即時運算（）是電腦科學中對受到「即時約束」的電腦硬體和電腦軟體系統的研究，即時約束像是從事件發生到系統回應之間的最長時間限制。即時程序必須保證在嚴格的時間限制內回應。通常即時回應時間會是以毫秒為單位，也有時是以微秒為單位。相比之下，非即時系統是一種無法保證在任何條件下，回應時間均符合實時約束限制的系統。有可能大多數的情形下，非即時系統都可以符合即時約束限制，甚至更快，只是無法保證在任何條件都可以符合約束限制。
在其他領域中也有用到「即時」这个词，但其含義不同：在仿真领域，即時是指「即時时钟同步」，此外在数据传输、多媒体处理和企业系统领域，实时意思是「感觉不到延迟」。
即時軟體必須使用一種或多種、实时操作系统以及建立在一個實時軟件應用程序上的即時網絡提供的基本框架。
防抱死系统是个即時運算系统的简单例子，在这个系统中的即時即時约束是為了避免车轮锁死，刹车必须释放的时间。即時電腦系統的反應最後期限一般和事件有關，若没能在最后期限前完成，即為即時電腦系統的失效。不論如何，即時電腦系統均需滿足最后期限的限制條件。
历史
术语即時的使用起源于仿真的早期，
即時運算标准
如果一個系統作業的正确性不仅取决于其逻辑的正确性，还与其执行的时间有关，此系统就被称为即時系统。即時系统和其最後期限可以依照若错最後期限所造成的後果來加以分類：
Hard
错过最后期限会导致整个系统故障。
Firm
能容忍偶爾的错过最后期限，若超过最后期限，其实用性为零。
Soft
超过最后期限的结果会造成实用性降低，从而降低系统的服务品質。
因此，硬體即時系统的目标是确保所有最后期限的完成。
即時數位訊號處理
实时和高性能
设计方法
参看
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参考文献
外部链接
技术委员会
*[https://web.archive.org/web/20111008235412/http://cs-www.bu.edu/pub/ieee-rts/ IEEE Technical Committee on Real-Time Systems]
*[http://www.ecrts.org Euromicro Technical Committee on Real-time Systems]
科学会议
*[http://www.ecrts.org/ ECRTS - Euromicro Conference on Real-time Systems]
*[http://www.rtss.org/ IEEE Real-time Systems Symposium]
*[http://www.rtas.org/ IEEE Real-time Technology and Applications Symposium]
*[http://www.informatik.uni-trier.de/~ley/db/conf/isorc/index.html International Symposium on Object-oriented Real-time distributed Computing]
*[http://www.rtcsa.org/ IEEE International Conference on Embedded and Real-Time Computing Systems and Applications]
*[http://rtecc.com/home/ Real-Time & Embedded Computing Conference]
期刊
- [http://www.inderscience.com/ijccbs International Journal of Critical Computer-Based Systems]
- [http://www.springer.com/computer/communication+networks/journal/11241 International Journal of Real-Time Systems]
研究小组
*[http://www.cister.isep.ipp.pt/ CISTER Research Unit, ISEP, Polytechnic Institute of Porto (IPP), Portugal]
*[https://web.archive.org/web/20110927050332/http://www.loria.fr/equipes/TRIO/english/index.html Real-Time Systems Research Group,INRIA,LORIA NANCY, France]
*[https://web.archive.org/web/20120224213521/http://www.cs.virginia.edu/~control/ Real-Time & Embedded Computing Laboratory (USMAN SHARIF BCS-SP03-37)]
*[https://web.archive.org/web/20111004212959/http://www.mrtc.mdh.se/ Mälardalen Real-Time research Centre]
*[https://web.archive.org/web/20100118184005/http://www.eecs.umich.edu/RTCL/ Real-Time Computing Laboratory]
*[http://www.ida.liu.se/~rtslab/ Real-Time Systems Laboratory]
*[https://web.archive.org/web/20140104130942/http://rtselab.org/ RTSE Laboratory]
*[https://web.archive.org/web/20050916141435/http://www.rts.uni-hannover.de/en/ Institute for Systems Engineering - Real Time systems Group]
*[http://retis.sssup.it/ Real-Time Systems Laboratory at Scuola Superiore Sant'Anna, Pisa, Italy]
*[http://rts.eit.uni-kl.de/ Technical University of Kaiserslautern - Institute for Electrical Engineering and Information Technology - Real-Time Systems Group]
*[https://web.archive.org/web/20111205114719/http://www.vmars.tuwien.ac.at/ Vienna University of Technology - Institute for Computer Engineering - Real-Time Systems Group]
*[http://www.cs.york.ac.uk/rts/ Real-Time Systems Research Group at the University of York, UK]
*[http://www.chalmers.se/cse/EN/research/research-groups/dependable-real-time Chalmers University of Technology - Dependable Real-Time Systems research group]
*[http://www.artes.uu.se/ ARTES: a national Swedish strategic research initiative in Real-Time Systems supported by the Swedish Foundation for Strategic Research (SSF), SE]
*[http://www.cs.unc.edu/~anderson/real-time/ Real-Time Systems at the University of North Carolina at Chapel Hill]
*[https://web.archive.org/web/20111008235408/http://www.real-time.ece.vt.edu/ Real-time Systems Laboratory at Virginia Polytechnic and State University, Blacksburg]
*[http://www.mc2labs.com/ mc2labs.com - ICT Research&Development, Mogliano Veneto IT - Realtime Internet programming infrastructures]
技术论文
*[http://www.opal-rt.com/technical-document/what-where-and-why-real-time-simulation The What, Where and Why of Real-Time Simulation]
Real-time
Системы реального времени
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