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Using a C-variable from ASM

Xarion Comoretto

Hey guys I see there are numerous threads and support docs with mixing C + ASM. but I am still pulling hair out. I can get C to use a register I declared in asm
eg:

CODLO DATA 39D

by using

extern unsigned char CODLO;
in c

but I found that the address isn't reserved and that the memory location 39D is actually used by another variable delcared in c later in the program. So I am trying to get asm to use a variable I have declared in C...

so, I declare

unsigned char CODLO;

in C and in ASM i put

EXTERN DATA CODLO.

I get the compilation error:

smartcode.a51(2): error A48: DATA-ADDRESS EXPECTED

hope someone can help :)

Xarion

Parents Reply Children
  • 0 in reply to Xarion Comoretto

    if i assign a register a name in ASM does the compiler know not to use that memory location for other c variables?

    No, it won't. The aspects of mixing C and assembly routines are described, in great detail (far greater than anything that could be described in a posting here), in the compilers manual. This includes things like calling conventions and register usage.

    http://www.keil.com/support/man/docs/c51/c51_ap_ctoasm.htm

  • 0 in reply to Christoph Franck

    My ASM and c still seem to be fighting with one another, another variable I have declared in C is being overwritten by the ASM code. I have followed the docs you posted to me (http://www.keil.com/support/man/docs/c51/c51_ap_globalvars.htm)

    it says ....

    To access these variables in assembler, you must create an extern declaration that matches the original declaration. For example:

    EXTERN DATA(jim)

    if I do that it does not compile and i get the errors in my original post..

    so what I have done now, I have declared the register in ASM :

    CODHI DATA 41D CH_REG DATA 40D CODLO DATA 39D

    and then at the top i put

    PUBLIC CODHI
    PUBLIC CODLO
    PUBLIC CH_REG

    and in the c i put:

    extern unsigned char CODHI ;
    extern unsigned char CH_REG;
    extern unsigned char CODLO ;

    this gives me the problem with the variables being overwritten. I have no idea where to turn from here :(

    thanx again