I'd classify the BeOS kernel as a modular-ish monolithic kernel. I'm a bit fuzzy on some of the more precise details, like the network stack (which they rewrote

I'd classify the BeOS kernel as a modular-ish monolithic kernel. I'm a bit fuzzy on some of the more precise details, like the network stack (which they rewrote 3 or 4 times...) but I know that all drivers, at least, ran in ring 0. Haiku's kernel is also monolithic, although we have a few hybrid tendencies (FUSE, but it's not widely used) and even more modularity than BeOS had, but we are still mostly source-compatible with the BeOS kernel. I don't know why people say it's a microkernel; people say the same of Haiku, but it's just not the case. Perhaps it's because the dynamic kernel module system is built-in and virtually all kernel modules are stored as shared objects and loaded on startup, vs. Linux's more static approach, at least historically? That's not at all what's meant by the term "monolithic", but popular perception is often wrong...