HP-41: upper bits in module ID fields

[hth]

Thank you for the information.

The background is that I need a way of telling if a module page contains a secondary FAT. That secondary FAT structure can start anywhere in bank 1. Its root pointer is the word at page address 0xfc2 (the address before the bank switchers). However, that location will obviously contain something else in a module that is not aware of the scheme, so I use the upper bits of 0xffe to indicate whether that location is such root pointer or not.

I take from the comments above that the upper bits in 0xffe are "free", though some occasional module might have bits set there. I guess that I just have to be incompatible with those then.

I forgot about the potential issue with 41CL, good that you pointed it out. Luckily it seems the word at 0x5ffe is 0x0014, so it is safe in this regard.