Keil Logo


The NXP 80C51MX provides a unified 16 MB address space. New instructions can access up to 16MB memory whereby CODE and XDATA space are mapped into one single address space.

The stack pointer can be configured as 16-Bit stack pointer that addresses the on-chip RAM area in the EDATA memory class.

The following table shows the memory classes used for programming the 80C51MX architecture. These memory classes are available when you are using the AX51 macro assembler and the LX51 linker/locater.

Memory Class Address Range Description
DATA 7F:0000 – 7F:007F Direct addressable on-chip RAM.
BIT 7F:0020 – 7F:002F Bit-addressable RAM; accessed bit instructions.
IDATA 7F:0000 – 7F:00FF Indirect addressable on-chip RAM; can be accessed with @R0 or @R1.
EDATA 7F:0000 – 7F:FFFF Complete on-chip RAM; can be used as stack space or can be accessed with @PR0 or @PR1.
XDATA 00:0000 – 00:FFFF 64 KB RAM (read/write access). Accessed with MOVX instruction.
HDATA 00:0000 – 7F:FFFF 8 MB RAM (read/write access). Accessed with MOVX instruction and extended DPTR.
CODE 80:0000 – 80:FFFF Classic 8051 compatible 64 KB ROM (only read access possible). Used for executable code or constants.
ECODE 80:0000 – 80:FFFF 8 MB ROM (only read access possible).
HCONST 80:0000 – 80:FFFF 8 MB ROM. Same as ECODE, this class is used by the CX51 Compiler for constants.
… BANK 63
80:0000 – 0xBF:FFFF
B0:0000 – B63:FFFF
Used by the CX51 Compiler to expand the program memory to more than 64KB.


  • Colons are used to improve the readability only. The addresses are entered in the tools as numbers without colon.
  • The memory prefixes D: I: X: C: B0: .. B31: cannot be used at Ax51 assembler level. The memory prefix is only listed for better understanding. The Lx51 linker/locater and several Debugging tools, for example the µVision2 Debugger, are using memory prefixes to identify the memory class of the address.
  Arm logo
Important information

This site uses cookies to store information on your computer. By continuing to use our site, you consent to our cookies.

Change Settings

Privacy Policy Update

Arm’s Privacy Policy has been updated. By continuing to use our site, you consent to Arm’s Privacy Policy. Please review our Privacy Policy to learn more about our collection, use and transfers
of your data.