Volatile means variable and variable, and the role is to limit the compiler to optimize certain variables. First look at a C51 program: Keil gets the following assembly code (partial not listed) when the optimization level is 8. It can be seen that when the value of the variable d is assigned to x, y, and z, only the value in d is directly read in x, and y=d, z=d directly assigns the value in the register to y, z. If the value of the variable d is changed during this process (for example, d is a hardware register), the data obtained in the y, z variables will be wrong, so there are hidden dangers in some applications. The code that was recompiled (partially not listed) is as follows: It can be seen that the value of this y,z variable is read from the storage area of ​​d. This is mainly done by the optimization of the compiler, not the compiler error. After modifying the variable d with a volatile variable, the compiler does not optimize the operation of this variable, and the execution of the code achieves the desired purpose.
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This type of problem is not a problem with the compiler. Since accessing the internal registers is more efficient than accessing the RAM speed block, the compiler optimizes the program when compiling a similar program. Except when the first compiled variable is in a continuous reading of a variable, the compiler simplifies the program whenever possible. The first read value will be placed in ACC or Rx, and the first read value will be used directly when reading the value of the variable later. If the value of this variable has been changed by a peripheral (such as a peripheral port as an external RAM address when reading an external device port) or other programs (such as an interrupt service routine) in this process, an error may occur. In order to solve this kind of problem, the common method is to reduce the optimization level of the compiler or use the volatile keyword. Obviously reducing the optimization level is not expected, so it is necessary to modify the relevant variables with the volatile keyword.
The example above adds d to the volatile keyword as follows: 
In general, the volatile keyword is used in the following places.
(1) Variables modified by the interrupt service routine for other programs need to be added with volatile.
(2) The flags shared between tasks in a multitasking environment should be volatile.
(3) Memory mapped hardware registers are usually also added with volatile instructions, because each time it is read or written may have different meanings.