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owl [soylentnews.org] writes:

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.1 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/M2 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.

Original Submission