C51: HOW TO USE 256 BYTES DATA SPACE

Information in this article applies to:

• C51 All Versions

QUESTION

I have an 8051 device that has 256 bytes DATA RAM, but it appears that I can only access the first 128 bytes. How can I enable the remaining memory?

ANSWER

The 8051 architecture differs between DATA and IDATA. When a device has 256 bytes on-chip RAM space, the first 128 bytes can be only accessed in DATA space whereas the entire 256 bytes space can be accessed as IDATA.

In the C compiler you must define variables that should be placed above the 128 bytes limit using the idata memory type.

Example:

unsigned char idata uc;  // variable in IDATA can be located above 0x80

MORE INFORMATION

• Refer to Memory Areas in the Cx51 User's Guide.
• Refer to Classic 8051 Devices in the LX51 User's Guide.

SEE ALSO

• C51: CORRECTLY DECLARING VARIABLES WITH MEMORY SPACES
• C51: PHILIPS 87C51RC 512-BYTE INTERNAL RAM SUPPORT
• BL51: ERROR 107 (ADDRESS SPACE OVERFLOW)

Last Reviewed: Wednesday, November 15, 2006