Access times This disk access time is for random I/O Once the first bit is read, the rest (sequential I/O) is almost free (only 0.02 ms) Ratio of random I/O to sequential I/O is, therefore, (12 / 0.02) 600 times Bulk transfer rates are calculated more precisely using gaps Time to access a byte 60 ns for DRAM (used for main memory) and 4 ns for SRAM (used for cache memory) Ratio of disk access time to memory access time 40000 for SRAM and 2500 for DRAM