https://www.embedded.com/flash-101-nand-flash-vs-nor-flash/
https://en.wikipedia.org/wiki/Execute_in_place
XIP：
- The storage must provide a similar interface to the CPU as regular memory (or an adaptive layer must be present).
- This interface must provide sufficiently fast read operations with a random access pattern.
- The file system, if one is used, needs to expose appropriate mapping functions.
- The program must either be linked to be aware of the address the storage appears at in the system or be position-independent.
- The program must not modify data within the loaded image.