c++ - 汇编语言计算错误

Question

与 C++ 相比，该程序在汇编语言中给出了错误的计算，我找不到错误。有任何想法吗？

// (2 * c + b + a - 5) / (a / 4 - 1)

#include <iostream>

using namespace std;

void print_of() {
    cout << "Overflow error\n";
    exit(1);
}

void print_zf() {
    cout << "Division by zero error\n";
    exit(1);
}

int main() {
    long int a, b, c;
    long int asm_answer;
    cout << "(2 * c + b + a - 5) / (a / 4 - 1)\n";
    cout << "Enter a, b, c: ";
    cin >> a >> b >> c;
    // eax, ebx, ecx - general-purpose registers, its specific abilities don't matter in this laboratory work
    __asm {
        ; a / 4
        mov rax, a; rax = a
        mov rbx, 4; rbx = 4
        cqo; Convert Quad to Octa (rax -> rdx:rax)
        idiv rbx; division with a sign (dividend should be in rdx:rax -> quotient in rax, remainder in rdx)
        
        ; a / 4 - 1
        dec rax; decrement
        jo of_error; overflow error
        jz zf_error; if rax = 0, there will be division by zefo error
         
        ; reset
        push rax; push (a / 4 - 1) in stack
        
        ; 2 * c
        mov rax, c; rax = c
        imul rax, 2; rax = rax * 2
        jo of_error; overflow error
        
        ; 2 * c + b
        mov rdx, b; rdx = b
        add rax, rdx; rax = rax + rdx
        jo of_error; overflow error
        
        ; 2 * c + b + a
        mov rdx, a; rdx = a
        add rax, rdx; rax = rax + rdx
        jo of_error; overflow error
        
        ; 2 * c + b + a - 5
        sub rax, -5; rax = rax - 5
        jo of_error; overflow error
        
        ; rax = 2 * c + b - 5, rbx = a / 4 - 1
        pop rbx; rbx = a / 4 - 1
        
        ; (2 * c + b + a - 5) / (a / 4 - 1)
        cqo; Convert Quad to Octa (rax -> rdx:rax)
        idiv rbx; division with a sign (dividend should be in rdx:rax -> quotient in rax, remainder in rdx)
        mov asm_answer, rax
        jmp ext; "exit" is reserved
        
    of_error:
        call print_of; print overflow error (exit in function!)
    zf_error:
        call print_zf; print division by zero error (exit in function!)
    ext:
        
    }
    cout << "asm result: " << asm_answer << "\n";
    cout << "C++ result: " << (2 * c + b + a - 5) / (a / 4 - 1) << "\n";
    return 0;
}

控制台输出示例：

输入 a、b、c：100 0 20 asm 结果：6 C++ 结果：5 程序以退出代码结束：0

另一个例子：

输入 a、b、c：0 0 0 asm 结果：-5 C++ 结果：5 程序以退出代码结束：0

标签： c++assemblyx86-64inline-assembly