attachInterrupt on Pin 21 & 20 won't work? (Arduino Mega 2560 R3)

Hi,
I would like to make closed loop stepper code for x,y,z router
i saw your codes for rotary encoder x,y,z that look perfect, may use this code for my stepper motors
or if you can send me stepper codes related to your encoder code files.

With best regards,
Suh