if rupa is memory then what is saññā?
This is not true. Please read, Difference between physical rupa and rupakkhandha.
What is saññā? Read, What is Sanna?
Saññā is the recognition of something based on our memory.
In contrast, Vedana (vipaka vedana), means becoming aware of something.
And I also wonder how buddha can recall his past name?
This is possible through abhinna powers. with which one can recall past memory records stored in nama gotta.
All rupa we have ever encountered extending to infinite past time is there in nama gotta.
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For Details of gandhabba concepts, study, Living Dhamma – mental body gandhabba – section
If you have not, please read the section Key Dhamma Concept – five-aggregates-pancakkhandha Section First.
After studying above section more details are in Key Dhamma Concepts – nama&rupa to namarupa section