Difference between time sharing and real time operating system
Answers
Real-time OS:
An OS (or any software for that matter) which offers a predictable response time i.e. The maximum time the OS/software can take to respond or finish a function, is well known. E.g. The documentation says, XYZ function will take no more than 5ms (milliseconds to complete).
Real-time does NOT mean in real time, as there is no such thing as zero time or in no time or instantly. Even light takes certain amount of time to travel.
RTOS (real-time OS), may be single or multitasking i.e. can execute only one program/process at a time or can execute multiple processes at the same time (Time Sharing, see below for more).
RTOS are generally highly optimized pieces of code, usually small in size (take less memory), and without the gimmicky bells/whistles now a days most OSs include.
RTOS is designed for fast execution, thereby offering low or very low latency (just another way to say - finish a function very quickly).
Time Sharing Systems:
An OS which allows more than one program/process to run at any given time.
Most OSs today are time-sharing systems - Windows, Linux, Unix, all mainframe systems (they were the first time shared systems).
MS-DOS, CP/M are not time-shared systems, as they can only execute one process at a time.
In a single processor/CPU system, in reality only one process is being executed at any one time. The OS time slices the CPU among multiple processes, to give user the illusion that multiple processes are being executed.
Time slicing is done by using interrupts. A time slice can vary across systems, may also be a configurable parameter. One time slice may equal a few microseconds or a few milliseconds; it is up to the designers of the OS. Lets say each time slice is 1ms (millisecond), so the CPU will switch to a new process 1000 times each second. In each millisecond it will execute some instructions of a new process, picked from the list of running processes.
Multi-tasking RTOS will likely have very short time slice, so that they can offer low latency.
Time shared systems were created to reduce ownership costs for hardware/software.
Explanation:
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