Eric Smith <eric(a)brouhaha.com> Exact hit Eric. Yes, but as a rule, mainframes and most minis catch undefined opcodes in a special exeption. Even the Intel x86 (starting Jep, the web article used by Cameron as reference is based on the Commodore 8502 used in the C64. Just lucky that Commo used the original MOS design for the 8502 :). Other 6502 compatible Processors, that didn't use the original design have different operations at the undefind opcodes. Eventualy this led to some problems Apple II programms encountered on individual clones or Apples with upgraded CPUs. Especialy when one switched for any of the CMOS types (65C02, 65SC02, 65G02, 65S02), becuse they used some of the free Opcodes for new/enhanced operations (like bitmaipulation etc.). Undefined opcodes ar implementation dependend like internal function of an operating system are version dependand. Just remember how ridicoulus the CPU detection algorythms on the x86 are since there was no standard cpu type command. Some of the enhanced chips can only be detected usind complex tryal and error strategies. I remember that a general purpose detector could take some seconds to determinate the CPU thru exeptions, timeouts, busfaults and arithmetic checking. And now try to distinguish the different 6502s. I have never seen any programm to perform this. Funy thing: History repeats with DirectX - Microsoft didn't include any version checking in DirectX 3.0 so, for example, if you want to use the enhanced sound functions later on, you have to perform time consuming operations to finaly find the needed function resulting in an error (or worse, depending on the function just performing diferent). Gruss H. -- Ich denke, also bin ich, also gut HRK