In whch yr hmbl svt writes a simple assembler for CP/M-65, which badly needs one. It's done in C, and is compiled with llvm-mos to run itself on CP/M-65. This means that it needs to be very small and very fast, but it also needs to be sophisticated enough to generate the relocatable binaries which CP/M-65 wants.

You can find the resulting source code in the CP/M-65 repository: https://github.com/davidgiven/cpm65 (Look in `apps/asm.c`.)