However, you may not want to rely on undocumented behavior. If the code could be run on an emulator, I don't think it's fair to expect an emulator have to replicate undocumented behavior.

Why not? If real hardware consistently produces same results and they happen to be undocumented, then they ...

Faster, shorter, no decimal mode problems.

1536: PHA
1537: LDA #2 ;color value
1539: STA 53272 ;store color register
1542: CLD ;clear decimal mode
1545: ADC #2
1548: STA 1538 ;modify the code itself
1545: PLA
1546: RTI Lee.

That's just one made-up example. Most of the games/applications are ...

Hi AtariKSI, and welcome!
STZ direct is opcode $64. At http://visual6502.org/wiki/index.php?title=6502_all_256_Opcodes we learn that this is indeed a NOP on the NMOS 6502 - it is unspecified behaviour, but turns out to be a nop. It's a 3-cycle NOP and you'll see that there are some 2-cycle NOPs ...

AtariKSI, I'm not familiar with the systems you're talking about but I'm a bit surprised that the ISRs don't clear the D flag if they need it clear. I can see your point though about saving the instruction time since they're interrupting 15,000 times a second on a 1MHz machine. But it does bring up ...

I'll look at that and see if that'll work also on the machines I have. It looks he is also using undocumented opcodes and a bunch of them.

The PLA and RTS are because this function is supposed to be called form BASIC, via something like A = USER(1536, B, C, D), where B, C, D are 16-bit integers passed on the stack. $D4 and $D5 are used to to set up the return value; see http://www.atariarchives.org/cfn/12/02/0091.php .

The original ...

I would enjoy learning how your proposed method functions, and discussing any pros & cons. It's true I'm not an Atari expert, and the same is true for a significant portion of the readership here. Would you still like to engage our interest as a group? This is a friendly bunch, and that's how we ...

Welcome AtariKSI.
I must say I find your proposed technique rather opaque, especially since you haven't included comments. As a courtesy, and since you're asking for assistance, it would be helpful if you were more informative.

Only code I wrote is "STZ 213" which is self-explanatory. And I did ...

However, you may not want to rely on undocumented behavior. If the code could be run on an emulator, I don't think it's fair to expect an emulator have to replicate undocumented behavior.

Why not? If real hardware consistently produces same results and they happen to be undocumented, then they ...

I wrote a 6502/65C02 cross assembler within MPDOS Pro (PC side) and wanted the fastest code to detect whether a 65C02 was present in any 6502 based system. On Atari 5200/800 systems, I have been able to test using this code:
$600: PLA
STZ 213
RTS

D=USR(1536) : REM call from BASIC and D returns ...