FAQ

See the comp.realtime newsgroup

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Comp.realtime: a (LONG) list of Real-Time Operating Systems

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This posting provides an overview of newsgroup comp.realtime by summarizing the history, common past topics, and frequently asked questions.

HTML versions (much easier to read) are available at:
http://www.dedicated-systems.com/encyc/publications/faq/rtfaq.htm
http://www.faqs.org/faqs/realtime-computing/faq/
http://www.cis.ohio-state.edu/hypertext/faq/usenet/realtime-computing/top.html

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What's new in the FAQ?

Added a definition for the term real-time.
Added a section for WindowsCE.
Added RT-Linux to the Research and Free Product section

I- INTRODUCTION

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What is the purpose of this FAQ ?

The purpose of this FAQ is to give sufficient knowledge to a new user in the Real-Time field and to serve as a reference to the engineer working in this field. This FAQ gives an overview about each topic and refers to other ressources (Internet, Publications, Company) for a more complete information.
A companion posting to this one, "Comp.realtime: Welcome to comp.realtime" @message-id realtime_welcome@, complements this one by providing a concise introduction to the group. Another posting, "Comp.realtime: A list of real-time operating systems", @message-id realtime_list@, provides references to available operating systems.

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What is the charter of comp.realtime?

The charter of comp.realtime is to provide a forum for discussion of both the theory and practice of real-time computer systems. The group is unmoderated; participation is open to all.
[If there was a formal charter for the newsgroup at the time of its creation, we don't have access to it at the moment. Readers?]

Note that the listing in the canonical "newsgroups" file is:
comp.realtime Issues related to real-time computing.

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Where should I ask questions about real-time systems?

Comp.realtime is certainly the place. However, if you are asking about a particular real-time system, see below for a (possibly) better place to start.

For topics that are only somewhat related to real-time systems, also consider comp.arch and/or comp.os.misc. For instance, topics about bus-based computer systems are best asked in comp.arch, or, if they're about the VMEbus, comp.arch.bus.vmebus.

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What is considered good net.etiquette on comp.realtime?

Here are some etiquette reminders that will help us all to make the group an ever-friendlier place:

-- Please, before posting, ensure that you've read the basic Usenet etiquette guide in news.announce.newusers.

-- Please set the Followup-To: line in your post. This is especially true if you are cross-posting. If you are requesting information, consider setting:
Followup-To: poster, and then summarizing the replies to the net.

-- When following up, please change the Subject: line if the subject has really changed.

-- Some sites that receive comp.realtime are on branches of the net that frown on overtly commercial announcements. These postings are welcomed on comp.newprod and anywhere in the biz.* hierarchy. However, short offers by vendors to provide further information by email are usually seen as acceptable.

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II- DEFINITIONS

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What exactly is meant by real-time?

There are _several_ definitions of real-time, most of them contradictory. Unfortunately the topic is controversial, and there doesn't seem to be 100% agreement over the terminology.

1. The canonical definition of a real-time system (from Donald Gillies mailto:gillies@ee.ubc.ca ), is the following:

"A real-time system is one in which the correctness of the computations not only depends upon the logical correctness of the computation but also upon the time at which the result is produced. If the timing constraints of the system are not met, system failure is said to have occurred."

Others have added:

"Hence, it is essential that the timing constraints of the system are guaranteed to be met. Guaranteeing timing behavior requires that the system be predictable. It is also desirable that the system attain a high degree of utilization while satisfying the timing constraints of the system."

A good example is a robot that has to pick up something from a conveyor belt. The piece is moving, and the robot has a small window to pick up the object. If the robot is late, the piece won't be there anymore, and thus the job will have been done incorrectly, even though the robot went to the right place. If the robot is _early_, the piece won't be there yet, and the robot may block it.

Another example is the servo loops in an airplane when on auto-pilot.
The sensors of the plane must continuously supply the control computer with proper measurements. If a measurement is missed, the performance of the airplane can degrade, sometimes to unacceptable levels.

