Which output control signal is activated after every six oscillator periods while fetching the external program memory and almost remains high during

Question

**Which output control signal is activated after every six oscillator periods while fetching the external program memory and almost remains high during internal program execution?**
(i) ALE
(ii) $\overline{PSEN}$
(iii) $\overline{EA}$
(iv) All of the above

Accepted Answer

**Answer: (ii) \overline{PSEN}**

**Explanation:** \overline{PSEN} (Program Store Enable) is the 8051 control output that:
- Pulses low once every 6 oscillator periods (once per machine cycle) when fetching code from **external** program memory.
- Stays high (inactive, because it is active-low) almost continuously when execution is from **internal** program memory, going low only if an external access is needed.
- ALE also pulses every 6 osc periods but is not specific to external code fetch and pulses even during internal execution (except when disabled). \overline{EA} is an input, not an output. Hence the description matches \overline{PSEN}.

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