Ken Shirriff dissolved a 1980 Intel chip in acid to read its adder. The author took questions (righto.com)
The design is a hybrid: a Manchester carry chain with carry-skip in 4-bit blocks, chosen because faster adders existed but cost too many transistors in 1980 NMOS. Addition takes two clock cycles.
Ken Shirriff answered questions in the thread — including a sharp one on how power reached circuits in the layer he'd just etched away. One reader noted nobody has ever rebuilt the 8087 in HDL, unlike the 8086.
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