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I have just started writing assembly code. I am using the simulator in Keil uvision5 to the run the code below. I dont understand why I can't see the src data value of 14 in register r1 when I ldr the data value from the address stored in the r0 register. I have tried to research the solution myself but cant find an exact reason why this happens. Any help would be gratefully appreciated.
THUMB AREA RESET, CODE, READONLY EXPORT __Vectors EXPORT Reset_Handler __Vectors DCD 0x20001000 DCD Reset_Handler AREA Code, CODE, READONLY ENTRY Reset_Handler LDR r0, =src LDR r1, [r0] terminate ; sit in an endless loop B terminate ; data area that can be modified (readwrite) AREA Data, DATA, READWRITE src DCD 14 END
This is due to the way you have the linker configured (scatter file), and that statics bound for RAM are placed into the back end of the FLASH/ROM portion with the expectation that start up code (either the C run time library, or your own) will copy the statics and zero other regions. When the processor resets the RAM is assumed to contain garbage, and must be loaded with the default/expected values.
Your code does not provide any copying function, the processor starts and goes directly to your code. The routine __main normally does this before calling your own main() function.
So either place your constants in FLASH, or take more ownership of copying them too RAM.
You might want to search on the linker documentation, and the concepts related to startup from ROM.
Thank you for your prompt and detailed answer. I have researched the topics that you have said for me to look at in your answer but I am not quite understanding this. I understand what you are telling me to do and why I can't initialise my constants I just don't know how to go about doing it. My research has not helped me understand how to do this I feel more confused! From what you are saying I don't have uvision configured correctly in particular the linker. When I go into project options and the linker tab under scatter file there is no file. Also how would you copy constants to RAM in code I dont understand what code you would write to do this. Does my whole problem stem from the incorrectly configured linker or do I also need to add code in once I get the linker correctly configured.
Thank you very much for all your help. I now understand what I have to do to initialise the data from reading the links you provided.
Thank you once again for your patience and help.
THUMB AREA RESET, CODE, READONLY EXPORT __Vectors __Vectors DCD 0x20001000 DCD Reset_Handler AREA Code, CODE, READONLY ENTRY Reset_Handler PROC EXPORT Reset_Handler IMPORT ||Lib$$Request$$armlib|| ; Pull in library IMPORT __scatterload ; Scatter Loader BL __scatterload ; Initialize statics in RAM ; jumps to __main_after_scatterload ENDP ; Reset_Handler __main_after_scatterload PROC EXPORT __main_after_scatterload LDR r0, =src LDR r1, [r0] ldr r1, =123 mov r1, #456 LDR r0, =foo LDR r1, [r0] B . ; sit in an endless loop ENDP ; __main_after_scatterload foo DCD 1234 ; In ROM ; data area that can be modified (readwrite) AREA Data, DATA, READWRITE src DCD 14 END
Thanks that code works great. Problem solved.