为什么输入p16=0xabef时，输出结果为0xef？？

这是因为在ARM Cortex-M系列中，Flash和SRAM存储器的字节序都是Little-Endian。这意味着在内存中存储的数据的低字节（即地址的最低位）在前面，高字节在后面。

假设p16指向SRAM的起始地址，输入*p16=0xabef时，这个16位数据在内存中的存储形式为：0xef（低字节）0xab（高字节），因为SRAM的字节序是Little-Endian，所以低字节在前面。

当你读取*p16时，读取到的是低字节0xef，因为它在内存中的地址比高字节更小，所以*p16的值为0xef，输出结果也是0xef。

相关问题

P1M0 = 0xff; P1M1 = 0xff;P3M0 = 0xff; P3M1 = 0xef; P1M0 = 0xff; P1M1 = 0xff;P3M0 = 0xff; P3M1 = 0xf7;怎么把这个编成数组

这是一个关于GPIO口配置的例子，其中`P1M0`, `P1M1`, `P3M0`, 和 `P3M1` 可能是某个微控制器（如Arduino、STM32等）中用于设置特定功能寄存器的位。`0xff`代表全部使能或不确定状态，而`0xef`和`0xf7`可能是另一种配置。将这些配置编成数组可以便于管理和操作。

假设我们有一个四元素的数组，每个元素代表一组配置：

uint8_t gpioConfig[] = {
    (0xff << 8) | (0xff),      // P1M0 = 0xff, P1M1 = 0xff
    (0xff << 8) | (0xef),       // P3M0 = 0xff, P3M1 = 0xef
    (0xff << 8) | (0xff),      // P1M0 = 0xff, P1M1 = 0xff
    (0xff << 8) | (0xf7)        // P3M0 = 0xff, P3M1 = 0xf7
};

这里，每个数组元素是一个16位值，高8位对应`P3M0`，低8位对应`P1M1`。注意，实际编程中需根据具体的硬件库或API来调整这个结构，因为不同平台的GPIO映射可能会有所不同。

#include<reg51.h> void delay(unsigned int xms){ //ÑÓʱº¯Êý unsigned int i, j; for(i=0; i<xms; ++i) for(j=0; j<110; ++j); } void main(){ while(1) { while(P0==0xFE) { P2=0xFE; delay(200); P2=0xFD; delay(200); P2=0xFB; delay(200); P2=0xF7; delay(200); P2=0xEF; delay(200); P2=0xDF; delay(200); P2=0xBF; delay(200); P2=0x7F; delay(200); } while(P0==0xFD) { P2=0xFE; delay(200); P2=0xFD; delay(200); P2=0xFC; delay(200); P2=0xFB; delay(200); P2=0xEA; delay(200); P2=0xDF; delay(200); P2=0xBE; delay(200); P2=0x7D; delay(200); } while(P0==0xFB) { P2=0xFA; delay(200); P2=0xF4; delay(200); P2=0xF6; delay(200); P2=0xF7; delay(200); P2=0xA4; delay(200); P2=0x56; delay(200); P2=0x47; delay(200); P2=0x7F; delay(200); } while(P0==0xF7) { P2=0xFE; delay(50); P2=0xFD; delay(50); P2=0xFB; delay(50); P2=0xF7; delay(50); P2=0xEF; delay(50); P2=0xDF; delay(50); P2=0xBF; delay(50); P2=0x7F; delay(50); } while(P0==0xEF) { P2=0xFE; delay(50); P2=0xFD; delay(50); P2=0xFC; delay(50); P2=0xFB; delay(50); P2=0xEA; delay(50); P2=0xDF; delay(50); P2=0xBE; delay(50); P2=0x7D; delay(50); } while(P0==0xDF) { P2=0xFF; delay(500); P2=0x00; delay(500); } while(P0==0xBF) { P2=0xEA; delay(200); P2=0xDF; delay(200); P2=0xBD; delay(200); P2=0x7E; delay(200); } while(P0==0x7F) { P2=0xFF; delay(500); } } }

这段代码看起来是在控制 LED 灯的亮灭，但是代码结构比较混乱，建议可以进行如下的优化：

1. 引入宏定义，将常数定义为宏进行使用，方便后续修改和维护。
2. 将 LED 灯控制的代码封装成一个函数，方便后续调用和维护。
3. 使用 switch-case 语句替换多个嵌套的 while 循环，提高代码可读性和可维护性。

下面是优化后的代码，供您参考：

#include <reg51.h>

#define DELAY_TIME 200
#define SHORT_DELAY_TIME 50
#define LONG_DELAY_TIME 500

void delay(unsigned int xms) {
    unsigned int i, j;
    for (i = 0; i < xms; ++i) {
        for (j = 0; j < 110; ++j);
    }
}

void controlLED(unsigned char value) {
    switch (value) {
        case 0xFE:
            P2 = 0xFE;
            delay(DELAY_TIME);
            P2 = 0xFD;
            delay(DELAY_TIME);
            P2 = 0xFB;
            delay(DELAY_TIME);
            P2 = 0xF7;
            delay(DELAY_TIME);
            P2 = 0xEF;
            delay(DELAY_TIME);
            P2 = 0xDF;
            delay(DELAY_TIME);
            P2 = 0xBF;
            delay(DELAY_TIME);
            P2 = 0x7F;
            delay(DELAY_TIME);
            break;
        case 0xFD:
            P2 = 0xFE;
            delay(DELAY_TIME);
            P2 = 0xFD;
            delay(DELAY_TIME);
            P2 = 0xFC;
            delay(DELAY_TIME);
            P2 = 0xFB;
            delay(DELAY_TIME);
            P2 = 0xEA;
            delay(DELAY_TIME);
            P2 = 0xDF;
            delay(DELAY_TIME);
            P2 = 0xBE;
            delay(DELAY_TIME);
            P2 = 0x7D;
            delay(DELAY_TIME);
            break;
        case 0xFB:
            P2 = 0xFA;
            delay(DELAY_TIME);
            P2 = 0xF4;
            delay(DELAY_TIME);
            P2 = 0xF6;
            delay(DELAY_TIME);
            P2 = 0xF7;
            delay(DELAY_TIME);
            P2 = 0xA4;
            delay(DELAY_TIME);
            P2 = 0x56;
            delay(DELAY_TIME);
            P2 = 0x47;
            delay(DELAY_TIME);
            P2 = 0x7F;
            delay(DELAY_TIME);
            break;
        case 0xF7:
            P2 = 0xFE;
            delay(SHORT_DELAY_TIME);
            P2 = 0xFD;
            delay(SHORT_DELAY_TIME);
            P2 = 0xFB;
            delay(SHORT_DELAY_TIME);
            P2 = 0xF7;
            delay(SHORT_DELAY_TIME);
            P2 = 0xEF;
            delay(SHORT_DELAY_TIME);
            P2 = 0xDF;
            delay(SHORT_DELAY_TIME);
            P2 = 0xBF;
            delay(SHORT_DELAY_TIME);
            P2 = 0x7F;
            delay(SHORT_DELAY_TIME);
            break;
        case 0xEF:
            P2 = 0xFE;
            delay(SHORT_DELAY_TIME);
            P2 = 0xFD;
            delay(SHORT_DELAY_TIME);
            P2 = 0xFC;
            delay(SHORT_DELAY_TIME);
            P2 = 0xFB;
            delay(SHORT_DELAY_TIME);
            P2 = 0xEA;
            delay(SHORT_DELAY_TIME);
            P2 = 0xDF;
            delay(SHORT_DELAY_TIME);
            P2 = 0xBE;
            delay(SHORT_DELAY_TIME);
            P2 = 0x7D;
            delay(SHORT_DELAY_TIME);
            break;
        case 0xDF:
            P2 = 0xFF;
            delay(LONG_DELAY_TIME);
            P2 = 0x00;
            delay(LONG_DELAY_TIME);
            break;
        case 0xBF:
            P2 = 0xEA;
            delay(DELAY_TIME);
            P2 = 0xDF;
            delay(DELAY_TIME);
            P2 = 0xBD;
            delay(DELAY_TIME);
            P2 = 0x7E;
            delay(DELAY_TIME);
            break;
        case 0x7F:
            P2 = 0xFF;
            delay(LONG_DELAY_TIME);
            break;
        default:
            break;
    }
}

void main() {
    while (1) {
        unsigned char value = P0;
        controlLED(value);
    }
}