Microsoft Basic for 6502 is free
Microsoft has released the version 1.1 source code of “Microsoft BASIC for 6502 Microprocessor” on GitHub, which means you can freely view the assembly code and download it if you want. [PCWorld]
What’s wild is that the entire source for BASIC is a single 6955 line long 6502 assembly file. Also bonus points for back-dating the checkin so it says it was checked in 48 years ago – long before github was even a glimmer in SVN’s eye.