8051 External Memory Interface — Address/Data Mapping Which ports of the 8051 are used as the multiplexed address/data bus for external memory interfacing?

Introduction / Context:
When the 8051 accesses external memory, it exposes address and data signals on its I/O pins. Understanding which ports carry which signals is essential for latching, decoding, and bus transceiving.

Given Data / Assumptions:

• P0 is multiplexed AD0–AD7 (low address and data).
• P2 provides A8–A15 (high address).
• Control strobes ALE, PSEN, RD, and WR handle timing.

Concept / Approach:
The 8051 uses a multiplexed bus to reduce pin count. The low address byte appears on P0 and is latched with ALE; during later phases the same P0 pins carry data. P2 continuously drives the high address lines during the cycle.

Step-by-Step Solution:
1) Associate P0 with AD0–AD7.2) Associate P2 with A8–A15.3) Conclude that ports 0 and 2 together implement the external address/data interface.

Verification / Alternative check:
Standard timing diagrams show ALE latching P0 into an external latch such as 74HC373 or 74HC573 to separate address from data.

Why Other Options Are Wrong:

• ports 1 and 2 / ports 1 and 3: P1 and P3 are not assigned to the external address/data bus.
• ports 0 and 3: P3 provides control signals, not the address/data bus itself.

Common Pitfalls:
Forgetting to demultiplex P0 with a latch; assuming P3 carries address bits.

Final Answer:
ports 0 and 2