I was using the venerable Motorola AS9 MC6809 assembler for firmware development. This was not ideal for several reasons: it's a DOS application, it lacks macro capability, and worst of all doesn't know the HD6309-specific registers and mnemonics. There is an AS9 linux port (by Albert van der Horst) but that still didn't address the HD6309 topic. Fortunately, I eventually found LWTOOLS, a nice open-source toolchain (assembler, linker, librarian) that addresses all of these issues. It works well and is, by and large, compatible with the syntax of the the Motorola assemblers. Its only shortcoming, for my purposes, is that the assembler lacks the ability to produce ASCII hex (S19 or Intel-hex) output. I threw together a command line utility to convert the (TRS-80 CoCo) DECB format to S19 and Intel hex. I submitted patches to lwasm to generate S19 and Intel hex, and the LWTOOLS author (William Astle) indicated that these options would make it into the next official release.  You can find LWTOOLS at  http://lwtools.projects.l-w.ca/