Original reverse compensation and size end

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Series column ：C Advanced language

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In the computer Integers There are three representations , namely Original code 、 Inverse and complement .

There are three ways of expression Sign bit and Value bits Two parts , The sign bits are all used 0 Express “ just ”, use 1 Express “ negative ”, And the number bit

The three representations of 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 .

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

Why? ？

In computer system , All values are represented and stored by complements . The reason lies in , Use complement , You can combine sign bits and numeric fields One processing ; 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 It's the same , No need for additional hardware circuits .

Let's look at the storage in memory ：

 

We can see that for a and b The complement code is stored separately . But we found something wrong with the order .  That's why ？

Introduction to big and small end

What is 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 , Low address saved in memory in ;

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 .

Suppose there is a number of 16 Into the system is 0x11223344

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 unit Each 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 position The processor of , for example 16 Bits or 32 Bit processor , Because the register width is larger than one byte , Then there must be a problem such as How to arrange multiple bytes . So This leads to large 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 by High byte ,0x22 Is low byte . For big end Pattern , 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 Structure is The small end model , 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 It is up to the hardware to choose whether it is the big end mode or the small end mode  

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