Dear Mr.Walter

At compilation of the text:

#include <bios.h>

void
main()
    {
    int a = _PRINTER_WRITE;
    }

The mistake is given out:

   int a = _PRINTER_WRITE;
                         ^
est.c(7) : Error: undefined identifier '_PRINTER_WRITE'
-- errorlevel 1

And in bios.h the constant is determined. Why?