Stored state and running state of program

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• MDK Compilation process and ARM Compile toolchain

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The application has a quiescent state （ Stored state ） And running state （ Running state ）. Static state is the state that the program is stored in nonvolatile memory before running , such as stm32 Internal Flash in , therefore , The program can also be saved normally after the system is powered off . When we download the program （ burning ） To STM32F429 Of Flash in , If you do not run the program , Then the program is at rest , The program still exists after shutdown and restart , Will not be lost . When the program starts to run , That is, when the program is running , Programs often modify some temporary data , In order to speed up , These data are usually loaded into memory （RAM） in , The data in memory will be lost after power failure . therefore , The program is in both static and running states , Their positions in the memory are different .

When the program is in the stored state ,RO section （RO section） and RW section （RW section） Is stored in a ROM District , such as STM32 Of Flash in . When the program starts to run , The kernel directly from ROM Read code from , And execute a loading code before executing the body code , hold RW section Data from ROM copy to RAM In the memory , And in RAM Add ZI section,ZI The data of the section is initialized to 0, Finished loading RAM After the district , Officially start the implementation of the main procedure .
stay 《MDK Compilation process and ARM Compile toolchain 》 In the article , The compiled information is divided into several fields （ Please refer to my article ）. In compilation information RW-data and ZI-data Corresponding to RW section and ZI section, The difference between the two is whether it needs to be saved after power failure . Because in RAM When creating data in , The default initial values are 0, such as ZI section , But some data require that the initial value is not 0, That is, it is initialized as non 0 The value of the data , This data needs to be processed through ROM To record the initial value （ Because the initial value is determined before the program runs , That is to say, this is not 0 The initial value should have the property of saving after power failure , Therefore, it is necessary to put non 0 The initial value data is saved in ROM/Flash in , And the initial value is 0 After the power failure, the data will be restored in RAM Loading in memory does not affect the initial value , So there is no need to save it after power failure , There is no need to save to ROM/Flash Record the initial value , But directly in the running state RAM You can create it in , namely ZI section）, And stored in ROM The data in will be copied to RAM.STM32 Of RO The area will not be loaded into RAM in , The kernel directly from Flash Read instruction . What needs to be distinguished here is , The computer will put RO Area loaded into memory , Because computers have MMU unit , Support virtual memory , You can run programs larger than physical memory .
When the program is downloaded to STM32 Of Flash（RAM） In the time , It takes up space code + RO-data + RW-data , If this sum is greater than STM32 Of Flash Storage space , There will be mistakes . Occupied during program execution SRAM Space for RW-data + ZI-data.

Program status	Compiled data field
Running state RO District	code + RO-data
Running state RW District	RW-data + ZI-data
Stored state ROM District	code + RO-data + RW-data

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