#include <stdio.h>
int main(void)
{
    unsigned int original = 0xABC;
    unsigned int result = 0;
    unsigned int mask = 0xF; /* Righmost four bits */

    printf("\n original = %X", original);

    /* Insert first digit in result */
    result |= original&mask;    /* Put right 4 bits from original in result */

    /* Get second digit */
    original >>= 4 ;            /* Shift original right four positions */
    result <<= 4 ;                /* Make from for next digit */
    result |= original&mask;    /* Put right 4 bits from original in result */

    /* Get third digit */
    original >>= 4;                /* Shift original right four positions */
    result <<= 4;                /* Make from for next digit */
    result |= original&mask;    /* Put right 4 bits from original in result */
    printf("\t result = %X\n", result);
    return 0;
}

请高手帮我解惑,这个result |= original&mask得到的值的二进制是0000 0000 0000 1100，
original = 0xABC 换成二进制也是0000 1010 1011 1100，这个值经过和mask的值0000 0000 0000 1111按位与运算得到上面的结果。但是我按着得到的结果算下去，最后输出的结果并不是CBA，请高手解释下，感谢！

[ 本帖最后由 format_yang 于 2012-8-25 16:25 编辑 ]

original&mask的值都是取original的最后4位，其余位为0，result先向前移4位，然后做|=操作，就相当于将前面与的结果的后四位，
放到result移出来的后4位上，推算结果如下：
#include <stdio.h>
int main(void)
{
    unsigned int original = 0xABC;                            //0000 1010 1011 1100
    unsigned int result = 0;                                  //0000 0000 0000 0000
    unsigned int mask = 0xF; /* Righmost four bits */         //0000 0000 0000 1111

    printf("\n original = %X", original);

    /* Insert first digit in result */
    result |= original&mask;    /* Put right 4 bits from original in result */ //result=0000 0000 0000 1100

    /* Get second digit */
    original >>= 4 ;            /* Shift original right four positions */      //original=0000 0000 1010 1011
    result <<= 4 ;                /* Make from for next digit */               //result  =0000 0000 1100 0000
                                                                               //与结果  =0000 0000 0000 1011
    result |= original&mask;    /* Put right 4 bits from original in result */ //后四位  =0000 0000 1100 1011

    /* Get third digit */
    original >>= 4;                /* Shift original right four positions */   //original=0000 0000 0000 1010
    result <<= 4;                /* Make from for next digit */                //result  =0000 1100 1011 0000
                                                                               //与结果  =0000 0000 0000 1010
    result |= original&mask;    /* Put right 4 bits from original in result */ //后四位  =0000 1100 1011 1010
                                                                               //结果刚好将ABC逆序了下变成CBA
    printf("\t result = %X\n", result);
    return 0;
}