作业3

3.3

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movb $0xF, (ebx) # %ebx是调用者保存的信息，不能用 %ebx 作为目标地址
movl %rax, (%rap) # 指令后缀与寄存器不匹配,应为movl %eax, (%rsp) 或 movq %rax, (%rsp)
movw (%rax), 4(%rsp) # 的两个操作数不能同时为内存引用
movb %al, %sl # %sl不存在
movq %rax, $0x123 # 不能用立即数作为目标地址
movl %eax, %rdx # 应为 movl %eax, %edx
movb %si, 8(%rbp) # 指令后缀与寄存器不匹配，应为movb %sil, 8(%rbp) 或者 movw %si, 8(%rbp)

3.5

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void decode1(long *xp, long *yp, long *zp)
{
    long x = *xp;
    long y = *yp;
    long z = *zp;
    *yp = x;
    *zp = y;
    *xp = z;
}

3.7

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long t = 5 * x + 2 * y + 8 * z

3.10

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long arith2(long x, long y, long z)
{
    long t1 = x | y;
    long t2 = t1 >> 3;
    long t3 = ~t2;
    long t4 = z - t3;
    return t4;
}

3.15

A. 0x4003fe

B. 0x400425

C. 400543 400545

D. 0x400560

3.18

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long test(long x, long y, long z) {
    long val = x + y + z;
    if (x < -3) {
        if (y < z)
            val = x * y;
        else
            val = y * z;
    } else if (x > 2)
        val = x * z;
    return val;
}

3.21

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long test(long x, long y) {
    long val = 8 * x + 0;
    if (y > 0) {
        if (x < y)
            val = y - x;
        else
            val = x & y;
    } else if (y <= -2)
        val = x + y;
    return val;
}

3.60

A. %rdi, %esi, %rax, %rdx

B. result = 0; mask = 1;

C. mask != 0

D. mask <<= n

E. result |= (x & mask)

F.

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long loop(long x, int n)
{
    long result = 0;
    long mask;
    for(mask = 1; mask != 0; mask <<= n) {
        result |= (x & mask)
    }
    return result;
}

3.61

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long cread_alt(long *xp) {
    return (!xp ? 0 : *xp);
}