关于原码与补码

#include <stdio.h>
unsigned int getbits(unsigned int value)/*补码函数*/
{
unsigned int z;
z=value&0x100000;
if(z==0x0100000)
  z=~value+1;
else
  z=value;
return z;
}
void main()
{
unsigned int a;
    unsigned int getbits();
printf("请输入一个八进制数: ");
scanf("%o",&a);
printf("它的补码是: %o\n",getbits(a));
}

z=value&0x100000;这句话是什么意思啊?小弟不懂,请高人指点一下!谢谢了
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同求！
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