8051 Instruction Set — Indirect Addressing to Port Which instruction copies the byte from internal RAM at the address contained in R0 to Port 1 (P1)?

Introduction / Context:
Indirect addressing with @R0 or @R1 allows the 8051 to access a RAM location whose address is held in a register. Understanding legal source and destination forms is key to moving data between RAM and special function registers like P1.

Given Data / Assumptions:

• R0 holds a valid internal RAM address.
• P1 is the Port 1 special function register at a direct address.
• Standard 8051 MOV instruction forms apply.

Concept / Approach:
Legal forms include MOV direct, @Ri and MOV direct, A and MOV direct, @Ri (source indirect to direct). To copy RAM[@R0] to P1, the destination must be direct (P1) and the source must be indirect (@R0). Thus MOV P1, @R0 is the correct encoding.

Step-by-Step Solution:
1) Determine direction: RAM[@R0] → P1.2) Select instruction form: MOV direct, @Ri.3) Substitute operands: direct=P1, @Ri=@R0.4) The legal statement is MOV P1, @R0.

Verification / Alternative check:
Assembler manuals list MOV direct, @Ri as a valid form. Simulators or assemblers will accept MOV P1, @R0 and produce the corresponding opcode.

Why Other Options Are Wrong:

• MOV @ P1, R0: Invalid syntax; destination cannot be @P1.
• MOV @ R0, P1: Moves in the opposite direction P1 → RAM[@R0].
• MOV P1, R0: Copies the contents of register R0, not RAM at address in R0.

Common Pitfalls:
Confusing the contents of R0 with the byte pointed to by @R0; forgetting that port SFRs are direct addressed destinations.

Final Answer:
MOV P1, @ R0