What is Real-Time Operating System?
A real-time operating system (RTOS) is a type of operating system. An OS is a program that serves as a bridge between the system hardware and the user. Furthermore, it manages all interactions between system software and hardware.
Real-time operating systems are employed in real-time systems when time limitations are fixed and followed strictly. It means that there is extremely little time for processing and responding. Furthermore, the system must perform the given task within a specific time frame, or else the system will fail. It is mainly used in control device apps like industrial control systems, automobile-engine fuel injection systems, medical imaging systems, weapon systems, etc.
| Architecture | Design | Certification | Other | |||||||||||
| Name | CPU Arch | Kernel Arch | Multicore? | Scheduling | Slack scheduling? | Mixed criticality | Synchronization | Addressing | Interrupts | POSIX? | ARINC 653? | DO 178-B? | MILS? | License |
| QNX Neutrino | x86, SH-4, PowerPC, MIPS, ARM | Microkernel | Yes | "Priority-based preemptive" | Yes, by allowing partitions to overrun budgets when underutilized. Called "adaptive partitioning" | Yes, through budgets and priorities | Semaphores and Mutexes | Uses MMU to partition memory | ISR priority higher than any task priority, blocking allowed | Conformant to 1003.13-2003 | No | No | No | Closed source |
| LynxSecure | x86, possibly others | Embedded hypervisor and separation kernel (likely monolithic) | Yes | Fixed-cyclic ARINC 653-based (default), dynamic scheduling policies also permitted. | Unknown | Unsure. Most likely. | Hypervisor supports some sort of communication between virtualized OSes. Unclear how this is done. | Partitioned address space for the VMs that most likely uses the MMU. | Not discussed. | In guest OSes, I suppose. | Yes | Says that the OS is “Designed to support” it | Yes | Closed source |
| LynxOS-178 | x86 and PPC | Monolithic | No (not mentioned) | ARINC 653-based fixed-cyclic scheduling | Unknown | Yes | Everything that ARINC 653 dictates, plus POSIX | Virtual memory with MMU | Split interrupts with kernel threads (see Lynx website for more). | Yes | Yes | Yes | Not mentioned | Closed source |
| RTLinux (early version) | x86 | Monolithic (Linux) | No | Provides priority-based preemptive and EDF, or user can write their own. RT tasks run in kernel at high priority. Linux kernel runs as a background task. | No | No | Simple FIFO buffers (RT-FIFO) | Same as linux, but RT tasks run in kernel mode as modules. | Hardware interrupts caught by emulator and deferred. | In non-RT tasks. | No | No | No | Varies, but a free version is now available. |
| VxWorks | ARM, ColdFire, x86, x86_64, XScale, MIPS, PowerPC | Monolithic | Yes | 256 User Static Priorities | No? | No? | Spinlocks, Atomic memory operations, Memory barriers, semaphores, message queues, others | Supports normal MMU and non-overlapping modes | Assigned to CPUs at boot time. Other details not clear. | Compliant | No | No | No | Closed source |
| LynxOS RTOS | x86, Motorola/IBM PPC, Freescale PowerQUICC II and III, AMCC 4xx PPC | Monolithic | SMP support | FIFO, priority quantum (256 priority levels), round-robin, or non-preemptive | Unknown | Unsure. | Message queues, semaphores, shared memory, POSIX (sockets, signals, pipes, mutexes, etc.) | Demand paged virtual memory using an MMU | Split interrupts with kernel threads (see Lynx website for more). | Yes | Maybe, using round-robin (see scheduling)? | Yes | Not mentioned | Closed source |
| DeOS | x86, PPC | Microkernel | No (possibly in progress) | Rate monotonic w/ slack pool | Yes | Yes | Mutexes, semaphores, shared pages, message passing | Uses MMU/Virtual Memory in the normal way | Can be handled directly in user applications. Unknown if kernel interrupts are split. | No | No | Yes | No | Closed source |
