NuttX

Many people first understand an RTOS as tasks, queues, semaphores, and a few driver callbacks.

NuttX does not fit that image well. It is better understood through a Unix-style model: POSIX-like APIs, VFS, device nodes, filesystems, sockets, shell, and applications form a small device system.

The first model is:

traditional MCU RTOS: application organized around tasks, queues, and driver APIs
NuttX: application organized around POSIX, VFS, device nodes, and system abstractions

That means comparing NuttX with FreeRTOS, RT-Thread, and Zephyr is not only a kernel-size comparison. It is a comparison of system abstraction.

When an MCU project chooses an RTOS for the first time, the first questions are often about size, speed, and API simplicity.

Those questions help, but they are not enough. Choosing an RTOS is not only choosing a scheduler. It is choosing how device software is organized: how tasks are created, how drivers are integrated, where networking and filesystems come from, how board differences are maintained, whether existing C or POSIX-style code can be reused, and whether the team can debug field failures.