sorry, I'm getting confused here....
writing, is the same as reading and no different for sram or sdram
using SDRAM rather than SRAM is down to how the array is allocated... eg.
static int8_t _array[LENGTH] __attribute__ ((section (".sdram")));
is this what your after?
I guess Im totally missing your point, as this was the same thing you asked and answer in a previous thread
size available are included in here: