关于比特序的问题

 不同的CPU，有大小端的问题（至少两个字节），也有比特序的问题。下面的例子运行在POWERPC和X86下运行的结果是不同的。这个在不同平台下，定义结构体要用到这个

例一：

#include "stdio.h"
#include "string.h"

struct entity_id
{
    uint8_t instance: 7;
    uint8_t logical: 1;
} __attribute__ ((packed));

 

 

 

int test_endian(void)
{
    struct entity_id id;
    uint8_t i = 0x96;
    memcpy(&id, &i, sizeof(uint8_t));
    printf("instance is %d/r/n",id.instance);
    printf("logical is %d/r/n",id.logical);
    return 1;
}

x86平台：
instance is 22
logical is 1

PPC平台：
instance is 75
logical is 0

 

 

例二、

#include "Stdio.h"
#include "Conio.h"

typedef unsigned short uint16_t;

struct _read
{
    uint16_t settable:8;
    uint16_t reserved:2;
    uint16_t unr:1;
    uint16_t ucr:1;
    uint16_t unc:1;
    uint16_t lnr:1;
    uint16_t lcr:1;
    uint16_t lnc:1;
};

void main(void)
{
    struct _read read;
    uint16_t n = 0x0036;
    memcpy(&read,&n, 2);
    printf("read.settable = 0x%x/r/n", read.settable);
    printf("read.reserved = 0x%x/r/n", read.reserved);
    printf("read.unr = 0x%x/r/n", read.unr);
    printf("read.ucr = 0x%x/r/n", read.ucr);
    printf("read.unc = 0x%x/r/n", read.unc);
    printf("read.lnr = 0x%x/r/n", read.lnr);
    printf("read.lcr = 0x%x/r/n", read.lcr);
    printf("read.lnc = 0x%x/r/n", read.lnc);
    getch();

}

这个程序运行在X86下，运行的结果是：
read.settable = 36
read.reserved = 0
read.unr = 0
read.ucr = 0
read.unc = 0
read.lnr = 0
read.lcr = 0
read.lnc = 0

 

这个程序运行在PPC下，运行的结果是：
read.settable = 0x0
read.reserved = 0x0
read.unr = 0x1
read.ucr = 0x1
read.unc = 0x0
read.lnr = 0x1
read.lcr = 0x1
read.lnc = 0x0

 

经验就留给大家去总结吧