David Sonnier ( mailto:dps@devnull.mpd.tandem.com ) adds the distinction:

In the robot example, it would be hard real time if the robot arriving late causes completely incorrect operation. It would be soft real time if the robot arriving late meant a loss of throughput. Much of what is done in real time programming is actually soft real time system. Good system design often implies a level of safe/correct behavior even if the computer system never completes the computation. So if the computer is only a little late, the system effects may be somewhat mitigated.

2. POSIX Standard 1003.1 defines "real-time" for operating systems as:

"Realtime in operating systems: the ability of the operating system to provide a required level of service in a bounded response time"

3. One will occasionally see references to "real-time" systems when what is meant is "on-line", or "an interactive system with better response time than we used to have". Often, this is just marketing hype. For instance, although some have queried whether running "rn" is real-time, it is not, as it is interacting with a human who can tolerate hundreds of milliseconds of delays without a problem. Similarly, on-line stock quotation systems interact with humans.

4. One will also see references to "real-time" systems when what is meant is just "fast". It might be worth pointing out that "real-time" is not necessarily synonymous with "fast"; that is, it is not the latency of the response per se that is at issue (it could be of the order of seconds), but the fact that a bounded latency sufficient to solve the problem at hand is guaranteed by the system. In particular, frequently, algorithms that guarantee bounded latency responses are less efficient overall than algorithms that don't.

5. One will also occasionally see discussions of "soft" vs. "hard" real-time systems. In many of these discussions, "hard" real-time means the type of real-time system discussed above, and "soft" real-time means systems which have reduced constraints on "lateness" but still must operate very quickly and repeatably. However, the definition is controversial, as some mean by "hard" and "soft" the degree of time constraints. For instance, a real-time process attempting to recognize images may have only a few hundred microseconds in which to resolve each image, but a process that attempts to position a servo-motor may have tens of milli-seconds in which to process its data.

6. Robert Bristow-Johnson adds the following distinction (in the case of DSP):
In a real-time DSP process, the analyzed (input) and/or generated (output) samples (whether they are grouped together in large segments or processed individually) can be processed (or generated) continuously in the time it takes to input and/or output the same set of samples independent of the processing delay.

Consider an audio DSP example: if a process requires 2.01 seconds to analyze or process 2.00 seconds of sound, it is not real-time. If it takes 1.99 seconds, it is (or can be made into) a real-time DSP process.

A common life example I like to make is standing in a line (or queue) waiting for the checkout in a grocery store. If the line asymtotically grows longer and longer without bound, the checkout process is not real-time. If the length of the line is bounded, customers are being "processed" and outputted as rapidly, on average, as they are being inputted and that process _is_ real-time. The grocer might go out of business or must at least lose business if he/she cannot make his/her checkout process real-time (so it's fundamentally important that this process be real-time).

The last definition (6) is the one used for real-time audio and video, for phone call over Internet, and so on. It means that the processing time is less than the time to get a sample. Note that in the case of Internet it is easy to get starvation because the sample arrivals depend on the bandwidth.

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What is POSIX 1003.1b (formerly 1003.4)? Where is it available?

POSIX 1003.4 was the working name for what is now the 1003.1b standard..

Recently, Dan Hildebrand posted:

The ratified POSIX standards that generally pertain to real-time OS's consist of: 1003.1 (OS, process, filesystem and device API), 1003.2 (utilities), 1003.1b (real-time), and 1003.1c (threads). POSIX 1003.1d (which defines some additional real-time extensions like standardized interrupt handler support) is not yet ratified, although some OS's already support portions of it.

The best way to get the most current status is to refer to some of these texts and contacts:
The POSIX 1003.1 standard is ISBN 1-55937-061-0. A good O'Reilly text is "POSIX Programmer's Guide: Writing Portable UNIX Programs". Donald Lewine. ISBN: 0-937175-73-0

For other standards, the IEEE's address is:
Secretary, IEEE Standards Board
445 Hoes Lane
P.O. Box 1331
Piscataway, NJ 08855-1331
USA
http://cs-www.bu.edu/pub/ieee-rts/Home.html

Many of the POSIX draft standards can also be obtained by calling the IEEE Draft Standards Office. Credit card in-hand, phone +1 202 371 0101 to place an order.

Another contact is the IEEE-USA Customer Service Center at 800 678 4333
(+1 908 981 1393 for outside of 800 zone); fax: +1 908 981 9667.

