C51: Offsets with Far Memory Access

Information in this article applies to:

• C51 Version 7

QUESTION

I need to manage an EEPROM on an Atmel device, and I am using the 'far' memory type as explained in Keil\C51\EXAMPLES\FarMemory\E2PROM on 80C51RD2. Within the EEPROM, I store offsets to other elements using WORD variables. However, when I try to access these objects, the code fails.

My access code looks like:

unsigned int far offset;    // an offset in EEPROM that points to other elements
unsigned char far *p, c;
  :
  p = (unsigned char far *) offset;
  c = *p;                        // this access fails

It seems to be impossible to access EEPROM space by just using an offset.

Do you have a solution to that problem?

ANSWER

Yes, there is a solution. You can use the FARRAY macros to define an EEPROM memory. The following example shows you how to do that.

#include

unsigned int far i[3];

unsigned char far *p;
unsigned char uc;

// macro that allows easy EEPROM access
#define EEPROM FARRAY (unsigned char, 0x20000)

void test (void)  {
// construct without macro
  p = (unsigned char far *) &(FARRAY (unsigned char, 0x20000)[i[0]]);

// using macros
  p  = &EEPROM [5];
  uc =  EEPROM [i[3]];
  p  = &EEPROM [uc];
}

MORE INFORMATION

• Refer to FARRAY in the Cx51 User's Guide.

SEE ALSO

• C51: Microchip/Atmel T89C51CC01 Internal EEPROM Support
• µVision DEBUGGER: Simulating On-chip EEPROM

Last Reviewed: Thursday, February 25, 2021