Question

Why MOVX instruction is used to access the ports of 8255?

• A. To use memory space to access I/O devices
• B. To write signals
• C. To provide serial communication
• D. To provide internal access

Hint:

Always remember: If the data is "eXternal" to the chip, you need the MOVX instruction. Internal data only needs the standard MOV instruction.

Answer 1

The Correct Option is A

1. Memory-Mapped I/O Concept: The Intel 8255 is a Programmable Peripheral Interface (PPI) chip often used to expand the I/O capabilities of the 8051. When such external chips are connected, they are typically mapped into the

external data memory space of the microcontroller.

2. The Role of MOVX: In 8051 assembly language:

• MOV is used for internal RAM and SFRs.

• MOVC is used for program (code) memory.

• MOVX (Move External) is specifically designed to access the 64 KB

external data memory space.

3. Connecting the 8255: Because the 8255 ports are wired to the 8051's external address and data buses, the CPU treats them as if they were locations in external RAM. This is known as using "memory space" to access I/O devices. The

MOVX instruction triggers the external RD/WR signals required to communicate with the 8255. Therefore, MOVX is used because it is the only instruction capable of reaching the external memory-mapped address where the 8255 ports reside.