Well, yes - FF can be either a data byte, part of the instruction (not only MOV R7,A, but also as ) or unused space. But, then, the unused space can be filled by ANY character - this depends on the device programmer ("burner") software and the programmer's attitude (indeed, in one product, I deliberately filled the unused space with garbage, to deter potential intruders; even better would be to fill it with real program sequences... :-) ).

Nevertheless, I still wonder WHY do you want to write a disassembler - have you seen Jeff Post's D52, just one click away from here (see "Disassembler" in the column on the left)? You hardly can beat that with your own in a reasonable time - there are hours and hours of work put into that one already. Actually, it even does have sort of a "autodetection" of unused space (all FFs), which can of course be overridden... Or do you have some particular reason not to use it?

JW