I read many many things these days, including interviews of Dan Spracklen & Kathe Spracklen. They have written computer chess history since their first tournament in 1978, that they unexpectedly win, they share many world champion prices, and they owe it. Sargon was their first and mainstream program for years, on Z80, 6502, 68000.
Today I receive the “Sargon book”, by Hayden editor, that contain full source-code of Sargon, fully annotated, for Zilog Z80 CPU (a faster clone of Intel 8080 that was cloned itself many times, for example the NSC 800 a buggy version on the Canon x-07 hand-held computer). This book needs to be read, annotated, by anyone willing to make an efficient chess program. I saw some code that might be rewritten to be faster (ie mul 8bx8b that loop 8 times even if multiplier has no more set bit after right rotation, and code should have been unrolled), but honestly it’s a masterpiece!
They even ended-up beating a 6 million dollar Amdahl mainframe with a personal computer that cost thousands less!
On my collection I have the Fidelity Electronics Chess Challenger Excel Mach III Excel (first USCF Master rated electronic chess computer!) with their program on it. It plays a beautiful chess, that is solid and elegant (for a 1987 program!) with a 2079 ELO rating.
The most surprising, given their incredible achievement, is their humility. Thanks for your incredible work on chess computing! An interview of Dan & Kathe Spracklen you have to read here (PDF)
PS: Full source-code in Z80 assembly language of Sargon chess program here. Enjoy, and appreciate the comment they put, some young programmers might take a lesson or two!