unsigned char nb_flags;
nb_flags variable holds the interpreter mode flags. nb_flags is checked to see if any flag is set before each statement is run and, if a flag is set, the flags are serviced.
Do not extend the flags in nb_flags without understanding what the implications are. If you add another flag, for instance, to indicate some interpreter mode, setting that flag will cause the interpreter's performance to degrade because on each statement the flag will be set and the interpreter will try to service the flag. Better would be to move that flag into another variable and keep nb_flags uncluttered for best performance.