Reverse-engineering the ModR/M addressing microcode in the Intel 8086 processor

One interesting aspect of a computer’s instruction set is its addressing modes, how the computer determines the address for a memory access. The Intel 8086 (1978) used the ModR/M byte, a special byte following the opcode, to select the addressing mode. The ModR/M byte has persisted into the modern x86 architecture, so it’s interesting to look at its roots and original implementation. In this post, I look at the hardware and microcode in the 8086 that implements ModR/M and how the 8086 designers fit multiple addressing modes into the 8086’s limited microcode ROM. One technique was a hybrid approach that combined generic microcode with hardware logic that filled in the details for a particular instruction. A second technique was modular microcode, with subroutines for various parts of the task. This is way above my pay grade, but I know quite a few of you love this kind of writing. Very in-depth.