A 32 bit wide main memory with a capacity of 1 GB is built using \( 256 \mathrm{~M} \times 4 \) bits DRAM chips. The number of rows memory cells in the DRAM chip is \( 2^{14} \) . The time taken to perform one refresh operation is 50 nanoseconds. The refresh period is 2 milli seconds The percentage (rounded to the closest integer) of the time available for performing the memory read/write operations in the main memory unit is \( \qquad \) .