A51: Instructions Generated by PDATA or XDATA Variable

Information in this article applies to:

• C251 Version 1.0 or later
• C51 Version 5.0 or later

QUESTION

I'm using PDATA and XDATA in my project. What assembly language instructions are generated for accesses to these memory areas:

ANSWER

PDATA refers to one 256-byte "page" of external data. Therefore, PDATA pointers may be stored in a single byte. PDATA memory is accessed using the MOVX @R0 and MOVX @R1 instructions. The upper byte of a PDATA address comes directly from the P2 register. Refer to the startup code in STARTUP.A51 for information on how to set P2.

XDATA refers to the full 64K of addressable external data. Therefore, XDATA pointers may be stored in a word (2 bytes). XDATA memory is accessed using the MOVX @DPTR instruction.

MORE INFORMATION

• Refer to MOVX in the 8051 Instruction Set Manual.

Last Reviewed: Thursday, February 25, 2021