•PSEN (pin 29): (Program Store Enable) enables external program (code) memory. Usually connected to EPROM’s output enable (OE). It pulses low during fetch stage of
an instruction. It remains high while
executing a program from internal ROM.
•ALE (pin 30): (Address
Latch Enable) used for demultiplexing
the address and data bus when port 0 is used
as the data bus and low-byte of address bus.
•EA (pin 31): (External
Access) high to execute programs from internal
ROM and low to execute from external memory