I found 2 resources on the web, Sargon I full source-code for Z80 (the book content in fact), and Sinclair ZX81 1KB chess source-code (Z80 too).
Naturally Sargon is impressive, but having a chess program fully running (even if primitive) on 1KB RAM (code+data) is hot! It’s not a good chess program and will only challenge beginners, true beginners, but still it plays chess!
PS: The original article from Claude Shannon “Programming a Computer for Playing Chess” in PDF in Philosophical Magazine. A must-read, probably before everything else, or maybe competing with Turing article on Chess!
Tags: sargon, source-code, zx81