8051 reset vector and code source On reset with EA (External Access) pin held HIGH, where does the 8051 start fetching instructions from (PC = 0000H)?

Introduction / Context:
The EA pin determines whether code fetches are from on-chip ROM/Flash or from external program memory when the address is within the internal block. Understanding reset behavior and the reset vector is a core 8051 concept.

Given Data / Assumptions:

• Reset vector is at address 0000H.
• EA = 1 selects internal code for addresses mapped on-chip.
• EA = 0 forces external code fetches for all addresses.

Concept / Approach:
When EA is HIGH during reset, the CPU fetches the first instruction from internal code memory at 0000H (if present). Subsequent fetches within the internal code range also come from internal memory unless accesses exceed the internal range, where external fetch may occur in some derivatives.

Step-by-Step Solution:

Verification / Alternative check:
Datasheets specify EA behavior and the reset vector mapping explicitly.

Why Other Options Are Wrong:

• External code memory: Chosen when EA = 0.
• Internal/external data memory: Data memories are not used for opcode fetch.

Common Pitfalls:
Confusing EA pin function with the PSEN pin or with data memory spaces.

Final Answer:
internal code memory