Ah, I think we saw this at the time, with
Bringing BASIC back: Microsoft’s 6502 BASIC is now Open Source
Oh oops. Feel free to suppress the dup.
It’s OK - it’s a birthday! Maybe someone will have some reflections.
The source code for several early versions of Microsoft BASIC interpreters has been available for some time.
If you are interested in understanding how they work, the best documentation I have come across is for the original Altair version. Microsoft released the source code some time ago, but this article explains how it works, particularly the floating-point mathematics.
Altair BASIC 3.2 (4K) – Annotated Disassembly
For subsequent versions of Microsoft BASIC, Microsoft used many of the techniques found in the original 8080 version in its later versions for both the 6502 and 6809 processors.
The CoCo community has also done an excellent job of reverse-engineering CoCo BASIC:
If you are interested specifically in Microsoft 6809 BASIC, the source code for the Dragon version was released a few years ago with Microsoft’s approval. The code is almost identical to the CoCo version, apart from the essential changes needed to accommodate hardware differences.
Most early microcomputers ran a version of Microsoft BASIC. I run a version adapted from the Dragon code on my Newbear 7809.
In addition to Microsoft BASIC, here in the UK the most important alternative was BBC BASIC, which has been reverse-engineered. I am not aware of the original source code ever having been released.
There was also BASIC09, which ran on OS-9-based machines such as the Positron 9000.