How to pass a value to an inline assembler script in a #define macro in cc65

Question

I'm trying to add a compact #define macro in a cc65 program for performing various low-level tasks efficiently. For some of the macros, I need to pass a non-constant value to the assembler part. I studied the cc65 guide about inline assembler but could not get it to work.

Here is a minimal example

#include <stdio.h>
#include <stdlib.h>

#define set_bgcolor(C) (asm("lda #%b",C),asm("sta $D021"))

int main (void)
{
  int x=3;
  set_bgcolor(1);   //this works
  set_bgcolor(1+1); //this works as well
  set_bgcolor(x);   //this fails to compile "Error: Constant integer expression expected"
  return EXIT_SUCCESS;
}

What is the proper definition of the macro in line 4 to make this work?

Answer 1

There is a pseudo variable __AX__ of type unsigned int that can be assigned a value that then ends up in register A (lo byte) and X (hi byte). So, the following macro definition works as intended:

#define set_bgcolor(C) (__AX__ = (C),asm("sta $D021"))

I found this under "Extensions" in the cc65 user guide.