nybblesPerRegister property Null safety

1. jrpn
2. model
3. Registers
4. nybblesPerRegister property

Registers class

1. Constructors
2. Registers
3. Properties
4. hashCode
5. helper68
6. index
7. indirectIndex
8. nybblesPerRegister
9. runtimeType
10. Methods
11. clear
12. incrementI
13. isZeroI
14. noSuchMethod
15. resetI
16. toString
17. Operators
18. operator ==
19. operator []
20. operator []=