Any way for declaring symbolic constants as an external number in C51 source code ?

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Read-Only Author Standa Bernatik Posted 9-Jan-2002 13:50 GMT Toolset C51	Any way for declaring symbolic constants as an external number in C51 source code ? Standa Bernatik Is there any way to declare a symbolic constant as 'an external number' in Keil C51 source code so that the compiler generates object file using instructions with # symbol prefix (e.g. mov r0,#symbol) and therefore the value of this symbol can be supplied on the linker level (from another obj file, where the symbol is given a value and declared as public) ? (Including file with #defines into the source code does not solve the issue - symbols must be defined at the compile time.) Note: in assembler it is easy ... file1.a51 : (file1.obj defines particular values of needed constants for a concrete project) ... public SYMBOL SYMBOL equ 5 ... file2.a51 : (file2.obj can be stored in a library being 'universal' and the SYMBOL value will be supplied by linker) ... extrn number (SYMBOL) ... mov a,#SYMBOL ...

Read-Only Author Andrew Neil Posted 9-Jan-2002 14:38 GMT Toolset C51	RE: Any way for declaring symbolic constants as an external number in C51 source code ? Andrew Neil I don't think so. Remember, the Linker deals with addresses - not values; but there might be a clue there...

Read-Only Author Standa Bernatik Posted 10-Jan-2002 14:16 GMT Toolset C51	RE: Any way for declaring symbolic constants as an external number in C51 source code ? Standa Bernatik Hi Andrew, "I don't think so..." - probably you are right (what a pity :-), I've thought it as well but I'm still not sure. In my opinion, the linker does not deal with addresses but with "numbers" associated with attributes - data(address), idata(address), xdata(address), ... and number. The attributes provide the linker with necessary information, so it can handle the numbers appropriately and can check their right use as well. If there is at least one assembler module with the equ directive used in a project, then you can find lines like: VALUE TYPE NAME ------------------------------ N:0002H PUBLIC PADDRESS N:FF90H SYMBOL BFUX10000H in the "SYMBOL TABLE OF MODULE" in the linker listing file (.M51). N means NUMBER. Further, in the "INTER-MODULE CROSS-REFERENCE LISTING": NAME ....... USAGE MODULE NAMES ---------------------------------------- ADST_NONE .. NUMB; REG3_30 BLNK_AD I have browsed .M51 files in my "purely C-based" projects and haven't found anything like that there. Btw. I have noticed an interesting thing - there is a word NUMBER in the C51.exe file (use an ASCII viewer, e.g. View in XTree) and it is located in a group of words like DATA, BDATA, etc. (looks like group of 'keywords'). But is there any way to make the compiler to use it ? (I'm sorry for such a long 'article') Thanks for your interest. Stan

Read-Only Author Christian Bienert Posted 9-Jan-2002 15:54 GMT Toolset C51	RE: Any way for declaring symbolic constants as an external number in C51 source code ? Christian Bienert you can use a include file ... #define number 5 /* my number */ ... A51 V6.14 can handle defines from C51 Chris

Read-Only Author Standa Bernatik Posted 10-Jan-2002 12:42 GMT Toolset C51	RE: Any way for declaring symbolic constants as an external number in C51 source code ? Standa Bernatik "you can use an include file" - Yes, I know that feature, but this is not the point. You haven't understood my query well, maybe you haven't read it carefully, I'm sorry. Thanks anyway. Stan

Read-Only Author Scott deWolski Posted 10-Jan-2002 16:06 GMT Toolset C51	RE: Any way for declaring symbolic constants as an external number in C51 source code ? Scott deWolski In C++ this is possible by declaring an item 'const'. CPP int const thisInt = 0x4893; HPP extern int const thisInt; The compiler/linker then can decide whether to use the item as a number or create it in constant space and use it as an addressable entity. 'C' only uses it as an addressable entity. This still may be useful. However, it cannot be used in some situations such as the size of a static array. For a good review of this topic see articles by Dan Saks in the last several issues of 'Embedded Systems Programming' magazine (http://www.embedded.com). Best luck

Read-Only Author Andrew Neil Posted 10-Jan-2002 17:11 GMT Toolset C51	RE: Any way for declaring symbolic constants as an external number in C51 source code ? Andrew Neil "The compiler/linker then can decide" that'd be a problem - he needs to be able to specify this, not just hope that the compiler happens to do what he wants!

