Yes, it’s POSIX-like, its system calls can even be implemented using interrupts like a normal OS (though that is optional). It has its own hardware abstractions

Yes, it’s POSIX-like, its system calls can even be implemented using interrupts like a normal OS (though that is optional). It has its own hardware abstractions and IOCTL interfaces around devices. Yes, it has a global memory allocator you obviously can use when processing interrupts. You can register your own interrupt handlers through the operating system, so you can still get real time guarantees. It does run microcontrollers. In the project I worked on we ran it on a STM32 device. The networking stack, threads, POSIX-like message queue, memory allocator, and synchronization primitives were fine. It has a little shell than runs on serial that can be useful debugging tool. It had a little file system that was easy to corrupt. Most of the other hardware abstractions we avoided and just talked directly to hardware. Personally if I were doing a project from scratch I would use a different RTOS that does not add extra layers of abstraction. But I understand the appeal of porting existing software to a POSIX-like target.