At the first day on my first professional programming job in 1980 or so, my new manager told me that I would be writing a brand-new 6502 macro assembler, in 6502 assembly language, to replace the slow & limited one supplied by the vendor (Ohio Scientific).

Under his guidance I wrote the assembler, and even earned a raise after simulating the hashing function in FORTRAN and finding it lacking.

I still have very fond memories of that job and of the 6502, and would be interested in purchasing a replica as soon as you go into production!