Volatile function of embedded C language

Important examples ：
tell compiler Can't do any optimization
For example, send two instructions to a certain address ：

int *ip =...; // Device address 
*ip = 1; // The first instruction 
*ip = 2; // Second instruction 

In general c In the program , This program is just assignment , The first instruction of the above program is completely unnecessary ,compiler It may be optimized ：

int *ip = ...;
*ip = 2;

As a result, the first instruction was lost . But in embedded or single chip computer , The first is necessary , If you use volatile, compiler No optimization is allowed , So as to ensure the original intention of the program , This is very important for embedded .

One 、 Changeable
volatile The main definition in the dictionary is “ Changeable ”.
Variables are stored in memory , But in order to improve the running speed , Sometimes you don't get values from memory , And from cpu cache / Value in register . In embedded / Or MCU development , This can sometimes be fatal .
volatile After defining variables ： Suppose you have read 、 Write two sentences , One by one volatile Variable to operate , Then the read operation of the latter item will not directly use the corresponding write operation of the previous item volatile Variable register Content , But from Memory Read the volatile The value of the variable .
As described above （ part ） The sample code is as follows ：

volatile int nNum = 0;  //  take nNum Declare as volatile
int nSum = 0;
nNum = FunA();      // nNum New content written , Its value is cached in registers 
nSum = nNum + 1;    //  From memory （ Not registers ） Read from nNum Value 

Two 、 Non optimizable
stay C/C++ In programming language ,volatile The second feature of is “ Can't be optimized ”.
volatile Will tell the compiler , Don't take volatile Declare variables for various radical optimizations （ Even eliminate variables directly ）, So as to ensure that the instructions written by the programmer in the code will be executed .
As described above （ part ） The sample code is as follows ：

volatile int nNum;  //  take nNum Declare as volatile
nNum = 1;    
printf("nNum is: %d", nNum);

In the above code , If the variable nNum There is no claim to be volatile type , Then the compiler will optimize it in the compilation process , Use constants directly “1” Replace （ After this optimization , The generated assembly code is very brief , Efficient execution ）.
And when you use volatile After making the statement , The compiler will not optimize it ,nNum Variables still exist , The compiler will take the variable out of memory , Put it into the register , Then call printf() Function to print .
3、 ... and 、 In sequence
stay C/C++ In programming language ,volatile The third feature of is “ Sequential execution feature ”, That is, it can guarantee volatile The order between variables will not be optimized by the compiler .
**C/C++ The basic optimization principle of compiler ：** Ensure the output of a program , There is no change before and after optimization . In order to have an in-depth understanding of this feature , Here are two variables （nNum1 and nNum2） For example （ Since it exists “ Sequential execution ”, The description object must be larger than one ）, Introduce volatile Sequential execution feature of , The sample code is as follows ：

int nNum1;
int nNum2; 
nNum2 = nNum1 + 1;    //  sentence 1
nNum1 = 10;           //  sentence 2

In the above code ： When nNum1 and nNum2 No use volatile When modifying keywords , The compiler will “ sentence 1” and “ sentence 2” Optimize the execution sequence of ： I.e. execute first “ sentence 2”、 Re execution “ sentence 1”; When nNum2 Use volatile When modifying keywords , The compiler may also be right “ sentence 1” and “ sentence 2” Optimize the execution sequence of ： I.e. execute first “ sentence 2”、 Re execution “ sentence 1”; When nNum1 and nNum2 All use volatile When modifying keywords , The compiler will not “ sentence 1” and “ sentence 2” Optimize the execution sequence of ： I.e. execute first “ sentence 1”、 Re execution “ sentence 2”; explain ： The above discussion can be verified by observing the assembly code generated by the code .