Question on LD Linker providing low 2 bytes of address as a constant

[Nick Clifton]

Hi Doreen,

> If I put the address of the table I wanted stored in HdlrTbl in my
> pre-initializer, the compiler reports "error: initializer element is not
> constant".  If I change HdlrTbl from 2 bytes to 4 bytes and leave the
> pre-initilizer the way it was, it compiles fine.  So it looks like I
> need a way to tell the compiler and linker that I only want the lowest 2
> bytes of the address (which is identical to the address since the 2 high
> bytes are 0).

Tricky.  There are several possibilities, none of them easy:

   1.  Don't do this.
       You are trying to play tricks on the compiler and linker and they
       are doing their best to prevent you.

   2.  Do not use static initialization.
       If you initialised your StructTbl structure at run time rather
       than at compile time (or at link time or at load time) then you
       could do pretty much whatever you want.

   3.  Invent a new M68K relocation type and a new assembler directive.
       Some architectures support a %lo() assembler directive which places
       the low 16-bits of a 32-bit word into the appropriate field in an
       instruction.  (eg the IQ2000).  You could add such a directive to
       the M68K assembler, along with a new reloc to support and new linker
       code to implement it and then access this directive from your source
       code.  Of course you would then be using a custom toolchain to build
       your app.

    4. Hard code the addresses of your HdlrTbl structures.
       You would have to make sure that you use a low memory area that is not
       going to be used by anything else and use two versions of the address,
       one 16-bit and the other 32-bit.

Cheers
   Nick