About RO, RW, ZI in ARM

I have always had a seemingly correct understanding of the RO, RW and ZI data described in the ARM system. During this period, I carefully studied them and found some rules. I understood some things that were in the books but not understood before. I think there should be many people who have the same confusion as me, so I wrote down some of my understandings about RO, RW and ZI, hoping to help everyone.
To understand RO, RW and ZI, you need to first understand the following knowledge:
The composition of ARM programs
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 contains 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
The composition of ARM image files
The so-called ARM image file refers to the bin file burned into the ROM, also known as the image file. It is called the image file below.
The image file contains RO and RW data.
The reason why the Image file does not contain ZI data is that ZI data is all 0, so it is unnecessary to include it. Just clear the area where ZI data is located before the program is run. 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 initialized variables in RW cannot be "created out of nothing" like ZI.
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 the 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 be stored in ROM.
In the initial stage of program execution, 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 saying the above, it may still be a bit confusing. What are RO, RW and ZI? Below I will give a few examples to explain what RO, RW, and ZI mean in C in the most intuitive way. 1;
RO
Look at the following two programs. There is a statement between them. This statement is to declare a character constant. Therefore, according to what we said before, they should only differ by one byte in RO data (character constants are 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 Size) + 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 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
Similarly, looking at the two programs again, the only difference between them is an "initialized variable". According to what was 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 compiling Prog3 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)
===
...
​
​
​
Prog3:
#include

void main(void)
{
;
}
Prog4:
#include

char a；
void
main(void)
{
;
}
The information after compiling Prog3 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 after Prog4 compiles 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 )
=== ... 3; The variables in C that have been initialized to non-zero values ​​will be compiled into RW type data. Appendix: Program compilation command (assuming the C program name is 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