zoukankan      html  css  js  c++  java
  • two's complement,2的补码

    Let's start with one question.

    Q: What's the output of below program?

    #include <stdio.h>
    
    void main(void)
    {
        char a = -1;
        printf("a = %d\n", (unsigned char)a);
    }
    

    Hope the rest of this article can help you answer this question.

    Q: What is two's complement?

    The two's complement of a is 2^N-a.

    Q: What is one's complement?

    The one's complement of a is (2^N-1)-a. (2^N-1) is all 1, e.g. 2^4-1 is 1111. So this is called one's complement.

    Q: In address 0x100, data 1010 1100 (let's name the value as a) is stored. From the value, can we know it's signed or unsigned type?

    The answer is definitely NO. a can be considered as unsigned char (value will be -84), it can be also considered as signed char (value will be 172). There is also no another flag indicating us how to consider it, so we don't know the memory a should be considered as signed or unsigned.

    We don't know, and the computer also don't know.
    But CPU has two flags (OF and CF), the overflow flag based on signed type is OF, the flag based on unsigned type is CF.
    Except the overflow flag, add and sub operation don't distinguish type of data.

    Example 1

    For example,

    • memory address 0x001 stores 1111 1100(let's name it as c),
    • memory address 0x002 stores 0000 0101 (let's name it as d).
      When the CPU do c+d, what will happen?
                    signed   unsigned    name
       1111 1100     -4       252     :   c
    +  0000 0101      5        5      :   d 
    --------------
      10000 0001      1       257
                     No       Yes     :  overflow?
    

    In upper example,

    • the result doesn't overflow if we consider them as signed number. So the result is right.
    • the result overflows if we consider them as unsigned number, so the result is wrong.

    Which result we should use?
    The answer depends on you, not depend on the CPU. The good thing is that the CPU will tell you the overflow flag based on signed and unsigned together.

    Example 2

    Another example as below.

    #include <stdio.h>
    
    int main(void)
    {
        /* range of char is [-128, 127], so `a` doesn't overflow.
           The `a` in memory is `0111 1111`(MSB -> LSB in binary format).
         **/
        char a = 127;
    
        /* C compiler knows `a` is `char` type, so it will decode
           memory value of `a` according to `char` type.
         **/
        printf("char a = 127; ==> a is %d\n", a);
        
        /* When perform `a = a + 1`, CPU just add `1` to the memory
           value of `a`. Value of `a` in memory is `1000 0000` after add operation.
         **/
        a = a + 1;
        printf("char a = a+1; ==> \n");
    
        /* C compiler will decode the memory value of `a` as `char` type.
           `1000 0000` in `char type` is `-128`.
         **/
        printf("\t1). a is %d (overflowed!!)\n", a); 
    
        /* C comiler will decode the memory value of `a` as `unsigned char` type,
           because we explicitly say by `(unsigned char)a`.
           `1000 0000` in `unsigned char` type is `128`.
         **/
        printf("\t2). (unsigned char)a is %d\n", (unsigned char)a); 
    
    }
    

    The output of upper program as below.

    char a = 127; ==> a is 127
    char a = a+1; ==> 
        1). a is -128 (overflowed!!)
        2). (unsigned char)a is 128
    

    本文为作者原创,允许转载,但必须注明原文地址:https://www.cnblogs.com/byronxie/p/10117265.html


    本文的数字签名如下:

    MGUCMQDYymaHZ/G/34Ds5GegHhsHgdMsoiGJ+Fu/An/IDiL3O1TwxbiPWMlTvCvdBiM9wR0CMDVUVIgk/YfWRRzyHQkKGp3DSiO5DKLpm8XSQg8HX0Ua+BcdHLOWiZOjPj/RzgmjNA==
    --- 2019-7-20 9:34:50

    数字签名验证方法

  • 相关阅读:
    pytest 框架生成 pytest
    pytest 之数据驱动参数化:pytest.mark.parametrize
    web 网站表单测试、搜索查询测试、删除测试、数据库测试知识点
    pytest 之 fixture 的前置后置功能
    pytest 框架之pytest-html报告生成
    Initialization of bean failed; nested exception is java.lang.
    使用jmeter测试Oracle&&MySQL数据库性能
    jmeter的日常特殊参数化
    压测出现各种奇葩问题,求围观
    weblogic性能调优
  • 原文地址:https://www.cnblogs.com/byronsh/p/10117265.html
Copyright © 2011-2022 走看看