從C語言到彙編（二）表達式

C語言變量，常量，運算符組成表達式。
在這裏我寫上幾個重要的表達式以及其彙編代碼。

數字交換

int main(void)
{
	int a=1;
	int b=0;
	int tmp;
	
	tmp=a;
	a=b;
	b=tmp;
	return 0;
}

彙編代碼

.text
.global main

main:
	pushl %ebp
	movl %esp,%ebp
	
	subl $12,%esp
	movl $1,-4(%ebp)
	movl $0,-8(%ebp)
	
	movl -4(%ebp),%eax
	movl %eax,-12(%ebp)
	movl -8(%ebp),%eax
	movl %eax,-4(%ebp)
	movl -12(%ebp),%eax
	movl %eax,-8(%ebp)
	
	movl $0,%eax
	leave
	ret

大寫轉小寫

int main(void)
{
	int ch='A';
	ch=ch-'A'+'a';
	
	return 0;
}

彙編代碼

.text
.global main

main:
	pushl %ebp
	movl %esp,%ebp
	
	subl $4,%esp
	movl $'A',-4(%ebp)
	movl -4(%ebp),%eax
	subl $'A',%eax
	addl $'a',%eax
	movl %eax,-4(%ebp)
	
	movl $0,%eax
	leave
	ret

3個字符組成數字

int main(void)
{
	int ch1='1';
	int ch2='5';
	int ch3='6';
	int val=0;
	
	val=(ch1-'0')*100;
	val+=(ch2-'0')*10;
	val+=(ch3-'0');
	
	return 0;
}

彙編代碼

.text
.global main

main:
	pushl %ebp
	movl %esp,%ebp
	
	subl $16,%esp
	movl $'1',-4(%ebp)
	movl $'5',-8(%ebp)
	movl $'6',-12(%ebp)
	movl $0,-16(%ebp)
	
	movl -4(%ebp),%eax
	subl $'0',%eax
	imull $100,%eax
	movl %eax,-16(%ebp)	
	
	movl -8(%ebp),%eax
	subl $'0',%eax
	imull $10,%eax
	addl %eax,-16(%ebp)
	
	movl -12(%ebp),%eax
	subl $'0',%eax
	addl %eax,-16(%ebp)
	
	movl $0,%eax
	leave
	ret