如何检测在大会语言特点X86溢出条件

问题描述

我在，我们必须写两个函数赋值。还必须检测使用处理器的条件codeS溢出情况，并返回 0 来表示已遇到一个错误。我能写的功能。

I have an assignment in which we have to write two functions. Also must detect overflow conditions using the processor's condition codes and return 0 to indicate that an error has been encountered. I was able to write the functions.

 .file  "formula.c"
    .text
.globl _nCr
    .def    _nCr;   .scl    2;  .type   32; .endef
_nCr:
        pushl   %ebp
    movl    %esp, %ebp
    subl    $56, %esp
    movl    8(%ebp), %eax
    movl    %eax, (%esp)
    testl %eax, %eax
    call    _factorial
    movl    %eax, -12(%ebp)
    movl    12(%ebp), %eax
    addl    $1, %eax
    movl    %eax, (%esp)
    call    _factorial
    movl    %eax, -16(%ebp)
    movl    12(%ebp), %eax
    notl    %eax
    addl    8(%ebp), %eax
    movl    %eax, (%esp)
    call    _factorial
    movl    %eax, -20(%ebp)
    movl    -16(%ebp), %eax
    movl    %eax, %edx
    imull   -20(%ebp), %edx
    movl    %edx, -28(%ebp)
    movl    -12(%ebp), %eax
    movl    %eax, %edx
    sarl    $31, %edx
    idivl   -28(%ebp)
    leave
    ret
.globl _factorial
    .def    _factorial;  .scl    2;     .type   32;     .endef
_factorial:
    pushl   %ebp
    movl    %esp, %ebp
    subl    $16, %esp
    movl    $1, -8(%ebp)
    movl    $1, -4(%ebp)
    jmp L3
L4:
    movl    -8(%ebp), %eax
    imull   -4(%ebp), %eax
    movl    %eax, -8(%ebp)
    addl    $1, -4(%ebp)
L3:
    movl    -4(%ebp), %eax
    cmpl    8(%ebp), %eax
    jle L4
    movl    -8(%ebp), %eax
    leave
    ret
    .def    ___main;    .scl    2;  .type   32; .endef
    .section .rdata,"dr"
    .align 4

这个功能基本上没有 N！/ R！（N-R）！。如果数量较大的溢出发生在阶乘。我只是不明白我怎么会设置溢出条件。

This function basically does n!/r!(n-r)!. The overflow occurs in factorial when the numbers get larger. I just do not understand how I would set the overflow conditions.

推荐答案

1）您的算术命令​​是可能设置溢出位的操作

1) Your arithmetic commands are the operations that could potentially set the overflow bit

2）JO溢出（跳）和JNO上不溢出（跳）允许您分支，根据是否产生溢出与否

2) The "JO" (jump on overflow) and "JNO" (jump on not overflow) allow you to branch, depending on whether an overflow occurred or not

3）你很可能只是联办后，将％EAX为0。

3) You'd probably just set "%eax" to 0 after "JO".

4）优秀的，如果你不熟悉它优秀的资源：

4) Excellent, excellent resource if you're not already familiar with it:

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