Bill Gates wrote the initial BASIC interpreter using Paul Allen's PDP-10 assembler macros so they could run the generated interpreter on a PDP-10, while also extracting a native binary for the targeted CPU/OEM hardware.
from Paul Allen's Idea Man biography:
... It gathered steam with the release of the 6502 chip from MOS Technology at an unheard-of $25. For each new microprocessor, I created a new set of development tools on the PDP-10, while Ric [Weiland, 2nd new-hire, 4th employee] helped with the BASIC interpreter rewrites.
Bill Gates was involved with the business-side of the company at this point, trying to sign new clients for the BASIC interpreter.So Ric Weiland would have done much of the 6502 BASIC interpreter.