006 移位运算

C语言精彩编程百例 第6个例子 移位运算

 

#include<stdio.h>
void main()
{
	unsigned a,b,c,d;
	int n;
	a=64;
	n=2;

	b=a>>(6-n);
	printf("b=%d\n",b);

	c=a<<n;
	printf("c=%d\n",c);

	d=(a>>(n-1)|a<<(n+1));
	printf("d=%d\n",d);
}

 

变量赋值部分

	movl	$64, -4(%ebp) # a
	movl	$2, -20(%ebp) # n

b=a>>(6-n);

	movl	$6, %eax   
	movl	%eax, %ecx    
	subl	-20(%ebp), %ecx # 6-n
	movl	-4(%ebp), %eax  # eax=a  
	shrl	%cl, %eax       # a>>(6-n) 右移6-n次
	movl	%eax, -8(%ebp)  # b = a>>(6-n)

 

c=a<<n;

	movb	-20(%ebp), %cl  # n 直接入cl寄存器
	movl	-4(%ebp), %eax  # a
	sall	%cl, %eax       # shl 左移AT&T写法是sall
	movl	%eax, -12(%ebp) # c=a<<n

 

d=(a>>(n-1)|a<<(n+1));

	movl	-20(%ebp), %ecx # n
	decl	%ecx            # n-1
	movl	-4(%ebp), %eax  # a
	movl	%eax, %edx      # edx =a      
	shrl	%cl, %edx       # edx = a>>(n-1)
	movl	-20(%ebp), %ecx # n
	incl	%ecx            # n+1
	movl	-4(%ebp), %eax  # a
	sall	%cl, %eax       # eax = a<<(n+1)
	orl	%edx, %eax      # eax = (a>>(n-1)) | (a<<(n+1))
	movl	%eax, -16(%ebp) # d = eax