用0xffffffff进行按位与运算的无符号长 [英] bitwise anding unsigned long with 0xffffffff

问题描述

下面是switch语句中的一些代码. getvalue()返回unsigned long. 有人可以解释为什么value与0xffffffff进行按位运算. 单片机是32位.

Below is some code from switch statement. getvalue() returns an unsigned long. Could somebody explain why value is bitwise anded with 0xffffffff. The mcu is 32 bit.

#define WriteMemory(A,V) *(volatile unsigned long*)(A)=(V)
static unsigned value;

case 'b':
value = getvalue();
value &= 0xffffffff;
WriteMemory(2147455555, value);
break; 

推荐答案

unsigned long由C标准不能保证为32位.只能保证能够容纳32位值.

unsigned long isn't guaranteed to be 32 bit by the C standard. It is only guaranteed to be able to hold 32 bit values.

与0xffffffff进行对接，以确保32位上的所有位都被清零.

and-ing it with 0xffffffff makes sure any bits over the 32 are zeroed out.

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