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Latches inside: Reverse-engineering the Intel 8086's instruction register

The Intel 8086 microprocessor is one of the most influential chips ever created; it led to the x86 architecture that dominates desktop and server computing today. But it is still simple enough that its circuitry can be studied under the microscope and understood. In this post, I explain the implementation of a dynamic latch, a circuit that holds a single bit. The 8086 has over 80 latches scattered throughout the chip, holding a variety of important processor state bits,1 but I’ll focus on the eight latches that implement the instruction register and hold the instruction that is being executed.

The photo above shows the silicon die of the 8086 processor under a microscope. I removed the metal and polysilicon layers to reveal the transistors, approximately 29,000 of them. The highlighted region indicates the 8086’s 8-bit instruction buffer, consisting of eight latches. (This 1978 processor is simple enough that a single 8-bit register occupies a substantial region of the die.) The closeup shows the silicon and transistors making up a single latch.”

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