Read-Only Author Scott deWolski Posted 10-Jan-2002 19:41 GMT Toolset C51	RE: Any way for declaring symbolic constants as an external number in C51 source code ? Scott deWolski Since Keil is a 'C' compiler the point is moot. In the '167 and '251 tool chains there is an 'ASSIGN' command for the linker. I'm not sure if it is in the '51 tools. Usage is; 'C' file extern unsigned char symbol_Var; { ... unsigned int destination_Var; destination_Var = _sof_( symbol_Var ); ... } Linker command line L167 ... ASSIGN( 'symbol_Var' (0x1234)) This produces; MOV R4, #1234h Downside is lack of portability. Check with Keil if 'ASSIGN' is available for your tool chain. Best Luck

Read-Only Author Andrew Neil Posted 13-Jan-2002 21:52 GMT Toolset C51	RE: Any way for declaring symbolic constants as an external number in C51 source code ? Andrew Neil I have seen that sort of trick used by a Client before; not only was it non-portable, but it was also very poorly documented! In the particular case I'm thinking of, the Linker did assign a symbol address rather than a value - which just added to the confusion, as the source code wanted a value! This particular Client also used the cunning trick of passing their Linker command file through the 'C' preprocessor before actually giving it to the Linker - thus enabling them to use 'C'-style symbolic definitions, instead of "magic numnbers" in the Linker command file! This was also undocumented and, needless to say, the whole tangled web took several days to unravel! What I'm saying is: if you really must resort to such tricks, *please* ensure that they are lucidly documented - preferably as comments directly in the files, rather than some separate "document" which will just get lost and/or out of date. But then, if people did that, they wouldn't need to hire people like me to sort it out later ... ;-)

Read-Only Author Jon Young Posted 10-Jan-2002 20:47 GMT Toolset C51	RE: Any way for declaring symbolic constants as an external number in C51 source code ? Jon Young This will work. extern char idata* c; #define symbol_c ( (char)(&c) ) void main( void ) { char C = symbol_c; }

Read-Only Author Scott deWolski Posted 11-Jan-2002 15:27 GMT Toolset C51	RE: Any way for declaring symbolic constants as an external number in C51 source code ? Scott deWolski extern char idata* c; #define symbol_c ( (char)(&c) ) This will work if you can create 'c' at a specific address in another module. How do you do this in C? I think you could create a separate module for each variable and use the section/class controls of the linker to put it in a phantom RAM location but it would be confusing, hard to document and maintain. Is there another way? Thanks

Read-Only Author Standa Bernatik Posted 12-Jan-2002 20:03 GMT Toolset C51	RE: Any way for declaring symbolic constants as an external number in C51 source code ? Standa Bernatik I think Scott is true. Your suggestion looks (at the first sight) as if it solved one half of the issue - the compiler generates instructions like 'mov a,#...'. But remember that 'c' is declared as an external pointer to idata (or anywhere else). Then it should be defined as a public item in a different C module and what is important - initialized to an arbitrary required value. However, this is normally linker's job. What is wrong - the item is not a simple number then, it occupies some space in memory. (This second half of the problem can be solved defining and 'publishing' symbol 'c' in an asm module using equ statement, linker is then satisfied with the value and won't use any memory space for it. It's a bit violent but one of my colleagues says "a little bit of violence is not any fascism yet" :-) Nevertheless, I have been interested in purely C-based solution - if it exists... Stan

Read-Only Author Jon Young Posted 13-Jan-2002 20:10 GMT Toolset C51	RE: Any way for declaring symbolic constants as an external number in C51 source code ? Jon Young Sorry, I was replying to your orignal question of generating a "mov r0,#symbol" where the symbol is supplied on the linker level. I just assumed you had a assembly program that generated these constants. I know of no standard "C" way of doing what you ask. The only way I know is ugly. You have to export the constant as an address. The following will export the constant 0x1234. char xdata symbol _at_ 0x1234; However the linker now thinks there is a char allocated. It would have been nice if the following would work. char xdata symbol[] _at_ 0x1234; This would have let you have your cake and eat it too.

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