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Module 2: The 8051 Architecture

20252m

Which port has dual functionality in 8051?

(i) Port 0
(ii) Port 2
(iii) Port 1
(iv) Port 3

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Answer: (iv) Port 3 (also Port 0 and Port 2 have dual function, but most dual is Port 3)

Explanation: All ports except Port 1 have dual functions:

  • Port 0: I/O + AD0-AD7 multiplexed address/data + external memory
  • Port 2: I/O + A8-A15 high address
  • Port 3: I/O + alternate functions (RxD, TxD, INT0/1, T0/T1, WR, RD)
    Port 1 is quasi-bidirectional I/O only. Among options, Port 3 is the classic "dual functionality" with 8 alternate functions, and in a single-choice question the expected answer is Port 3 (if multiple, Port 0 & 2 also have dual, but Port 3 is the most prominent). If forced to one: Port 3.

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