RTX / RTX64 are real-time operating system (RTOS) by the firm IntervalZero.

They are software extensions that convert Microsoft Windows operating system into a RTOS.

[1] It was the first Windows real-time solution on the market.

[2] RTX and RTX64 real-time software extensions to Windows combine the familiar user experience of Microsoft Windows (rich GUI, available drivers and software, ...) and the direct control of embedded hardware by providing hard real-time determinism and symmetric multiprocessing (SMP) abilities.

[6] The systems are used in different markets such as industrial automation, testbed and simulation, digital audio, digital video, aerospace military, medical devices, electrical grid, electricity generation, and other uses.