Cirrus Logic CS485 Manual de usuario descargar pdf (Pagina 24)

CS485G Spring 2015 24

(b) %ebx, %esi, %edi are non-volatile (callee-save) general-purpose

registers.

5. Example

1 int bitCount(unsigned x) {

2 if (x == 0)

3 return 0;

4 else

5 return (x & 1) + bitCount(x >> 1)

6 }

bitCount:

pushl %ebp # save %ebp (non-volatile)

movl %esp, %ebp # new %ebp

pushl %ebx # save b (non-volatile)

subl $4, %esp # room for p1

movl 8(%ebp), %ebx # b = x

movl $0, %eax # a = 0

testl %ebx, %ebx # x ==? 0

je .L3 # if (x==0) jump

movl %ebx, %eax # a = x

shrl %eax # a = x >> 1

movl %eax, (%esp) # p1 = x >> 1

call bitCount # a = bitCount(p1)

movl %ebx, %edx # d = x

andl $1, %edx # d = x & 1

leal (%edx,%eax), %eax # a = (x&1) + bitCount(p1)

.L3:

addl $4, %esp # remove p1

popl %ebx # restore b

popl %ebp # restore ebp

ret # return

24 Code for local variables, pointers

1. Lecture 14, 2/18/2015

1 2 ... 19 20 21 22 23 24 25 26 27 28 29 ... 66 67

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Cirrus Logic CS485 Manual de usuario Pagina 24