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What makes an OS a RTOS?

1. A RTOS (Real-Time Operating System) has to be multi-threaded and preemptible.
2. The notion of thread priority has to exist as there is for the moment no deadline driven OS.
3. The OS has to support predictable thread synchronisation mechanisms
4. A system of priority inheritance has to exist
5. OS Behaviour should be known
So the following figures should be clearly given by the RTOS manufacturer:
1. the interrupt latency (i.e. time from interrupt to task run) : this has to be compatible with application requirements and has to be predictable. This value depends on the number of simultaneous pending interrupts.
2. for every system call, the maximum time it takes. It should be predictable and independent from the number of objects in the system;
3. the maximum time the OS and drivers mask the interrupts.
The following points should also be known by the developer:
1. System Interrupt Levels.
2. Device driver IRQ Levels, maximum time they take, etc.

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What is a good RTOS?

A good RTOS is not only a good Kernel ! A good RTOS should have a good documentation, should be delivered with good tools to develop and tune your application. So even if some figures like the Interrupt latency, Context switch time are important, there are a lot of other parameters that will make a good RTOS. For example a RTOS supporting many devices will have more advantages than a simple very good nano-kernel.

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What is RMA?

"Rate-Monotonic Analysis" is a term coined by researchers at CMU. It refers to the real-time performance analysis of a system design that uses static-priority driven scheduling, in particular, the "rate-monotonic" static priority assignment, where tasks with shorter periods get the higher priorities, in a typical static-priority driven real-time operating system like pSOS VxWorks VRTX etc.

Research (mostly) from CMU and U-York and Illinois gives you the mathematical tools to answer "what if I designed it this way???" analysis on your system workload. You need to break your software into "tasks" with "periods" and "deadlines" (relative to the period) and you must be able to guess or prototype a rough "execution time" for each task. Also, for more precise analysis, it helps to know all the critical sections and their runtimes and who shares them, or at the least, the length of the longest critical section in all of your software.

You can plug all this data into a "schedulability analyzer" tool (PERTS, MIMOSA, Scheduler 1-2-3 (CMU), TimeWiz (TimeSys), Software Engineer(?s) Notebook (CMU, newer)), or even the back of an envelope "Utilization Test" or "Formula Test" and find out if your workload will meet all its different deadlines.

If you workload does not meet deadlines, the better tools can help you to explore changes to your workload, in order to meet all the deadlines.

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III- PUBLICATIONS COVERING REAL-TIME TOPICS

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Here are some references to the theory and practice

Several people recommended as a starting place the article "Tutorial on Hard Real-Time Systems", edited by John A. Stankovic and Krithi Ramamritham, IEEE Computer Society reprint series, Computer Society order number 819.

Kopetz, H.: Real-Time Systems, Design Principles for Distributed Embedded Applications. Kluwer Academic Publishers, Massachusetts, 1997.
A good book indeed. It covers:
Real-Time Environment, Distributed Solutions, Global Time, Modeling Real-Time Systems, Real-Time Entities and Images, Fault Tolerance, Real-Time Communication, The Time-Triggered Approach, Input/Output, Real-Time Operating Systems, Real-Time Scheduling, Validation, System Design, Time Triggered Architecture

PRACTITIONER'S HANDBOOK FOR REAL-TIME ANALYSIS: Guide to Rate Monotonic Analysis for Real-Time Systems. Klein,Mark; et al, Year 1993, Definitive developer's guide. Ten chapters in 4 parts: Introduction;Concepts & Techniques; Analyzing Real-Time Systems; & Using the Handbook on Realistic Systems. KLUWER ACADEMIC, Pages 712, ISBN: 0-7923-9361-9

STRATEGIES FOR REAL-TIME SYSTEM SPECIFICATION, Hatley,D.J. & Pirbhai, I.A, Year 1988, Casebook & practical reference for modeling requirements & architecture. Topeics include: Process; Control; Finite State Machines, Timing; Dictionary; & Examples, DORSET HOUSE, Pages 408, ISBN: 0-932633-11-0

