One of my ref books says a task is re entrant if it modifies itself. whatsoever reason may be. let me study and give you the exact auther and title."
abhishek...this refers to software..not hardware...
You are implying that the processor rewire's its PCB when entering a section of code..
No, Rob it is not hardware.
I have written self modifying code for the SCC990 (an old very slow minicomputer) in order to make it produce just a little bit of throughput.

An example of self modifying code would be that in a parameter driven program you analyze parametres at start and modify the code to branch around code not used since the parametre is 0.

Of course in a ROM based '51 this is totally impossible.

Erik