Consider a single-rank memory system that has a capacity of 8GBytes and is comprised of 16 banks - these banks are operating together (same address, control, CS...) because it's single-rank. Assume that each row in each bank has a capacity of 4Kbytes. Each row must be refreshed once in every 64 milliseconds. To carry out the necessary refresh activity, a refresh command is issued once every 7.8 microseconds. Each refresh command triggers a parallel refresh operation in every bank,

1. Calculate the total number of rows refreshed in one refresh command.
2. Assume that it takes 30 ns on average to refresh one row. Calculate the latency of a single refresh command (tRFC).
3. Calculate the fraction of time for which the memory system is unable to service memory requests due to refresh activity. The fraction of time of time is tRefLatency/TRefPeriod or tRFC/tREFI and tREFI is 7.8microsec.