[advanced C language] integer storage in memory

This series , We will explore C The deeper content of language ！

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List of articles

• Preface
• One 、 What integers include ？
• Original code , Inverse code , Complement code
• Two 、 How to store
  • 1. What is the big and small end
  • 2.unsigned and signed The difference between integer classes
• summary

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Preface

  In the daily process of typing code , We often use integer constants to assign values to variables , But we may not have considered how different variables are stored in memory ！ Today I'd like to explore the mystery with you .

The internal capacity may be slightly larger , We can't finish it. We can collect it , Then look at ！

One 、 What integers include ？

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      1. Classification of integers

         Integers include char,short,int,long,long long. Of course, someone might ask , Why? char It's also an integer ？

That's because char Every character has its corresponding ASCII Code value , So of course it's an integer ！ This is the integer family ！

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       2. Integers store in memory the basics you must know （ Original inverse complement ）

    In memory , Whether it's positive or negative , All storage forms are stored in the form of complement ！！

    First , When stored in memory , We all store binary integers . Then use binary to represent the original code of integer , Inverse code and complement code ！

        There are three kinds of integers in the computer 2 Decimal representation , The original code 、 Inverse and complement .

      The three representations have two parts: sign bit and numeric bit , The sign bits are all used 0 Express “ just ”, use 1 Express “ negative ”.
      A positive number 、 back 、 The complement is the same .
      The three representations of negative integers are different

      Original code
        The original code can be obtained by directly translating the numerical value into binary in the form of positive and negative numbers .
      Inverse code
        Change the sign bit of the original code , The reverse code can be obtained by inverting other bits in turn .

      Complement code
        Inverse code +1 You get the complement .

Illustrate with examples ：int a= -20;（ The first bit is the sign bit ）

Original code ：10000000 00000000 00000000 00010100

Inverse code ：11111111 11111111 11111111 11101011

Complement code ：11111111 11111111 11111111 11101111

Complement is the way to store in memory ！！

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       3. Classification of integers

      Integer types can be divided into signed and unsigned types ！ That is to say signed and unsigned.

     signed and unsigned The difference is that Sign bit Remember or not Value bits ！

      In general ,int,char An equal integer is usually a signed number .

Illustrate with examples ：

unsigned  int a= -20;

An unsigned number , Treat the first sign bit as a numeric bit , So this is also a positive number . The former is the same as the latter

Original code ：10000000 00000000 00000000 00010100

Inverse code ：10000000 00000000 00000000 00010100

Complement code ：10000000 00000000 00000000 00010100

Two 、 Specific storage methods

1. Large and small end

              We can find out , When stored in memory ,int a and int b Shouldn't it be stored with binary complement ？  Should not be 32 Bit storage form of binary bits ？

              Next , We explore ！

0x yes 16 Hexadecimal representation , So we guess , Probably stored in binary complement , With 16 The system is displayed , So give it a try ！

int b=20;

The former is the same as the latter ：0000 0000 0000 0000 0000 0000 0001 0100

（ Binary number every four bits , It's a bit. 16 Hexadecimal number ）

16 Hexadecimal said ：00 00 00 14

  A surprise discovery , It's the same ！！！！！

But there are still some differences , The order is different ！

So that leads to what we are going to talk about today （ Large and small end （ Byte order ）） That is, big end and small end

  Big end ： The low bit of the data is saved to the high address in the memory , The high bit of the data is saved to the low address of the memory

  Short paragraph ： The low order of the data is saved to the low address in the memory , The high bit of the data is saved to the high address of the memory

  therefore , After understanding and mastering , Try the compiler you are using for storage ！

Like blogger I use vs2022, Then let me have a try ！！

 

 

2.unsigned and signed The difference between integer classes

Let's start with a question , Explore through the topic ！

char a= -1;

    Original code ：10000000 00000000 00000000 00000001

    Inverse code ：111111111  11111111  11111111 11111110

    Complement code ：11111111 11111111 11111111 11111111（ In memory storage mode ： Complement code ）

    however char Take up a byte

  11111111

  To use %d Output , Then it will involve integer promotion .

                          Rules for integer Promotion ： And lower than its own type or some operation , Integer promotion will occur

                         1.signed type ： When a positive number increases , repair 1; Negative numbers make up 0;（ Complement as the highest sign bit ）

                         2.unsigned type ： In ascension , Direct compensation 0;

that ,char a= -1, With %d When the output , When integer lifting ,

11111111

Aftertreatment ：11111111 11111111 11111111 11111111 Stored in memory as complement , Output in original code

Original code ：10000000 00000000 00000000 00000001

（ Complement to original , It is still the first to take the opposite （ The sign bits remain the same ！）, One more ）

So the end result is -1

 

signed char a = -1 And char a = -1 It's the same .

In many environments ,char,int Are signed numbers ！

So it turns out that -1

unsigned char a = -1

    Original code ：10000000 00000000 00000000 00000001

    Inverse code ：111111111  11111111  11111111 11111110

    Complement code ：11111111 11111111 11111111 11111111

    Take only the last byte

  11111111

    Improve the overall shape （ Unsigned numbers are added when they are raised 0 ）：00000000 00000000 00000000 11111111（ Memory to make up         Code storage ）

    The highest bit is 0, Positive number , Both positive and negative complements are the same , So the result is   255.

Let's see the results

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  I believe you already know , And familiar with it , Of course, more practice is needed ！

Strike while the iron is hot ！

 

（%u Output for unsigned ）

primary 10000000 00000000 00000000 10000000

back 11111111 11111111 11111111 01111111

repair 11111111 11111111 11111111 10000000

char 10000000

Improve the overall shape 11111111 11111111 11111111 10000000

result ：4294967168

answer ：-10

  Is the result right ？

summary

      1. In memory , Whether it's positive or negative , All storage forms are stored in the form of complement ！！

      2. In general ,int,char An equal integer is usually a signed number .

      3. Large and small end

            Big end ： The low bit of the data is saved to the high address in the memory , The high bit of the data is saved to the low address of the memory

            Short paragraph ： The low order of the data is saved to the low address in the memory , The high bit of the data is saved to the high address of the memory

                                                                              Don't forget to think about it ！