AT9G45 Only have two ddr bank line,but 256MByte have 8 bank.how at9g45 drive 256MByte DDR (8BANK)?

The "at91sam9G45-Ekes" and "at91sam9G45M10-EK" for example, uses 4 x 64 MB DDR2 chips, these memories have four banks each. And it uses a 32-bit interface routing the lower 16 bits to a chip, and the high 16 bits to the next memory chip. Remember you have two DDR2 memory controller to use (2 x DDR2 memory controller).