[deep analysis of C language] - data storage in memory

Preface

In the previous blog , We've covered the basic built-in types , As follows

char	// Character data type 
short	// Short 
int		// integer 
long	// Long integer 
long long 	// Longer integers 
float	// Single-precision floating-point 
double	// Double precision floating point 

Today we will analyze these built-in types on this basis , Let us have a deeper understanding of it

1️⃣ Data type introduction

1. The integer family

char
unsigned char
signed char
short
unsigned short [int]
signed short [int]
int
unsigned int
signed int
long
unsigned long [int]
signed long [int]

among char It's also part of the integer family , because char In memory, I use ASCII Worth storing in form , It can be regarded as a special integer .

2. Floating point family 🧡

float
double

3. Construction type

 An array type 
 Type of structure  struct
 Enumeration type  enum
 Joint type  union

4. Pointer types

int pi;
char pc;
float pf;
void pv;

2️⃣ The storage of data in memory

1. The storage of integers in memory 🤎

Before you know how to store shaping , Let's introduce ： Original code 、 Inverse code 、 Complement code .

 There are three ways to express the number of symbols in a computer , The original code 、 Inverse and complement . The three methods have two parts: sign bit and value bit .
 Original code ： Directly translate binary into binary in the form of positive and negative numbers .
 Inverse code ： Change the sign bit of the original code , The other bits can be inverted in turn .
 Complement code ： Inverse code  +1  You get the complement .
 The original code of a positive number 、 Inverse code 、 The complement is the same .  

For plastic surgery ： Data stored in memory is actually stored in the complement .

There is also a reason ：

1. Use complement , Symbol bits and value fields can be treated in a unified way ;

2. Addition and subtraction can also be handled in a unified way （CPU Only adders ）;

3. Complement code and original code are converted to each other , Its operation process is the same , No need for additional hardware circuits .

2. Floating point storage in memory

1. According to international standards IEEE 754, Any binary floating point number V It can be expressed in the following form ：
   · (-1)^S * M * 2^E.

   · (-1)^S The sign bit , When S == 0, V Is a positive number ; When S == 1, V It's a negative number .

   · M Represents a significant number , Greater than or equal to 1, Less than 2、

   · 2^E Indicates the index bit .

2. IEEE 754 Regulations ： about 32 Bit floating point , The highest 1 Bits are sign bits S, And then 8 Bits are exponents E, The rest 23 Bits are significant numbers M.

E Two special values of ：

1. E All for 0：

   When E All for 0 When , namely 2 To the power of 0 - 127 by 2^-127 Power , therefore , When s = 0 when , A positive number 2^-127 Power , It is a wireless approach from the right of the number axis to 0 The number of ; And when s = 1 when , A negative number 2^-127 The power is from the left of the number axis to 0 The number of .

   So when E All for 0 When , Actually, it means ±0, So floating point numbers cannot appear in programs And 0 To compare （ Floating point numbers == 0）, But to compare with a range .

2. E All for 1：

   When E All for 1 When （ If M All for 0）, namely 2 To the power of 255 - 127 = 128, So when S = 0 when , Express 1 * 2^128 Power , When s = 1 It means -1*2^128 Power .

   So when E All for 1 when , In fact, it represents the value range of this floating-point number .

3️⃣ summary

The above is a description of how data is stored in memory

If there is anything improper , Hope to point out ！