组装检查数字是否为偶数 [英] assembly check if number is even

问题描述

我有功课要编写汇编代码，以检查数字是否为奇数或偶数.我有这个代码

I have homework to write assembly code for checking if number is odd or even. I have this code

code_seg SEGMENT
    ASSUME cs:code_seg, ds:data_seg;

    mov ax, 11;
    test ax, 1;

end: jmp end;
code_seg ENDS

然后检查数字是否为偶数，我看是否设置了零标志.我知道测试指令就像逻辑与，如果结果为0，它将设置零标志.我的问题是:如何检查数字是否为奇/偶?其实我不知道为什么有些偶数(二进制)和(逻辑与)1给出0的结果?

And to check if number is even I look if zero flag is set. I know that the test instruction is like logical AND and that if result is 0 it sets zero flag. My question is: how this checks if number is odd/even? Actually I can't figure out why some even (binary) number and (logical and) 1 gives result of 0?

推荐答案

未签名和已签名的数字( Two的补码)是奇数的:

Both unsigned and signed numbers (Two's complement) are odd if the lowest bit is set:

00000001 = 1    (odd)    // unsigned, or signed positive
11111111 = 255  (odd)    // unsigned
01111111 = 127  (odd)    // unsigned, or signed positive
10000001 = -127 (odd)    // signed negative
11111111 = -1   (odd)    // signed negative

因此 test指令

test al, 1

检查是否设置了AL/AX/EAX/RAX的最低位.如果是，则数字为奇数.
可以使用 Jcc说明进行检查，尤其是那些测试?ZERO标记为

checks if the lowest bit of AL/AX/EAX/RAX is set. If it is, the number is odd.
This can be checked using the Jcc instructions, especially those testing the ?ZERO flag with

JNZ target    ; jump if odd  = lowest bit set
JZ  target    ; jump if even = lowest bit clear = zero

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