www.pudn.com > tcpip5151.rar > Cksum.c

/* 
;------------------------------------------------------------------------- 
; CKSUM.A51 
; 
; This computes the internet checksum on an 8051 
; 
; Call from C program: UINT cksum(UCHAR xdata * buf, UINT length); 
; Where * buf is pointer to first char (must be in xdata space) 
; and "length" is number of bytes (not words) to sum over 
;------------------------------------------------------------------------- 
*/ 
#include "net.h" 
 
UINT cksum(UCHAR xdata *check,UINT length) //计算校验和 
{ 
	LONG sum=0; 
	UINT i; 
	UINT xdata *ptr;  
    ptr=(UINT xdata *)check; 
	for (i=0;i<(length)/2;i++) 
	{ 
		sum+=*ptr++; 
	} 
	if (length&0x01)//表示长度为单数 
	{ 
		sum=sum+((*ptr)&0xff00); 
	} 
	sum=(sum&0xffff)+((sum>>16)&0xffff);//高16位和低16位相加 
	if(sum&0xffff0000) 
	{//表示有进位 
		sum++; 
	} 
	return ( (UINT)((sum)&0xffff)); 
//	return ( (UINT)(~((sum)&0xffff))); 
}