and it just happened to have a 0 stored there, solution was just bump up the data address a few bytes.

 

Anybody know anything about this? I am glad I caught this one before the cartridge went into production. I know very few have 400/800 computers unmodified, but it better to be safe with these things now.

[a8isa1]

400/800 OS checks memory location $9FFC to see if it is 0, if it is, it attempts to run the cartridge at $9FFE, $9FFF.

Ah. I didn't know that's what was being checked.

 

When I had an SRAM cartridge, which was fixed at 16K, I had trouble running 8K images. I discovered if I mirrored the data of $BFFA-$BFFF to $9FFA it would work.

 

Only posting because my experience seems to confirm what you are seeing

Edited January 22, 2018 by a8isa1

[SS]

I know very few have 400/800 computers unmodified, but it better to be safe with these things now.

I don't know if I'd say that. I know of quite a few people who use stock 400/800s as their primary machines. I'd also wager that a great many of the people who use modded machines also have an unmodded one as a back-up.

[Kyle22]

Or an Incognito which may be switched between 800 and XL/XE mode.

[peteym5]

The whole purpose of my "16K for 16K" series of games is to ensure games run on as many Atari 8-bit and 5200 machines as possible. 16K ROM running on machines with 16K RAM, no OS version specific dependencies, and allow them to be ported over to the 5200. I know just about everyone has their machines expanded to 48K or more. With the aid of deflate/inflate, I am able to compact games that would require over 20K on a cartridge ROM. They are less expensive to make than larger games that need bank switching or flash cartridges.

[VladR]

Is the 20 KB->16 KB compression ratio a typical number you usually get across multiple games or just a special occurrence?

 

What kind of compression ratio you get just for code?

[peteym5]

Inflate is about 1.6 to 1.8 to 1. Only the parts that I need to load into RAM are normally compressed. How much varies between each game, some games only expand a section depending what is going on. Venture was almost a 32K game that I had to optimize some to get it down to about 27K, before compressing it into a 16K cartridge.

[VladR]

1.6:1 sure sounds great!

 

Did you write your own LZE depacker or is there a library available on 6502 ?

 

How long does it take to unpack, say, 16 KB ?

[tep392]

The process of checking for carts in the left or right slot by looking for 0 in 9FFC and BFFC is well documented in books like Mapping the Atari. De Re Atari has a good flowchart of the boot process. I like to reserve 9FFC with a non-zero value so I don't run into any surprises as things move around while I edit/debug code.

[peteym5]

1.6:1 sure sounds great!

 

Did you write your own LZE depacker or is there a library available on 6502 ?

 

How long does it take to unpack, say, 16 KB ?

Nope "Inflate/Deflate" compression is something entirely different.

 

http://atariage.com/forums/topic/264720-inflatedeflate-improving-the-speed-of-decompression/?hl=inflate&do=findComment&comment=3792369

 

If you downloaded MADS assembler package, look under examples -> compression, there you see inflate. Widely available and well use for Atari programmers. I was recently involved with some modifications that improved the speed it deflates (expands) into RAM.

[VladR]

Thanks. This is very useful!