Use of ARM Linker - Initialization of the application running environment

Generally, executable programs include code segments and data segments. They can also be simply viewed as consisting of two parts: the RO segment and the RW segment. The RO segment generally includes the code segment and some constants, and is read-only during runtime. The RW segment includes some global variables and static variables, which can be changed (read and write) during runtime. If some global variables are initialized to zero, the RW segment also includes the ZI segment.
RO: Read Only  Code Segment
RW: Read Write Initialized global variables
ZI: Zero Init   Uninitialized global variables

    Because the RO segment is read-only and cannot be changed during operation, the RO segment can reside in Flash (or SDRAM or SRAM) during operation. The RW segment is readable and writable, so it must be loaded into SDRAM or SRAM during operation.
    When compiling with ADS, you need to set the RO BASE and RW BASE. Those who have used ADS should know this. By setting the RO BASE and RW BASE, you can tell the linker the starting address (RO BASE) and the address of the RW segment (RW BASE) of the program. If a program only has the RO segment and no RW segment, then the program can be run completely in Flash without using SDRAM or SRAM. If it includes the RW segment and the RO segment, then the RW segment of the program must be copied to SDRAM or SRAM before being accessed to ensure that the program can run correctly. The following figure illustrates the status of a program before execution (load view) and during execution (execute view). As can be seen from the figure, the entire program is placed in ROM before execution. During execution, the RW segment is copied to the appropriate location in RAM.

　　The program is always stored in ROM/Flash at the beginning. The RO part can be executed in ROM/Flash or transferred to the faster RAM; while the RW and ZI parts must be transferred to the writable RAM. The so-called initialization of the application execution environment is to complete the necessary data transfer from ROM to RAM and clear the content.

　　Different tool chains will provide different mechanisms and methods to help users complete this step, mainly related to the linker. The following is a direct implementation of a commonly used memory model in the ARM development tool environment ADS:

　　 

     LDR r0, = |Image$$RO$$Limit| ;Get the address of the next byte at the end of the RO segment, the starting address of RW in ROMLDR
　　r1, = |Image$$RW$$Base| ;Get the initial address of the RW segment in RAMLDR
　　r3, = |Image$$ZI$$Base| ;Initial address of global variablesCMP 
　　r0, r1 ;
　　BEQ LOOP1
LOOP0
　　CMP r1, r3 ;  Is it the end address of the RW segment in RAM? If not, keep copying ROM (FLASH   variables and data segments to
　　RAMLDRC r2, [r0], #4  ;[R0]=[R1]
　　STRCC r2, [r1], #4 ; 
　　BCC LOOP0
LOOP1
　　LDR r1, = |Image$$ZI$$Limit| ; LOOP1 and LOOP2 execute to initialize ZI to 0
　　MOV r2, #0
LOOP2
　　CMP r3, r1
　　STRCC r2, [r3], #4;
　　BCC LOOP2

    In ADS, there are some predefined variables available (linker defined symbol). In the following implementation, several predefined variables are used:
Image$$RO$$Base. This variable specifies the BASE of the RO segment.
Image$$RO$$Limit. This variable specifies the Limit of the RO segment.
Image$$RW$$Base. This variable specifies the BASE of the RW segment.
Image$$RW$$Limit. This variable specifies the Limit of the RW segment.
Image$$ZI$$Base. This variable specifies the BASE of the ZI segment.
Image$$ZI$$Limit. This variable specifies the Limit of the ZI segment
. Note: For details, please refer to the ADS Linker Guide.
Image$$RO$$Limit minus Image$$RO$$Base equals the size of the RO segment. Image
$$RW$$Limit minus Image$$RW$$Base equals the size of the RW segment.
Image$$ZI$$Limit minus Image$$ZI$$Base equals the size of the ZI segment.
(Image$$RO$$Limit minus Image$$RO$$Base)
+ (Image$$RW$$Limit minus Image$$RW$$Base)
= Equal to the size of the entire program
Note: The ZI segment is included in the RW segment.