It's per memory cell. And that's what all modern memory does. Ignoring chop (afaik, no modern widely used memory controller makes use of it), the smallest reque

It's per memory cell. And that's what all modern memory does. Ignoring chop (afaik, no modern widely used memory controller makes use of it), the smallest request you can make from a 64-bit DDR3 stick is 64 bytes. All those 512 bits are accessed in parallel, and when done, sent over the memory bus over 8 bus transfers. While the controller is waiting for those bits to become available, you can issue commands to different banks within the same chip. (And in fact must do so to get anywhere close to the nominal bandwidth). Modern memory controllers interleave requests both between banks of the same chip and between different chips on the same channel.