KERNAL API: BASIC command and function extensions

[dansanderson]

BASIC 10 has an internal vector-based API for looking up commands and functions. It may be possible to formalize and document (and, if needed, repair) this as an external BASIC extension API, such that third-party software packages can add BASIC commands and functions while remaining compatible with future ROM revisions.

Historically, adding commands has been a popular use case for monkeypatching the C64 ROM, and monkeypatching the MEGA65 KERNAL is not feasible while it is under active development. The existing keyword lookup mechanisms have the potential to be inexpensively formalized for similar third-party extensions, without monkeypatching.

Doing this sooner rather than later will help us figure out what other parts of the KERNAL/BASIC need to be part of the API surface to make useful extensions, e.g. accessors for state variables. It currently looks like significant parts of the parser would need to be formalized to make this feasible: keywords are responsible for parsing their own arguments.

See b65.src starting $02f7.