< Then the high order half of X would be $FF. Not sure if thta happens on <other versions of the 6809. Then there are all the undocumented opcodes o <the 6502 ... 8080, 8085, z80, z180, z280 and others also have undefined or irregular opcodes that are either unsupported or unofficially supported (all versions have them but they may not ;-)). The 8085 and z80 ones are most unique as I know of no version that doesn't support them and it's across vendors (even those that dont swap masks!). So It' surprizes me none at all to see that in moto chips or any other. A few emulators like MYZ80 do infact support all the z80 ops faithfully but thats an example of a well tested package and a stable set of opcodes. The 6502(and cmos versions) are legions for odd and spotty opcodes off the basic set. I know the 6800 family vary some as do the 6809. It's my understanding it was years before the 68000 instruction set was stable. Emulators, useful, interesting but NOT the real thing. Allison