需要帮助理解与MASM条件指令 [英] Need help understanding conditional directives with MASM

问题描述

我试图用MASM指令来实现MASM下面的C code：

I am trying to implement the following c code in MASM using MASM directives:

if ( a > b )
    a = a - 1;
else
    if ( b >= c )
        b = b − 2;
    else
        if ( c > d)
            c = c + d;
        else
            d = d / 2;

这是我的尝试：

.if (a > b)
sub a, 1
.elseif b >= c1
sub b, 2
.elseif c1 > d
add c1, d
.else
mov eax, d
cdq
mov ebx, 2
idiv ebx
mov d, eax
.endif
.endif

我觉得我的逻辑是合理的但无论怎样我改变周围的，以保持它的完整，我收到错误信息。我相信我有误解的东西，但不要不确定的东西。

I feel like my logic is sound yet no matter what I change around to keep it intact I am getting errors. I am sure I have misunderstood something, but don't unsure about what.

推荐答案

首先，因为你只有一个开口。如果，你只需要一个 .endif注意。其次，至少在 A ， B ， C1 ，和 D 都是正常的内存操作数，你有大部分指令不能使用两个内存操作数（直接）的一个问题。对于典型的比较，操作数的至少一个最多可以在寄存器中。

First of all, since you have only one opening .if, you only need one .endif. Second, at least if a, b, c1, and d are normal memory operands, you have a problem that most instructions can't use two memory operands (directly). For your typical comparisons, at least one of the operands most be in a register.

顺便说一句，我也缩进。如果（等等）code，就像你通常会code在一个更高层次的语言。至少通常情况下，我还用月而不是子X，1 ，大概 SHR 而不是 IDIV 来除以2。

As an aside, I'd also indent .if (and such) code just like you normally would code in a higher level language. At least normally, I'd also use dec instead of sub x, 1, and probably shr instead of idiv to divide by 2.

考虑上述所有的考虑，你最终是这样的：

Taking all of the above into account, you end up with something like this:

.model flat, c
.data
    a dd ?
    b dd ?
    c1 dd ?
    d dd ?

.code
junk proc
     mov eax, a
     mov ebx, b
     mov ecx, c1
     mov edx, d

    .if eax > ebx
        dec a
    .elseif ebx >= ecx
        sub b, 2
    .elseif ecx > edx
        add ecx, edx
        mov c1, ecx
    .else
        shr edx, 1
        mov d, edx
    .endif
junk endp

    end

这组装就好了，至少对我来说。

This assembles just fine, at least for me.

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