What type of FLASH (NAND or NOR) is used on microcontoller?
Undoubtly, for program memory, the memory of choice is NOR Flash. NAND flash has several issues that make it unsuitable for program storage (NAND has to be accessed in blocks and has the bad habit of corrupting some of its data, so a management system is needed to keep track of blocks usage and of bad records. You don't want to put your program on an unreliable media).