pSOS (Portable Software On Silicon) is a real-time operating system (RTOS) developed for embedded systems, especially those using Motorola 68000 microprocessors. Introduced in the early 1980s, pSOS became popular for its modular architecture, small footprint, and fast context-switching capabilities, making it suitable for mission-critical and time-sensitive applications in telecommunications, aerospace, and industrial control. It offered multitasking, inter-process communication, and device management features. pSOS was widely adopted throughout the 1980s and 1990s before being gradually replaced by newer RTOS options. Its legacy persists in some long-lived embedded systems and as a reference point for modern RTOS development.