AMD FX Series: 8 cores or 8 threads in 4 cores?

I've looked at FX-8XXX series processors on Windows, and they show as 4 core, dual-threaded processors. (4 cores w/ 2 threads per core)

The processors are advertised as 8-core. Do they actually have 8 cores, or just 4?

The AMD FX-8000 series processors actually have 8 physical cores. Note that AMD cores and Intel cores are not directly comparable because AMD cores share some resources (such as FPU and AVX units).

The thing that's most comparable to a core in an Intel CPU is likely the module in an AMD CPU, and there are only four of those. Just like an Intel physical core can contain two virtual cores, AMD physical modules contain two physical cores. But Windows has only one place to put a core count.

In Intel hyper-threading, there is no additional execution unit; instead, the architectural state of the processor (control and general-purpose registers) is duplicated so as to allow the processor core to more fully utilize parts of the execution unit when they are not in use, such as when the pipeline is stalled.

In AMD FX, each module actually has two execution units, each fully capable of executing processes independently. However, each module share a (relatively large) floating-point unit and L2 cache.