Assemblers are mostly very large tables of data about instructions. If you're lucky, the architecture will have XML format descriptions of the ISA available tha

Assemblers are mostly very large tables of data about instructions. If you're lucky, the architecture will have XML format descriptions of the ISA available that you can derive an assembler from. I think x86-64 even has versions with experimentally determined instruction latency in the same data. A C assembler with the bulk of the implementation mechanically derived from XML is probably a reasonable project to attempt. I haven't done it yet but it's planned.