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i trying to understand startup files, here an example start_mx.a51
;----------------------------------------------------; START_MX.A51: This code is executed after processor reset. ; You may add this file to a uVision2 project.
according to this defintion, always after reset this codes will execute
here is an another part:
PDATASTART EQU 0H ; the absolute start-address of PDATA memory PDATALEN EQU 0H ; the length of PDATA memory in bytes.
for example if i determine program data memory 100H PDATALEN EQU 100H ;
IF PDATALEN <> 0 MOV R0,#PDATASTART MOV R7,#LOW (PDATALEN) CLR A PDATALOOP: MOVX @R0,A INC R0 DJNZ R7,PDATALOOP ENDIF
according to this code all the program memory will clear after reset. why this codes necessary? why the programmers wants clear their program memory, if i clears program memory , program dosent work. is it true?
"according to this code all the program memory will clear after reset."
No - all the data memoyr will be cleared!
"why this codes necessary?"
'C' specifies that memory should be initialised to zero.
"why the programmers wants clear their program memory"
Usually, it's nice to have memory start at some known "clean" state. It's not essential - that's why the source code is there for you to modify as you wish!
"if i clears program memory , program dosent work. is it true?"
If you really do clear program (ie, CODE) memory, then the microcontroller will have no program to run!
If your program doesn't work after this initial RAM clear, it probably means you have a bug; eg, using uninitialised variables...
If you mave memory-mapped peripherals, you may need to exclude their addresses from being cleared...
im sorry, i thinked, pdata means is code memory, i understand now, thank you.
"i thinked, pdata means is code memory"
No - the fact that it's called pdata is the clue there!
Maybe you should review the so-called "bible" for the 8051, to dispel any other misconceptions...
Chapter 1 - 80C51 Family Architecture: www.nxp.com/.../80C51_FAM_ARCH_1.pdf
Chapter 2 - 80C51 Family Programmer's Guide and Instruction Set: www.nxp.com/.../80C51_FAM_PROG_GUIDE_1.pdf
Chapter 3 - 80C51 Family Hardware Description: www.nxp.com/.../80C51_FAM_HARDWARE_1.pdf
for example i have got a global variable , and i want to locate it an absolute adress.
extern xdata unsigned char my_global; xdata unsigned char my_global _at_ 0x0A00 ; main(){//etc..}
how can i initilize my_golbal to 0. but i want to do this only one time, when program is loads. therefore in startup file i dont add 0x0A00 to clearing lines. becouse i dont want to clear this variable all of after resets.
" want to do this only one time, when program is loads."
Do you mean that you want the variable to retain its value even after the system has been turned off, then turned back on?
In that case, you need some sort of Non-Volatile memory...
yes. i means, variable stars with zero and it can change in program flow, but it must be keep this changes all the turn off, turn on and resets. for this i think i can use batery backup sram. but i dont be success about initilize this variable to zero.
This is a special case, and you really should have explained it in the first place.
Yes, you obviously need to exclude any non-volatile memories from the startup initialisation.
But you also need to consider what will happen in a "virgin" system the very first time it is started. You need to provide some means to reset everyting to a known starting condition - or at least mark everything as "undefined"
You might include this as part of your manufacturing pocess...
thank you very much for your helps, may be i can use similar this;
main(){ xdata unsigned char initilize_flag _at_ 0x0AFF /*also located at nvram*/ if (initilize_flag != 253){ initilize_nvram_variables(); initilize_flag = 253; } //others parts.. }
it is only a little possible that there is 253 value in nvram(0x0AFF) at the beginning