Notes on the Intel 8086 processor's arithmetic-logic unit
Summary
The article dives into the Intel 8086 ALU, explaining how 16-bit operations are implemented via microcode, with an XI mechanism that substitutes ALU operations from machine instructions. It describes the die-level ALU circuitry, including the two LUT-based control paths, the PLA that decodes 27 signals, and the flip-flop state that preserves the selected temporary registers and operations across micro-instructions. It also covers the microcode glue, the XI multiplexer, and the bootstrap drive circuitry that powers the LUT signals, ending with reflections on CISC complexity versus RISC simplicity and the benefits of x86 design choices.