This 6502 version of Tiny BASIC is being used for my Simple 65816 project.
I originally obtained the source code from https://github.com/jefftranter/6502/tree/master/asm/tinybasic, and per the remarks in http://www.ittybittycomputers.com/IttyBitty/TinyBasic/index.htm, I would "license" it as CC0, even though GitHub doesn't offer that option.
I am now heavily commenting the code with minimal alterations needed to get it assembled and "working" on my Simple 65816 emulator. It is still a work in progress.
In the future I may fork my version to support a few more stuff plus fixing any bugs. Who knows. But I shall properly document the internals for future. For that reason I also include the Tiny BASIC manual as is, taken from Itty Bitty website website shown above.