This is news to me. I thought all of them were implemented. I know for a fact that LAX and DCP are implemented because I'm using them in Atari Climber. I'm think I'm using NOP zp too but I'd have to go through the source to be sure.
I suppose I will see if I can recode my game to use regular opcodes while still fitting in 1k of space, as ARR and SBX are seldom used anyway. But still, if 100% compatibility with the 2600 is the goal with the Rev. C FB2, maybe the FB2 engineering team should consider implementing all undocumented opcodes, even the most obscure of them.
I think it's true. My game Zirconium definitely does not work on the FB2 (crashes after only a few frames) so I recoded it to take out SBX. Then it worked but had glitches in the code where I used ARR (calculating score pointers). But the changes require space that I don't have to spare in the 1k.
I use DCP, LAX, ISB, and SAX and they work just fine. But these are all in the same family (combinations of two nearby instructions.) Maybe the FB2 team did this family only.
SBX and ARR are in a different family, which includes ASR, ANE and LXA. Perhaps none of these were done. Granted, some of the opcodes in this family (ANE, LXA) I think are unpredictable and therefore basucally useless, and SBX does some really funky things if you happen to be in decimal mode. Regardless, I think SBX is useful, and perhaps the most useful in this family.
I said SBX was obscure since I've never heard of anyone else actually using it in a game, but I have seen it mentioned before. I used it twice in my game, actually, and I used it in the first bB kernel until people reported problems with PCAE (which, of course, is a limitation of PCAE but I opted for compatibility in this case.)
In my game, I have a loop like this:
;do some stuff here
TO recode, you need to do this:
;do some stuff here
the whole routine above was in a loop that was executed up to 10 times, thus saving 2 bytes and up to 160 cycles.
I wonder if they bothered with SHS, SHA, SHY, SHX and the like, which are also in a family of their own. I doubt anyone has used these yet, but I think Supercat has at least found a possible use for some of them.
Anyway, it wouldn't be hard to write some code to test each instruction and report the results somehow... Maybe I'll do this.