C51: Absolute Access for Far Variables

Information in this article applies to:

• C51 Version 6.14 and later

QUESTION

How do I access far absolute memory locations?

ANSWER

The far memory area is supported for devices with more than 64K of code/xdata space (like the Dallas 390, NXP 51MX, and Analog Devices ADuC812).

In C51 V6.14, we have added a new absolute access macro called FVAR. This macro allows you to define a memory type and its far address. For example:

#include
#define IO_PORT   FVAR(unsigned char, 0x200000)

void main (void)  {
  unsigned int i;                     /* Delay var */
  unsigned char j;                    /* LED var */

  IO_PORT = 0x55;
  i = IO_PORT;
}

In C51 V7.07 and later, we have added support for the _at_ keyword. This means that it is now possible to define absolute variables directly using the _at_ attribute:

unsigned char far IO_PORT _at_ 0x200000;

void main (void)  {
  unsigned int i;                     /* Delay var */
  unsigned char j;                    /* LED var */

  IO_PORT = 0x55;
  i = IO_PORT;
}

MORE INFORMATION

• Refer to Data Storage Formats in the Cx51 User's Guide.
• Refer to Absolute Memory Locations in the Cx51 User's Guide.
• Refer to Absolute Access Macros in the Cx51 User's Guide.
• Refer to The _at_ Keyword in the Cx51 User's Guide.

SEE ALSO

• C51: Absolute Memory Accesses to Maxim 390 Far Memory

Last Reviewed: Thursday, February 25, 2021