"You dont need !RD for accessing external CODE memory, only for DATA memory. "

True but unfortunatley not complete information. Let me try to summarize the situation. ( considering only !RD , !WR and PSEN pins )

1. Only PSEN needed for accessing external code memory WITHOUT a movx instruction.

2. Both PSEN and !RD needed for accessing external code memory WITH a movx instruction.

3. Both !RD and !WR required for reading from or writing to external data memory (RAM).

4.Only !RD required for reading data from an external memory (ROM).

Trust the above is complete now.

Raghu