8051 external data memory interface (MOVX space) What is the maximum external data memory capacity that can be interfaced to the 8051?

Introduction / Context:
The 8051 can address on-chip RAM plus external data memory through the MOVX instruction. Understanding the external memory address space is important for system design and memory mapping.

Given Data / Assumptions:

• Classic 8051 external data memory bus with 16-bit addressing.
• Separate program and data spaces (Harvard-like architecture).
• Focus is on external data memory capacity.

Concept / Approach:
With a 16-bit address for external data memory, the addressable range is 2^16 bytes = 65,536 bytes = 64 KB. This is accessed via MOVX using DPTR or Ri (for paged modes on some variants).

Step-by-Step Solution:

Verification / Alternative check:
Memory interfacing examples in 8051 textbooks consistently show 64 KB external data space with latches to demultiplex address/data lines (ALE).

Why Other Options Are Wrong:

• 32K: half the actual address space.
• 128K or 256K: exceed the 16-bit addressing limit of the standard external data space.

Common Pitfalls:
Confusing external data memory size with external program memory size; both are 64 KB separately, but they are distinct spaces in the classical architecture.

Final Answer:
64K