Ken Shirriff @righto.com · Feb 14 Most of the instructions in the 8087 are implemented in low-level software called microcode. The microcode inside the 8087 chip uses conditional jumps for many purposes including changing the behavior for different instructions. Here's the PLA that stores target addresses for microcode jumps.
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Instruction decoding in the 8087 has many more complications: hardcoded instructions handled by the "Bus Interface Unit", instructions that load a constant from the constant ROM, and handling the instruction queue. For more details, see my latest blog post: www.righto.com/2026/02/8087...