8086 Processor's microcode pipeline from die analysis
It&#x27;s interesting to read these articles about the implementation of an actual CPU, compared to the very idealised perspective that a lot of CS (and even some hardware-oriented) textbooks have. There&#x27;s a lot of complexity that those who are used to the latter would otherwise not have seen.<p>Also worth noting is that, like the Z80 and 8080&#x2F;8085, but unlike the 6502, the designers of the 8086 opted for instructions that took more cycles and compensated for that with increased clock frequency, instead of the 6502 that has shorter instructions and a lower clock frequency.<p><i>and the hardware fills in the particular ALU operation from bits 5-3 of the machine instruction.</i><p>This is clearly seen when the instructions are represented in octal:<p><pre><code>    0u4 alu AL, imm8
    0u5 alu AX, imm16
</code></pre>
The ALU operations are respectively: ADD, OR, ADC, SBB, AND, SUB, XOR, CMP.<p>According to the datasheets, the 80186 can execute the same instruction in 3 cycles, and the 80286 can do it in 2.