My first chess program was Sargon, running on Apple Computer. It was very impressive and beat me easily. My next two Sargon programs (II and III) were for Commodore 64. I guessed even my Fidelity Chess Computer ran on one of these versions of Sargon. I was always curious about the internal working mechanism of these programs. I tried to study the machine codes (6502 and 6510) of Sargon and Sargon III, but gave up about 10% into the process. I found this book from my school library, but at that time my programming experience was weak and had no tools to test the program in the book. Later, on the job, I found some discarded Z80 motherboards, and I was back to study this Sargon program. It is a very good way to understand how the internal program works. Now I have studied two more programs GNU Chess 5.00 and Phalanx, Sargon was still a "good-date" to remember. Sargon was written for micro-machine when memory was at the premium; therefore the code was in machine code. It took lots of efforts to go through it but it is worth it. With high-speed CPUs and plenty of memory newer computers provided, most programs are now in C or higher-level languages. These higher-level languages help a lot to speed through the programs. I have a copy of Sargon book. It is one of the historical documents of chess playing programs in particular and of artificial intelligence in general.
One more point, Sargon was the first program written for microcomputer to compete with other more dedicated and/or specialized chess engines, and Sargon had won some honorable prizes. If you can afford one for your chess program library, keep one.