STRUCTURED DEVELOPMENT FOR REAL-TIME SYSTEMS, Combined Version, Vols 1,2 & 3., Ward, P.T. & Mellor, S. J., Year 1987, PRENTICE HALL, Pages 468, ISBN: 0-13-854654-1

Caxton Foster's "Real-Time Programming: Neglected Topics," despite the title, is a very good introduction to the basic topics of real-time control,
starting with simple things like interrupts and debouncing switches, all the way through digital filters. It's a thin paperback (Addison Wesley MicroBooks), and a (somewhat) experienced programmer can get through it in a couple of days.

iRUG. Proceedings of the Intel Real-Time User's Group. Annual, back copies available from iRUG, P.O. Box 91130, Portland, OR 97291, (800) 255-4784. Annual conference proceedings dealing primarily with Intel's family of real-time OSs, iRMX.

Books references in The Online Dedicated Systems Encyclopaedia(there is always a comment there) http://www.dedicated-systems.com/encyc/publications/books.htm
J.E. Cooling, Software Design for Real-time Systems, SBN 0-412-34180-8, published by Chapman and Hall.
Yann Hang Lee and C.M. Krishna, Readings in real-time systems, ISBN 0-8186-2997-5, 1993, published by IEEE Computer Society Press.
Mathai Joseph, Real-Time Systems, University of Warwick, ISBN 0-13-455297-0, 1996, published by Prentice Hall Professional Technical Reference.
Krishna M. Kavi , Real-time systems, abstractions, languages and design methodologies, ISBN 0-8186-3152-X, 1992, published by IEEE Computer Society Press.
Phillip Laplante, Real-time systems design and analysis, an engineer's handbook, ISBN 0-8186-3107, 1993, published by IEEE Computer Society Press
David L. Ripps, An implementation guide to real-time programming, ISBN 0-13-451873-X, 1989, published by Yourdon Press, Prentice-Hall Building, now out of print!
Ken Shumate and Marilyn Keller, Software specification and design, a disciplined approach for real-time systems, ISBN 0-417-53296-7, 1992, published by John Wiley and Sons, Inc.

Another list of books with comments http://www.realtime-os.com/rtresour.html
A publisher: http://www.powells.com

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Peter Desnoyers <mailto:peterd@merlin.dev.cdx.mot.com> sends along:
The classic reference in the area of timers is:

George Varhese and Tony Lauck, "Hashed and Hierarchical Timing Wheels: Data Structures for the Efficient Implementation of a Timer Facility", Operating Systems Review 21, no. 5 (Proceedings of 11th ACM Symposium on Operating Systems), 1987.
Their results show O(1) times for insert and delete of 13 and 7 instructions for one of the schemes, and decent performance with large numbers of outstanding timers.

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Christian Ebner mailto:ebner@vmars.tuwien.ac.at sends along a classic reference in priority inheritance algorithm:
Sha, L., Rajkumar, R. and Sathaye, S.: Priority Inheritance Protocols: An Approach to Real-Time Synchronization. IEEE Transactions on Computers, Vol. 39(9). pp.1175-1185.
Analysis shows that the priority inheritance protocol can lead to chained blocking and deadlocks. To solve this problem, the priority ceiling protocol was developed by L. Sha, R. Rajkumar and S. Sathaye.

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Here are some other suggestions from various net.sources, in publishing date order:
Mellichamp, D. A. Real-Time Computing. New York: Van Nostrand Reinhold, 1983. 552 pp.
Twenty chapters by 11 authors on topics ranging from signal processing to managing real-time computing facilities.

A. K. Mok, The Design of Real-time Programming Systems Based on Process Models, in Proc. 1984 Real-Time Systems Symposium, Dec.1984, pp5-17.

E. Kligerman and A. Stoyenko, Real-Time Euclid: A Language for Reliable Real-Time Systems, in TOSE, Sep. 1986, pp 941-949, vol SE-12.

D. W. Leinbaugh and M.-R. Yamini, Guaranteed Response Times in a Distributed Hard-Real-Time Environment, in TOSE, Dec.1986, vol SE-12.

A. Stoyenko, A Real-Time Language With A Schedulability Analyzer, Computer Systems Research Institute, University of Toronto, Dissertation, Dec. 1987.

