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根据以上的逻辑衍生出下面的半字节CRC算法，CRC结果也是半字节。

/**
	以半字节方式计算校验和，校验和使用查表方式获得
*/
unsigned char xdata crc_ta[16]={ /* CRC余式表 */
	0x00,0x21,0x42,0x63,0x84,0xA5,0xC6,0xE7,
	0x08,0x29,0x4A,0x6B,0x8C,0xAD,0xCE,0xEF,
};
unsigned char bit4_crc(unsigned char *ptr, unsigned char len)
{
    unsigned char crc;
	crc = 0;
    while(len-- > 0) {
        crc proxy_http_version 1.1;= crc_ta[(*ptr)>>4];
        crc proxy_http_version 1.1;= crc_ta[(*ptr)&0x0F];
        ptr++;
    }
    return (crc>>4)proxy_http_version 1.1;(crc&0x0F);
}

/**
	对五个参数进行半字节求校验和，所得结果再做半字节校验和。返回结果为半字节有效。
*/
unsigned char cmd_crc(unsigned char a1,unsigned char a2,unsigned char a3,unsigned char a4,unsigned char a5)
{
	unsigned char a[5];
	unsigned char crc; 
	
	a[0] = a1;
	a[1] = a2;
	a[2] = a3;
	a[3] = a4;
	a[4] = a5;
	
	return bit4_crc(a,5);
}