What are RO, RW and ZI generated by Keil compilation?

To understand RO, RW and ZI you need to first understand the following:

1. Composition of ARM program:
        The "ARM program" mentioned here refers to the program being executed in the ARM system, not the bin image file saved in ROM. Please pay attention to the difference.
         An ARM program consists of 3 parts: RO, RW and ZI. RO is the instruction and constant in the program; RW is the initialized variable in the program; ZI is the uninitialized variable in the program.
         From the above 3 points, it can be understood that RO is readonly, RW is read/write, and ZI is zero
 
          . 2. Composition of ARM image file
         The so-called ARM image file refers to the bin file burned into ROM, also called image file. It is called Image file below.

         Image files contain RO and RW data. The reason why image files do not contain ZI data is that ZI data is all 0, so there is no need to include it. Just clear the area where ZI data is located before running the program. Including it will waste storage space.

            Q: Why must RO and RW be included in the Image?
            A: Because the instructions and constants in RO and the variables initialized in RW cannot be created out of nothing like ZI.
 
         3. The execution process of the ARM program
         From the above two points, we can know that the image file burned into the ROM is not exactly the same as the ARM program in actual operation. Therefore, it is necessary to understand how the ARM program reaches the actual running state from the image in the ROM. In fact
            , the instructions in RO should at least have the following functions:
            1. Move RW from ROM to RAM, because RW is a variable and variables cannot exist in ROM.
            2. Clear all RAM areas where ZI is located, because the ZI area is not in the Image, so the program needs to clear the corresponding RAM area according to the ZI address and size given by the compiler. ZI is also a variable. Similarly: variables cannot exist in ROM. In the
            initial stage of program operation, the C program can access variables normally only after the instructions in RO complete these two tasks. Otherwise, only code without variables can be run.


            After all the above, you may still be confused. What are RO, RW and ZI? Below I will give a few examples to explain what RO, RW and ZI mean in C.
1. RO
            Look at the following two programs. There is a statement between them, which is to declare a character constant. Therefore, according to what we said before, there should only be one byte difference in RO data (character constant is 1 byte).
            Prog1:
            #include
            void main(void)
            {
            ;

            }

            Prog2:
            #include
            const char a = 5；
            void main(void)
            {
            ;
            }
            The compiled information of Prog1 is as follows:
            =====================================================

            Code RO Data RW Data ZI Data Debug
            948 60 0 96 0 Grand Totals

            =====================================================
            Total RO Size(Code + RO Data) 1008 ( 0.98kB)
            Total RW Size(RW Data + ZI Data) 96 ( 0.09kB)
            Total ROM Size(Code + RO Data + RW Data) 1008 ( 0.98kB)
            =======================================================
            The information compiled by Prog2 is as follows:
            =====================================================
            Code RO Data RW Data ZI Data Debug
            948 61 0 96 0 Grand Totals
            ===================================================
            Total RO Size(Code + RO Data) 1009 ( 0.99kB)
            Total RW Size(RW Data + ZI Data) 96 ( 0.09kB)
            Total ROM Size(Code + RO Data + RW Data) 1009 ( 0.99kB)
            ===================================================
            From the information after the above two programs are compiled, we can see that:
            The RO of Prog1 and Prog2 contains two types of data: Code and RO Data. The only difference between them is that the RO Data of Prog2 has one more byte than that of Prog1. This is consistent with the previous speculation.
            If an instruction is added instead of a constant, the result should be a difference in the size of the Code data.


2. RW
            Again, look at the two programs again. The only difference between them is an "initialized variable". According to what has been said before, the initialized variable should be counted in RW, so the size of RW should be different between the two programs.
            Prog3:
            #include
            void main(void)
            {
            ;
            }
            Prog4:
            #include
            char a = 5；
            void main(void)
            {
            ;
            }
            The information after Prog3 is compiled is as follows:
            ======================================================
            Code RO Data RW Data ZI Data Debug
            948 60 0 96 0 Grand Totals
            =========================================================
            Total RO Size(Code + RO Data) 1008 ( 0.98kB)
            Total RW Size(RW Data + ZI Data) 96 ( 0.09kB)
            Total ROM Size(Code + RO Data + RW Data) 1008 ( 0.98kB)
            ======================================================
            The information compiled by Prog4 is as follows: 
   ======================================================
            Code RO Data RW Data ZI Data Debug
            948 60 1 96 0 Grand Totals
            =========================================================
            Total RO Size(Code + RO Data) 1008 ( 0.98kB)
            Total RW Size(RW Data + ZI Data) 97 (0.09kB)
            Total ROM Size(Code + RO Data + RW Data) 1009 (0.99kB)
            ==================================================
            It can be seen that there is only one byte difference between Prog3 and Prog4 in RW Data, which is caused by an initialized character variable "a".
 
3. ZI
            Looking at the two programs again, the difference between them is an uninitialized variable "a". From the previous understanding, it can be inferred that the only difference between the two programs should be the size of ZI.
            Prog3:
            #include
            void main(void)
            {
            ;
            }
            Prog4:
            #include
            char a；
            void main(void)
            {
            ;
            }
            The information after Prog3 is compiled is as follows:
            ========================================================
            Code RO Data RW Data ZI Data Debug
            948 60 0 96 0 Grand Totals
            ============================================================
            Total RO Size(Code + RO Data) 1008 ( 0.98kB)
            Total RW Size(RW Data + ZI Data) 96 ( 0.09kB)
            Total ROM Size(Code + RO Data + RW Data) 1008 ( 0.98kB)
            ==========================================================
            The information compiled by Prog4 is as follows:
            ========================================================
            Code RO Data RW Data ZI Data Debug
            948 60 0 97 0 Grand Totals
            ============================================================
            Total RO Size(Code + RO Data) 1008 ( 0.98kB)
            Total RW Size (RW Data + ZI Data) 97 ( 0.09kB)             Total             ROM             Size
            (Code + RO Data + RW Data) 1008             (             0.98kB             )
            === ...             Appendix:             Program compilation command (assuming the C program is named tst.c):             armcc -c -o tst.o tst.c             armlink -noremove -elf -nodebug -info totals -info sizes -map -list aa.map -o tst.elf             The compiled information of tst.o is in the aa.map file.             ROM mainly refers to: NAND Flash, Nor Flash             RAM mainly refers to: PSRAM, SDRAM, SRAM, DDRAM