Lawrence, P. D. and Mauch, K. Real-Time Microcomputer System Design. New York, McGraw-Hill, 1987. 568 pp.
The emphasis is on the design of I/O circuits and assembly language interfaces for small microprocessors used for embedded systems.

H. Kopetz and A. Damm and Ch. Koza and M. Mulazzani and W. Schwabl and Ch. Senft and R. Zainlinger, Distributed Fault-Tolerant Real-Time Systems: The MARS Approach, in IEEE Micro, vol.9, Feb.1989, pp25-40.

Burns, A. and Wellings, A. Real-Time Systems and Their Programming Languages. Wokingham: Addison-Wesley, 1990. 575 pp.
Ada, Modula-2, and occam 2 are used throughout the book, which covers topics ranging from basic programming techniques, fault tolerance, exception handling, concurrency, resource management, and distributed designs.

Vickery, C. Real-Time and Systems Programming for PCs. New York: McGraw-Hill, 1993. 604 pp. The thesis is that the development environment for real-time systems is ideal for studying systems programming, too. After some introductory material, the book deals exclusively with Intel's iRMX operating systems, with particular emphasis on iRMX for Windows.

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Real-Time and Embedded Systems related magazines

List from The Real-Time Encyclopaedia http://www.dedicated-systems.com/encyc/publications/magazine.htm

Embedded Systems Engineering: Embedded Systems Engineering is a UK based magazine dedicated to embedded systems and development tools. Although highly focused on embedded systems this publication also covers real-time products. For more info email to Jeremy Kenyon. (mailto:100142.1323@compuserve.com)

Details: 10 issues/year, 60 pages, English.

Embedded Systems Programming: Embedded Systems Programming is the leading magazine on embedded systems design in the US. Although covering mostly embedded systems, a lot of the editorial is dedicated to real-time systems.

Details: 12 issues/year, 106 pages, English.

Mezzanines: Mezzanines is the official publication from GRoupIPC, the international user & manufacturer group promoting IP & PMC mezzanine solutions. This fancy publication contains technical articles, application notes and offers a new products section besides a currently updated product directory.
Dedicated Systems Magazine: Dedicated Systems Magazine (ex- Real-Time Magazine) is THE European reference magazine for the real-time systems developer. Each magazine is dedicated to a special theme, such as Buses (VME, PMC, CompactPCI,...), RTOS, Tools (debugging, monitoring, simulation, design, bus hardware analyzers), Real-Time Networks, etc. A must for real-time engineers who don't have the time (and money) to spend on courses and workshops. The magazine also contains Dedicated Systems Gazette, the supplement which contains new products information.

Details: 4 issues/year, 124 pages (+16 page supplement), English.

Real-Time Systems: Real-Time Systems is a journal on time-critical computing systems. Although very real-time focused, this publication is very theoretical and more targeted toward researchers.

Details: 6 issues/year, 320 pages A5, English.

RTC Magazine: The RTC Magazine (before "The Real-Times"), not to confuse with Real-Time Magazine, is a more commercial publication which supports the RTC shows in the US and Europe. The magazine helps the exhibiting companies to better promote their products.

Details: 6 issues/year, 92 pages, English.

VITA Journal: The VITA Journal is a VMEbus related publication and the official publication from VITA, the VMEbus International Trade Association. As VME is an important standard in real-time, we shouldn't omit this publication in our list.

Details: 4 issues/year, 46 pages, English.

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What other net.resources are available on real-time systems?

There are at least two other newsgroups devoted exclusively to a particular vendor's real-time operating system:

news:comp.os.lynx The LynxOX real-time operating system.
news:comp.os.os9 Discussions about the os9 operating system.
news:comp.os.qnx The QNX real-time operating system.
news:comp.os.vxworks The VxWorks real-time operating system.
news:comp.sys.harris The Harris NightHawk & CX/UX & CX/RT operating systems.

