SDRAM refresh time
basically refresh is re-charging the caps inside the SDRAM, you can do it using internal counter, by issuing refresh command every 64mS, which is setting up row address. Assume you do video decoding in this case yo are constantly accessing to the SDRAM sequentially, row by row, which is eventually, selecting each row, now in any kind of video processing you need to have at least two buffers, one buffer is using to download your stream, other buffer you playing , in case in MPEG you will have up to 4 buffers, which you are constatlly accessing , and you do full buffer scanning
not like if you executing code and stick in the loop and sometimes executing some subroutines