We have not considered the time element in our decoding. An important time for decoding is the time from the address strobe (AS) to when the data is required in a read.
Time AS -> data = 250 ns -> 4 MHz
This time is available for the decoding and accesses of RAM, EPROM, I/O. the fastest ram we have at minimal cost, has an access time of 60ns and EPROM 120ns. Therefore we have to make sure that the memory chosen matches the hardware timing.