Here are some other related newsgroups:
news:alt.industrial.computing
news:comp.arch Computer architecture.
news:comp.arch.bus.vmebus Hardware and software for VMEbus Systems.
news:comp.os.misc General OS-oriented discussion not carried elsewhere.
news:comp.robotics All aspects of robots and their applications.
news:comp.sys.m68k Discussion about 68k's.
news:sci.engr.control The engineering of control systems.
news:sci.engr.manufacturing

There are too many other newsgroups devoted to computer operating systems that support some form of real-time scheduling to list here. The interested reader is advised to check the "newsgroups" file on her or his local machine.

There is a realtime-related mailing list for embedded computer systems developers. It is not strictly real-time, but there is some overlap. To subscribe, send your email address to mailto:embed-request@synchro.com.

A mailing list for discussions concerning the use of Futurebus+ now exists.
Appropriate topics include the design, implementation, integration and operation of the hardware and software that are related to Futurebus+. To subscribe, send the one-line email message (in the body of the message, not the header; the Subject line is ignored) as shown below to mailto:majordomo@theus.rain.com.

subscribe fbus_users <your_email_address>

To get more information about the mailing list, send the one-line command shown next to mailto:majordomo@theus.rain.com:

info fbus_users

The info page is automatically sent when you subscribe.

A mailing list intended for the discussion of topics relating to the pSOSystem and other products of Integrated Systems Inc., Software Components Group, has been started. Send articles to mailto:psosuser@isi.com and administrative (subscription) requests to mailto:listserver@isi.com. The list administrator is Radek Aster who can be reached at mailto:raster@isi.com.

Dan Hildebrand <mailto:danh@qnx.com> has a posting listing a number of the embedded PC standards and further references. If enough folks are interested, it's sufficiently detailed enough to make a separate FAQ of its own.

Russ Hersch <mailto:sibit@datasrv.co.il> is now maintaining two _extensive_ FAQs about specific microcontroller families, and one about microcontrollers in general. Here's the pointers:

Subject: 68hc11 microcontroller FAQ
Summary: This article is a collection of information sources on the Motorola 68hc11 line of microcontrollers.
Archive-name: 68hc11-microcontroller-faq
Posting-Frequency: monthly

Subject: 8051 microcontroller FAQ
Summary: This article is a collection of information sources on the Intel 8051 line of microcontrollers (and variants).
[He's working on the archiving of this one.]
Posting-Frequency: monthly

Subject: Microcontroller Primer FAQ
Summary: This article is a primer and general FAQ about microcontrollers.
[He's working on the archiving of this one.]
[Posting-Frequency: monthly, I think]

He also states that Tom Kellett is working on a FAQ on the PIC micro-controller line, and adds that "hopefully, this will lead towards a much needed collection of microcontroller FAQs."

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Which Web Sites give information about real-time?

The Dedicated Systems Encyclopaedia about everything you want to know about Real-Time (http://www.dedicated-systems.com)
E. Douglas Jensen Web Site (http://www.realtime-os.com/rtresour.html)
Frank Miller Resource list http://www.cs.umd.edu/~fwmiller/etc/realtime.html
EEToolbox Resource link list http://www.cera2.com/realtime.htm
The RTC Group (http://www.rtcgroup.com)
Embedded Systems (http://www.embedded.com/net.htm)
VITA (http://www.vita.com)
Mezzanines International (ex- GRoupIPC) (http://www.mezzanines.org)
A good collection of links (http://www.ifi.unizh.ch/groups/ailab/embedded.html)

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IV- POLEMIC TOPICS

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Is Windows NT (or windows 95, or even Windows CE now) a Real-Time Operating System?

This question appears repeatedly in this news group. Here are the key points:
- Despite a real-time class process, the Win32 API is not suitable to be used for a Real-Time System:
1. Too few priorities for processes and threads
2. No priority inheritance mechanism
3. Some calls are synchronous with process from the Dynamic Class
- Despite a good interface to hardware for CLASSICAL applications, this interface is not suitable to develop a Real-Time System:
1. Most of the job in a device driver is done at the DPC level. And most COTS DD take too much time in the DPC.
2. The DPC problem could have been avoided by increasing the number of DPC levels, but this is not the case.
3. Pentium Power Management interrupt can preempt your system for an unpredictible amount of time (depending of the BIOS)
- Real-Time clock
There is a lack of programmable timer.

