Seeking explanation for 2-port BIOS game incompatibility

[+Ryan Witmer]

Another topic came up mentioning this, and it's been bothering me for a long time so I'll throw this out there.

 

Does anyone know specifically why some games are incompatible with the 2-port BIOS?  I'm asking this from the perspective of a programmer because I'm paranoid that I'll fall into whatever trap the incompatible games got stuck in.  I don't have a 2-port 5200 to test on, so this is of great importance to me.

 

I've seen disassemblies of both BIOS variants and as far as I can tell, the only real difference with the 2-port is the addition of PAL detection code.  This code will lock the system in the case of a region mismatch and it's long been my hypothesis that the incompatible carts were being misidentified as PAL carts.

 

So, what is it about the 2-port BIOS that causes some games to not work?

[+-^CrossBow^-]

I think it has something to do with the controller mappings as well. I say this because when I've used the atari diagnostics cart on 2 port units, it detects the 2nd controller as being controller 3 I believe in the diagnostic test whereas on an actual 4 port unit is will detect it properly as port 2. So besides the hardware, I believe the bios was modified as well to try and get around this. But some games don't seem to like it too much. 

 

Again, it could be completely unrelated, but I know that port 2 on a 2 port unit does NOT show up as port 2 being in use on the older diagnostics testing. I can't recall if it works properly on the Pete's Test cart though.

 

[+Mitch]

There are only three carts with issues on the 2port. The reason is that they are making illegal references to the original bios.  The locations that they were refering to got changed in the 2port bios.

 

Mitch

[Shawn]

If you want a copy of the 4port bios for your 2 port system I can make you a copy. It's simple to replace as the BIOS chip is socketed on the 5200 motherboard. The 2532 eprom is a pin for pin compatible replacement. No messing around with rewiring anything.

[+Ryan Witmer]

4 hours ago, Mitch said:

There are only three carts with issues on the 2port. The reason is that they are making illegal references to the original bios.  The locations that they were refering to got changed in the 2port bios.

 

Mitch

This was the only other explanation I could think of, but I thought the PAL thing was more likely.  Some code did move around with the addition of the region check, so I can totally see this happening.  So it sounds like as long as I only touch the parts of the BIOS that you're supposed to touch, like the interrupt routines, all should be well.