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

[Nick Clifton <nickc at redhat dot com>]

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