Yes, you need to map the Flash to reset vector address. Your NFlash(NOR/NAND) may need to be configured to fetch instruction at address 0. Regarding choosing the higher address or not, it is upto the memory map. The memory map address 0x00000000 is reserved for the vector table, a set of 32-bit words. On some processors the vector table can be optionally located at a higher address in memory (starting at the offset 0xffff0000). Operating systems such as Linux and Microsoft's embedded products can take advantage of this feature