If you're interested in programming your own assembler, I would recommend googling "how to write an assembler".
There is lots of information out there; one link I found quickly was this:
How To Write An Assembler - comp.lang.asm.x86 | Google Groups
Ultimately, however, this is a programming question in an electronics forum--there is lots on programming in here, but mostly centred around programming microcontrollers, not necessarily programming the tools used to program microcontrollers.
You may have better luck searching in some programming forums.
Good luck,
Torben