C Advanced - data storage (Part 1)

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1 Data type introduction

Basic built-in type ：

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

The meaning of type ：

1. Use this type to exploit the size of memory space （ Size determines the range of use ）.
2. How to look at the perspective of memory space .

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1.1 Basic classification of types ：

Plastic surgery 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]

Floating point family ：

float
double

Construction type ：

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

Pointer types :

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

Empty type ：

void Indicates empty type （ No type ）
Usually applied to the return type of a function 、 The parameters of the function 、 Pointer types .

2. Shaping storage in memory

   The creation of a variable is to open up space in memory . The size of space is determined according to different types .
   Next, let's talk about how data is stored in the opened memory .

such as ：
int a = 20; int b = -10;

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2.1 Original code 、 Inverse code 、 Complement code

   There are three ways to represent integers in a computer , 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 ”, The three representations of numeric bit negative integers are different .

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 .

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   A positive number 、 back 、 The complement is the same .
   For plastic surgery ： Data stored in memory is actually stored in the complement .

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In computer system , All values are represented and stored by complements . The reason lies in , Use complement , Symbol bits and value fields can be treated in a unified way ;
meanwhile , Addition and subtraction can also be handled in a unified way （CPU Only adders ） Besides , Complement code and original code are converted to each other , Its operation process is the same , No need for additional hardware circuits .

Let's look at the storage in memory ：

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2.2 Introduction to big and small end

What big end, small end ：

Big end （ Storage ） Pattern , The low bit of data is stored in the high address of memory , And the high end of the data , Stored in a low address in memory ;
The small end （ Storage ） Pattern , The low bit of data is stored in the low address of memory , And the high end of the data ,, Stored in memory
Address .

Why there are big end and small end ：

Why are there big and small end patterns ？ This is because in a computer system , We are in bytes , Each address corresponds to a byte , A byte is 8bit. But in C In language, except 8 bit Of char outside , also 16 bit Of short type ,32 bit Of long type （ It depends on the specific compilation
Translator ）, in addition , For digits greater than 8 Bit processor , for example 16 Bits or 32 Bit processor , Because the register width is larger than one byte , So there must be a problem of how to arrange multiple bytes . So it leads to big end storage mode and small end storage mode .
for example ： One 16bit Of short type x , The address in memory is 0x0010 , x The value of is 0x1122 , that 0x11 For high byte , 0x22 Is low byte . For big end mode , will 0x11 Put it in the low address , namely 0x0010 in , 0x22 Put it in a high address , namely 0x0011 in . The small end model ,
Just the opposite . That we use a lot X86 The structure is small end mode , and KEIL C51 It's the big end mode . A great deal of ARM,DSP It's all small end mode . There are some ARM The processor can also choose the big end mode or the small end mode by the hardware .