Re: Passing an address to a #define in C

[Andi Hellmund <mail at andihellmund dot com>]

Hi Gus,

> I need to put an specific machine code in any part of the memory. This code
> is for some hardware module that can understand it. Of course, gcc will not
> understand it, so I need to use .word 0xfc to create that code.

> Now, I would like to create a pseudo assembler for my module, in following
> way:
>
> #define       LOAD         asm(".word 0xFC \n\t");
> #define       INSTRUCTION2   asm(.word  0x4b \n\t");
> #define       GOTO(label)         asm(".word " #label " \n\t");

Yes, this will not work this way. #label will stringify the input and 
will not produce an address.

Please do also keep in mind that .word might have different size 
depending on the target architecture the assembler is configured for.

> void mycode(void) {
>
>    LOAD
> Label1:
>    INSTRUCTION2
>    GOTO(Label1)
> }

It is generally possible to do use the asm() instruction for this 
purpose, but I'm not sure if this is the best approach.

I would possibly try to separate this code from the code generated by 
the compiler for the underlying CPU e.g. by putting these artificial 
instructions into a separate section and make this section executable by 
a linker script if executable permissions are required or to put this 
into the data segment if only read permissions are required.

The only benefit using the asm() to me is to put the data into an 
executable section, but the handling might be complex for your purpose 
since you don't use assembly mnemonics beside .word.



Hope that helps,
Andi