actually it's not true - there is SALVO Lite which is totally free. It has only some limitations like - maximum number of tasks, maximum number of events, semaphores. And of course you don't get the source code - only the library. Notice that this library is 100% functional and will never expire. One tip - if you're student you can claim for academic version which is quite cheap (~150$). You should not hesitate to use SALVO RTOS, personally for me the impact of introducing SALVO in my projects was smashing - the feeling was just like moving from assembler to C.