我如何从4倒计时，并汇编为0停止 [英] how do i count down from 4 and stop at 0 in assembler

问题描述

我怎么指望从4下来，汇编为0停止。到目前为止，我已经试过这一点。我想则显示3210文本但是它打印3333333333333。
我试图用存储STR然后使用新号码减1再减去之后的新的结果。但它似乎是使用4所有的时间。

how do i count down from 4 and stop at 0 in assembler. I have tried this so far. I would like the text to be displayed then 3210. However It prints 3333333333333. I am trying to store the new result after the subtraction using STR then using the new number subtract 1 again. But it seems to be using 4 all the time.

        B main

    verses  DEFW    4
    temp    DEFW    0
    text    DEFB    " men went to mow a meadow ",0

        ALIGN

main    ADR R0,verses
        SWI 4

        ADR R0,text
        SWI 3



loop    ADR R0,verses
        SUB R0,R0,#1
        STR R0,verses
        SWI 4

        B loop



        SWI 2

推荐答案

这是错误的：

ADR R0,verses

SWI 4 预计的值的在打印R0 ，而不是 的值的地址。所以，你应该使用类似 LDR R0，经文来代替。你得到4的原因，当你做 ADR R0，经文是因为诗恰好位于地址4。但是，通过使用 ADR 你会得到4不管你存储的在的诗。

SWI 4 expects the value to print in R0, not the address of the value. So, you should use something like LDR R0,verses instead. The reason you get 4 when you do ADR R0,verses is because verses just happens to be located at address 4. But by using ADR you'll get 4 whatever you store at verses.

另外，你的循环是无限的，因为你没有添加有史以来退出循环任何code。它应该是这个样子，而不是：

Also, your loop is infinite because you haven't added any code that ever exits the loop. It ought to look something like this instead:

LDR R0, verses
loop    SUBS R0,R0,#1
        BMI done
        SWI 4
        B loop
done

请注意：我不知道，如果 SWI 取值preserve 的值R0 。如果他们不这样做，你将不得不采取照顾自己，例如通过保存 R0 在堆栈中。

Note: I don't know if the SWIs preserve the value of R0. If they don't you'll have to take care of that yourself, e.g. by saving R0 on the stack.

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