8051 program (code) memory capacity What is the maximum external code memory that can be interfaced to a standard 8051?

Introduction / Context:
Classic 8051 architectures separate program memory (code) and data memory. Knowing the maximum addressable external program space is a fundamental device capability question.

Given Data / Assumptions:

• Program counter is 16 bits in the baseline 8051.
• External code memory is addressed over a 16-bit bus via PSEN.
• EA pin selects internal versus external code fetch at reset and beyond.

Concept / Approach:
With a 16-bit program counter, the address range is 0x0000 to 0xFFFF, which is 65,536 bytes = 64 KB. Therefore, the maximum external code memory space is 64K bytes for a standard 8051 (without banking or extended addressing in derivatives).

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer datasheets and 8051 architecture references consistently state 64K external code memory for the standard core.

Why Other Options Are Wrong:

• 32K: Understates the 16-bit space.
• 128K / 256K: Exceeds baseline 8051 PC width; applies only to enhanced derivatives with banking or paging.

Common Pitfalls:
Confusing external code size with external data (xdata) size or with internal code ROM sizes in specific derivatives.

Final Answer:
64K