cksum.c

来自「Hitech microchip PIC C18 Compiler」· C语言 代码 · 共 53 行

#include <htc.h>
#include "cksum.h"

// Determine whether an addition or subtraction algorithm is used.
#if (_ALGORITHM < 0)
 #define _SUM(x,y) x -= y
 #define _BYTEOP (_ALGORITHM * -1)
#else
 #define _SUM(x,y) x += y
 #define _BYTEOP _ALGORITHM
#endif

// Calculate the number of iterations through the loop.
#define _SIZE (( _END - _START + 1 ) / _BYTEOP)

// the return type is based on _RESULT_WIDTH in cksum.h
_D_TYPE
cksum(void){
   _D_TYPE  sum;
#if _SIZE > 0xFFFF
   unsigned long counter;
#else
   unsigned int counter;
#endif

	sum = _OFFSET;		// initial offset (usually zero)
	counter = _SIZE;

   TBLPTR=(far char *)_START;	// point to start of checksum range
   while(counter){
      asm("tblrd*+");
      _SUM(sum,TABLAT);		// read and accumulate a byte
#if _BYTEOP >= 2
      asm("tblrd*+");
      _SUM(sum,(TABLAT<<8));
#endif
#if _BYTEOP >= 3
      asm("tblrd*+");
      _SUM(sum,(TABLAT<<16));
#endif
#if _BYTEOP == 4
      asm("tblrd*+");
      _SUM(sum,(TABLAT<<24));
#endif
      counter--;
   }
#if (_END >= 0xFFFF) && defined(SMALL_DATA)
   TBLPTRU=0;	// if this register is non-zero, it must be zeroed in this mode
#endif
   return sum;
}