Accessing of memory by the CPU doesn't happen automatically. It takes some time before the CPU can access the data or program it needs from the memory. Since all systems around the microprocessor run with the system clock, the read/write operation can also be understood better from this vantage point.
legend: a - address set-up time
b - memory access time
c - data set-up time
d - data hold time
e - READY set-up time
f - READY hold time
legend: a - address set-up time
b - data width >= write time of memory
c - minimum time for DS
d - data hold time
e - READY set-up time
f - READY hold time
As can be seen from the graphs, data can only be read or written when the address is already valid. The address strobe, the Read/Write, and the data pin must be high in order for the read/write operation to get through. Try to study again the read/write sequence.
NEXT STOP: Memory: Read Only Memory
