Reverse-engineering the vintage Intel 8087's tangent algorithm: more than CORDIC
Summary
Ken Shirriff’s article reverse-engineers Intel 8087's tangent instruction FPTAN, showing that the tangent computation blends a CORDIC-based approach with a Padé rational approximation to achieve high accuracy with speed. It details the microcode flow, the role of the 80-bit temporary real format, ROMs, and the sequence of pseudo-division and pseudo-multiplication steps, supplemented by diagrams and references to the 8087 repository.