Ken Shirriff reverse-engineers Intel 8087 69-bit adder

Reverse engineering vintage silicon remains one of the best ways to appreciate the sheer ingenuity of early processor design. The Manchester carry chain was an elegant solution to the ripple-carry problem that allowed carry signals to propagate at wire speed rather than being gated by logic delays. Additionally, NMOS constraints dictated the electrical design by forcing engineers to use precharging tricks since pulling a signal high was much slower than pulling it low. The 69-bit width accommodated guard, round, and sticky bits to ensure rigorous mathematical precision, laying the groundwork for the math co-processors that defined computing for a decade.