If anyone is interested in the inner workings, including the 6502 assembly source code, for this amazing 8-bit Basic, you can view it online at
The Atari Basic Source Book. It has some very interesting reading, if you're a try basic junkie like myself
You can also order this amazing book at amazon.com, used of course, for about $75, although you can occasionally find it for $20-25.
It explains syntax checking and tokenization using BNF descriptions, expression evaluation and reordering, and other areas that are common to most basic's. It's descriptions are all in plain English, and the book is a real treat.
Enjoy!