http://www.pagetable.com/?p=774
This is the original 1978 source code of Microsoft BASIC for 6502 with all original comments, documentation and easter eggs:
http://www.pagetable.com/docs/M6502.MAC.txt -- 1978-07-27, 6955 lines, 161,685 bytes
This is currently the oldest publicly available piece of source written by Bill Gates.
Like the 8080 version, the 6502 version was developed on a PDP-10, using the MACRO-10 assembler. A set of macros developed by Paul Allen allowed MACRO-10 to understand and translate 6502 assembly