The subject of run-time non-valid instructions might appear to be of no practical interest, but maybe this question lead us to a very interesting aspect: how is the instruction decoder implemented in a particular 8051 family member?. Why might one valid instruction (that is, one that agrees with syntax and uses a valid address mode) fail in execution-time?.

I don´t know if the following question has been already considered in this forum (I am here since a couple of monthes only): I wonder why there is no 'dec dptr' instruction, but one reserved machine code does exist. Maybe the first implementation included a dec dptr that failed to work (using that reserved code?).

Alfredo.