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Hi, it seems that using "_at_" keyword with extern variables does not work well. The code below shows what I wish to share with you guys.
///////////////////////////////////////////// sample.c: xdata volatile unsigned char xdata_var_bad _at_ 16; xdata volatile unsigned char xdata_var_ok _at_ 16; void test( void ) { xdata_var_bad = 0; xdata_var_ok = 0; } ///////////////////////////////////////////// ///////////////////////////////////////////// sample.h: extern xdata volatile unsigned char xdata_var_bad _at_ (16); extern xdata volatile unsigned char xdata_var_ok; ///////////////////////////////////////////// ///////////////////////////////////////////// main.c: #include "sample.h" void main(void) { xdata_var_bad =0; // disassembly for above assignment is: // MOV DPTR,#0x0000 // MOV A,#0 // MOVX @DPTR,A xdata_var_ok =0; // disassembly for above assignment is: // MOV DPTR,#0x0010 // MOV A,#0 // MOVX @DPTR,A } /////////////////////////////////////////////
In my opinnion, seems to me that, using "_at_" with "extern" for external variable usage should be pointless - or sould I say "fully pointless". Many of us programmers like to copy variable declaration from .c to .h file - so there is probability that _at_ keyword with absolute memory storage address is also pasted. Altrough, in the help is noted how to use _at_ for extern variable reference... but there is not noted that using it like I was doing will produce wrong reference. That is why I posted description of non-documented compiler behavior, to avoid that other programmers go my way. And, I like to see one new compiler warning message in future release of my favorite IDE for u-controllers ;-) Boris
"Many of us programmers like to copy variable declaration from .c to .h file" When you do that, you have to remove any initialisations, don't you? Removing any _at_ keywords is just the same! Remember: the thing in the .c file is a definition; the thing in the .h file is a declaration - understanding the distinction is key to understanding what's going on here!