The difference lies in the depth of the layers of interaction with the hardware.
The precompiled kernel of most Linux distributions are not predisposed to act in real time. But unlike other high-level OS, the Linux kernel can be recompiled to get their direct contact with the hardware and act preemtive and deterministic way. Thinking of this, Linux and other general purpose OS are not designed for a specific environment for this market are several Linux-based solutions optimized for use as an RTOS.
Another major difference are the minimum requirements of the hardware.
Many RTOS born to ensure high performance because they are studied for the microcontroller using native instructions. Linux NOT!
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