c6000 emif Capacity

in c6416 EMIFA IS 64bit data Width& (3-23bit or 20bit) address Width.

2^20=1MBIT===>memoryaddressingcapability !!!!------>((2^20)*64BIT)=64MBIT memory space & no 256Mbye space.Why is address lines is 3 to 23?Where is the addresses 0 to 2??are they internally connected??

but Ithas 4*CE with 1024MByte Addressablespace(4*256Mbyte).How is it possiblewith20address lines???Is it reallyallthat spaceisavailable?Whatisthemeaning of256MByte,howit is possibleto do this with20address lines??

i want connect c6416 EMIFA to external sdram(4*32x*512k).acording to ((TMS320C6000 DSP External Memory Interface (EMIF) Reference Guide)) page 48 for 64MBit sram(4*32*512k) Addressable Space is 8Mbyte?Whatisthemeaning of8MBYTE??

Thank you  for your attention