| L4 (Fiasco) | x86, ARM, PowerPC | Microkernel | Yes | Not clear, but does periodic tasks natively | No? | No? | POSIX Threads capabilities | Paging in software with virtual memory | Appears to support two different modes, one of which may be polling | Compliant | No | No | No | GPL |
| VxWorks 653 | PowerPC, x86 | Separation Kernel (Microkernel), Choice of Partition OS | No (though Core 2 is a target) | Time partitioning (multiple modes supported), static priorities within partitions | Yes (can priority schedule a different partition when a partition is out of work) | Yes | Mutexes, memory locking, message queues, semaphores, signals, blackboards | MMU-based hardware partitioning | Not clear | Compliant | Yes | Yes | No | Closed source |
| Integrity | ARM, Power, Blackfin, OCTEON, Coldfire, x86, MIPS, DaVinci, OMAP | Separation kernel (Microkernel) | Yes | "Priority-based" | No? | Yes | Everything POSIX, highest locker semaphores to prevent priority inversion | Uses MMU to partition memory | Claims <1 nanosecond latency, never blocks or masks | Conformant | Available (probably precludes multicore) | Available (probably precludes multicore) | Available (precludes multicore) | Closed source |
| FreeRTOS | x86 (real mode only), 25 or more embedded devices | Extremely small monolithic | No | Dynamic priority round robin with support for an arbitrary number of compile-time priority levels. | No | No | Mutexes, semaphores, queues. | Architecture dependent. | Nothing special. | No | No | No | No | Modified GPL (no code re-release required) |
| VxWorks MILS | PowerPC | Separation Kernel (Microkernel) and can use choice of guest OS or "bare metal" High Assurance Environment | No | Time partitioning (multiple modes supported), static priorities within partitions | Yes, very limited (applications can donate time to device drivers) | Yes | Mutexes, memory locking, message queues, semaphores, signals, blackboards (within partitions) | Static space partitioning | Not clear | No? | No? | No? | Yes | Closed source |
| PikeOS | many | Microkernel | Yes | Uknnown; schedules virtual machines | Unknown | Unknown | Unknown | Unknown | Unknown | Yes | Yes | Unknown | No | Closed source |
| Nucleus OS (Successor to VRTX, apparently) | ARM, MIPS, Freescale, Power, PowerPC, probably others | Not clear | Yes | Not clear | No? | Yes | Includes at least semaphores but appears to require manual priority inheritance | "Full MMU Support" | Not clear. | Compliant | No | Yes (Perhaps not with multicore?) | No | Closed source |
| LynxOS-SE RTOS | x86 | Monolithic | No (not mentioned) | ARINC 653-based fixed-cyclic scheduling | Unknown | Yes | Everything that ARINC 653 dictates, plus POSIX | Unknown, likely the same as LynxOS RTOS | Split interrupts with kernel threads (see Lynx website for more). | Yes | Yes | Says that the OS is “Designed to support” it | Not mentioned | Closed source |
| VxWorks Hypervisor | x86, x86_64?, PowerPC | Separation Kernel (Microkernel) with choice of partition OS | Yes | Time partitioning or static priorities | No? (but could happen within a partition) | Yes | Just OS within partitions | Separate virtual memories | Not clear | Just OS within partitions | No? | No? | No | Closed source |
| eCos | ARM, CalmRISC, FR-V, Hitachi H8, IA-32, Motorola 68k, Matsushita AM36, MIPS, NEC V8xx, NIOS II, PowerPC, SPARC, SuperH | Microkernel | Yes (At least experimental SMP in kernel now) | Can do Multilevel Queue (FIFO with 32 static priority levels), bitmap (only one task allowed at each of 32 static priority levels), or, with special configuration, Lottery! Preemptive except when scheduling flag disabled. | No | No | Mutexes, Semaphores, Condition Variables, Flags, Message Boxes, Spinlocks | MMU-enabled, but all software shares an address space | Split interrupt handling, but all interrupts prioritized over threads | Compliant | No | No | No | GPL |
| VxWorks Cert (basically just a certified version of VxWorks) | Not clear | Monolithic? | No | 256 User Static Priorities? | No? | No? | Semaphore, Signal, Others? | Supports normal MMU and non-overlapping modes? | Not clear | Compliant | No | Yes | No | Closed source |