In Time Sharing Systems
time line is divided in to ``slots'', or ``rounds'', each one of maximum length equal to a fixed time quantum
if the executing job done not block on a I/O operation before the end of the quantum, it is suspended to be executed later
In time sharing systems
Each process executes approximately as it were alone on a slow processor
The OS (by scheduler) ``virtualizes'' the processor one single processor is seen as many (slower) parallel processors (one for each process)
OS can virtualize many hardware resources memory, disk, network, etc.
Time sharing systems are not predictable the amount of execution time received by one process depends on the number of processes in the system