CX51: ARRAY INDEX ARITHMETIC

Information in this article applies to:

• CX51 Version 7.10

QUESTION

The CX51 Compiler does not allow the definition of objects that are bigger then 64KB. Is there a way to define big objects in assembler and use pointer arithmetic in the CX51 Compiler?

ANSWER

Yes. CX51 Version 7.11 implements long pointer arithmetic and allows index calculations across the 64KB segment limits. You may define objects in assembly language (using HDATA or HCONST segments).

With (unsigned) long variables (or cast to long) the compiler generates address arithmetic that allows you to access huge arrays. You just need to assign the start address of the array to a far pointer.

Example:

unsigned char far *cp;        // allows to access any memory location

struct s  {                        // example with a struct
  unsigned int  a[10];
  unsigned char c;
};

struct s far *sp;                  // may be anywhere in memory

unsigned long l1, l2;
unsigned char c;
unsigned int  i;

void main (void)  {
  c = cp[l1 + 0x34000L];           // address calculation
  cp[l1 + l2] = c;
  i = sp[l1].a[(unsigned long)c];  // using long cast operations,
                                   // even arrays inside a struct may
                                   // cross 64KB segment boundaries.
}

MORE INFORMATION

• Refer to Application Note 160: Programming the Philips 51MX Architecture with the Keil PK51.
• Refer to Far Memory in the Cx51 User's Guide.

SEE ALSO

• CX51: USING 24-BIT MATH WITH FAR POINTERS
• C51: CONSTANT ARRAYS LARGER THAN 64KB

FORUM THREADS

The following Discussion Forum threads may provide information related to this topic.

• Reinhard, Jon can you answer: Struct across 64 boundary
• Philips 89c669 extended addressing problem
• passing 64k boundary

Last Reviewed: Thursday, October 06, 2005