Exponentiation mod n - the program

I discussed the logic for this program in a previous blog. Here is the logic converted into a program for the fx-50F:-

Lbl 0:?→X:?→A:?→B:While X≥B:X-B→X:WhileEnd:X→Y:2→C:1→D:
If C>A:Then X→D:Goto 2:IfEnd:Lbl 1:X²→X:While X≥B:X-B→X:
WhileEnd:2C→C:C≤A=>Goto 1:C÷2→C:A-C→A:DX→D:While D≥B:
D-B→D:WhileEnd:Y→X:2→C:C≤A=>Goto 1:A=1=>DY→D:While D≥B:
D-B→D:WhileEnd:Lbl 2:D▲Goto 0:

This program is 163 steps in length.