- Silicon reverse-engineering: the Intel 8086 processor’s flag circuitry (righto.com)
Status flags are a key part of most processors, indicating if an arithmetic result is negative, zero, or has a carry, for instance. In this post, I take a close look at the flag circuitry in the Intel 8086 processor (1978), the chip that launched the PC revolution.1 Looking at the silicon die of the 8086 reveals how its flags are implemented. The 8086’s flag circuitry is surprisingly complicated, full of corner cases and special handling. Moreover, I found an undocumented zero register that is used by the microcode.