Hi Gil,

Every memory chip has its own response time which we can found out by their related datasheets. Even RD and WR time can be different.
We have to check how much time our CPU allow for external RD or WR cycle. Usually for 89C52 you would not have problem, but in faster uP you may need to insert WAIT (NOP) instructions in between. How many? depends on how long your memory takes to respond. Its CPU speed versus real time equation.

NOTE: i haven't read all answers, someone might have already replied this before.

Take care

Rauf