这里参考wiki 的英文，加上自己的理解翻译下RTOS的定义：
A real-time operating system (RTOS) is an operating system (OS) intended to serve real-time application process data as it comes in, typically without buffering delays. Processing time requirements (including any OS delay) are measured in tenths of seconds or shorter increments of time. They either are event driven or time sharing. Event driven system switches between task based on their priorities while time sharing switch the task based on clock interrupts.
RTOS 就是一种实时的操作系统，当有新的任务一到来就立即响应的操作系统， 不需要缓冲延迟。 处理时间(包括系统延迟)在1/10 秒内或者更短的时间。
RTOS 一般分为时间驱动或者时间片分享两种。事件驱动基于事件优先级，而时间片轮转基于时钟中断。
A key characteristic of a RTOS is the level of its consistency concerning the amount of time it takes to accept and complete an application's task; the variability is jitter.[1] A hard real-time operating system has less jitter than a soft real-time operating system. The chief design goal is not high throughput, but rather a guarantee of a soft or hard performance category. A RTOS that can usually or generally meet a deadline is a soft real-time OS, but if it can meet a deadline deterministically it is a hard real-time OS.[2]
RTOS的一个典型特点就是系统从响应任务到执行完成任务这一整个时间变化，叫做jitter，我们也叫做周期性抖动，这个周期就是指任务完成的时间周期. RTOS分为硬实时和软实时， 硬实时jitter比软实时更小。 RTOS的首要设计目标就是不要太高的吞吐量，保证硬实时或者软实时这两种要求(TODO). 通常能满足一般性的deadline最后期限的os 叫软实时系统，而能满足确定性的最后期限的OS叫硬实时操作系统.
以WinCE 系统为：它的反应时间是1~2ms, 但是要达到硬实时的要求，响应时间要达到150us以内。软实时通常超过最后期限后，系统程序还可以容忍一小段的误差时间。比如当来电话的时候，必须接电话建立通话连接，但是通常我们会等一下，一定时间还没有接通后才会要求挂断。
A common example of an RTOS application is an HDTV receiver and display. It needs to read a digital signal, decode it and display it as the data comes in. Any delay would be noticeable as jerky or pixelated video and/or garbled audio. ARINC Specification 653 defines the RTOS interface standard used in aviation embedded system designs. The RTOS is developed and supplied by multiple suppliers in an open market.
一个很普遍的实时应用的例子是高清电视接收和显示。它收到数据后需要读取数字信号，解码然后显示。任何一点晓得延时都会导致视频乱码或者音频乱码。
ARINC653号规范定义啦航空嵌入式系统设计的RTOS接口标准。 在市场上有多家类型的RTOS操作系统。
A RTOS has an advanced algorithm for scheduling. Scheduler flexibility enables a wider, computer-system orchestration of process priorities, but a real-time OS is more frequently dedicated to a narrow set of applications. Key factors in a real-time OS are minimal interrupt latency and minimal thread switching latency; a real-time OS is valued more for how quickly or how predictably it can respond than for the amount of work it can perform in a given period of time.[3]
RTOS有调度先进的算法。调度灵活性使进程优先级更广泛，但是一个实时操作系统更频繁地专用于狭窄的一些应用。在实时操作系统的关键因素是最小的中断延迟时间和最小的线程切换的延迟;一个实时操作系统的评估是依据它是更为如何迅速或如何可预测它完成的时间而不是它可以在一个特定的时间期间执行的工作总量。