Fully agree on QNX: very elegant OS, unique concepts like "syscalls" via synchronous (!) messaging, real-time from the bottom up. We used it for a robot we buil

Fully agree on QNX: very elegant OS, unique concepts like "syscalls" via synchronous (!) messaging, real-time from the bottom up. We used it for a robot we build in the 90s. Everything from the low-level control to the image processing on some fine 386 boards with QNX-native networked IPC. Too bad that Blackberry was probably the death-spell for it. Also agree on Mach, although somehow NeXT/Apple seem to have managed to make a very good modern OS out of it - with a lot of Mach cruft hidden in the basement. The original NeXTStep Kernel suffered seriously from memory leakage and instability. Unbelievable it became the basis of iOS much later.