问题描述
我正在尝试理解MARIE汇编语言.我不太了解skipcond做类似<或>的事情，或者乘或除.
I am trying to understand the MARIE assembly language. I don't quite understand skipcond fordoing things like <, or >, or multiply or divide.
我正在学习这个简单的程序:
I am taking this simple program:
x = 1
while x < 10 do
x = x +1
endwhile;
我不了解的是如何使用某些跳过条件:
What I don't understand is how to use certain skip conditions:
Skipcond 800 if AC > 0,
Skipcond 400 if AC = 0,
Skipcond 000 if AC < 0
现在，我知道我会从10中减去x并使用skipcond进行测试.
Now, I know I would subtract x from 10 and test using skipcond.
我不确定是哪一个，为什么.我想如果我知道它们是如何工作的，也许会更容易理解.为什么用来与零进行比较?
I am not sure which one and why. I guess if I knew how they really work maybe it would be easier to understand. Why is it used to compare to zero?
这就是我所拥有的:
100 load one
101 store x
102 subt ten
103 skipcond400 if x-10 = 0? // or skpcond000 x -10 < 0??
推荐答案
while x < 10 do
x = x + 1
x等于10时，
将跳出循环.如果从x减去10，您将得到一个负值，直到x等于10(且值为0).因此，使用skpcond000是错误的，因为它跳得太早了.所以skpcond400是正确的.
will jump out of the loop as soon as x equals 10. If you subtract 10 from x, you'll get a negative value until x equals 10 (and the value is 0). So using skpcond000 would be wrong as it would jump out too soon. So skpcond400 is correct.
如果更改C代码，也许更容易理解，因此它更接近汇编代码:
Perhaps it is easier to understand if you change the C code so it will be closer to the assembly code:
Original: while (x < 10) do
Subtract 10: while ((x - 10) < 0) do
Use != instead of <: while ((x - 10) != 0) do
还请注意，必须增加条件后的x 才能再现与while循环相同的行为.
Also note that you have to increase x after the condition to reproduce identical behaviour to the while loop.
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