I've gotten quite a few questions now about my assembler and workflow for the MC3. I've used the same old 6800/01/04/05/09/11 assembler for years. I think it's made by E J Rupp originally. It's very basic and does not handle macros and such but it's clean and I do have the source code and have made a few modifications over the years. Mainly adding a version (as1h) that supports the very useful XGDX opcode for 6301/6303. The output is Motorola S-record (S19) that is directly compatible with most monitors and PROM programmers. Below is an archive of the assembler with both source and Linux binaries.
Download assembler source and binaries
I have included a build script (build.sh) for compiling under GCC.
Assemble and generate s19-file
$ as1h program.asm
Assemble and generate listing + s19
$ as1h program.asm -L > program.lst
As for assembler editor I use Emacs all the way. It handles 68xx assembly nicely.