For a more complete view, look at article:
http://www.dedicated-systems.com/magazine/97q2/winntasrtos.htm

Some companies are now providing Real-Time Extensions to fill up the hole let opened by Microsoft. (cf the RTOS list)
To do so three main approaches exists: include NT as the lowest level process in an existing RTOS, put a WIN32 API on top of an existing RTOS, make NT coexists with a RTOS by modifying the HAL. For complete view, look at article:
http://www.dedicated-systems.com/magazine/97q2/winntext.htm
One might also be interested in the vendors proposing such products
http://www.qnx.com/whitepaper/qnxwin32.html
http://www.vci.com
http://www.radisys.com
http://www.nematron.com
http://www.lp-elektronik.com

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Is Windows CE 2.0 a real-time operating system?

This question appears frequently since the release of Windows CE 2.0. It is not suitable for real-time system development because:
1.The number of priority levels is too low;
2. Interrupts can not be nested;
3. Interrupt latency is too high;

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Which methodology should I use to design a Real-Time System?

At least you should use one. It is high time people are convinced of the interest of building a house with the plan. It is very strange that people still think that Software development is equal to writing or even hacking code. There are at least three big classes of methodologies :
- The one related to Ada (Booch)
- The one based on data flows
- The OO Methodology (OMT)
One could add the formal approach too (SDL, MSC).
The choice will be based on the inhouse knowledge, level of education and the client knowledge of software development. For each methodology you have tools that are more or less good.

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Which programming language should I use to develop a Real-Time System?

Of course you can choose to use assembler. You can always use tools from the Ancient Age. Nevertheless it would be much better to use a higher level programming language. Most of them will fit. The Ada Community will always try to convince you their language is the best to use in any cases. Here is not the news group to argue about this (news:comp.lang.ada is THE place). Others will try to convince you to use an OO language. Then you have to be carefull with the memory management unpredicitbility (Is there a garbage collection ? Is it under the developpers control?). The best solution is to avoid the use of dynamic object creation. Just create them at startup. You have to know that the most used languages are (in alphabetical order) :
Ada, C, C++ for realtime system development.
Most of the time small parts of the system are still written in assembler (small parts of Device Driver).

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What kind of processor should I use for my Real-Time System ?

CISC vs RISC: there has been a lot of discussion in the past about this issue. Because of the high number of registers in a RISC processor, people said the context switch was not compatible with real-time systems requirements. This is not true as compilers can optimise the use of registers to reduce the size of the context switch. A lot of points could be added here, but as a conclusion we can say that both can be used for Real-Time System.
This FAQ is not the place to start a new war, but you can send any additions to mailto:jcmon@rtusi.com.

PowerPC 60x versus 40x
The PowerPC 60x family is well suited for calculation, but to deal with the external world (through Hardware Interrupt) the family 40x should be prefered as the interrupt management is much more oriented to hardware whereas in the 60x family it is more oriented to software.

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What kind of bus should I use for my Real-Time System ?

VMEbus vs Compact PCI
Just remember these few things :
1. you can have 21 boards on the same VME bus.
2. you have 7 priority levels for Interrupting on the bus
3. you have 4 level to take the bus
4. last but not least: the installed VME bus based applications is huge.

The Compact PCI offers a bigger bandwidth, is based on a widely spread standard and the boards should be less expensive to produce (the interface chips are cheaper).

Other busses:
The choice is big : FutureBus+, Multibus, VXIbus, PCI, ISA, ...
The choice will depend on the type of application, the type of hardware to use (price/performance) and the target maket.

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What Mezzanine technology should I use ?

Industry Pack, PMC, M-Module, S-Bus ...
For IP and PMC, a good place to look at is http://www.groupipc.com/

For M-Modules, CXC Modules, a good place to look at is http://www.vita.com/mezzprod/mezzdirindex.html

It should finally be noticed that the choice will depend on the type of I/O you deal with. If the key point is price, then IP or M-Modules is the answer, if performance is the key PMC or S-Bus Module should be choosen. Another point is the availability of the products: here PMC and IP is THE choice. They are much more widespread than any other. They are also supported by a usergroup organisation : GRoupIPC

------------------------------
What real-time network should I use for my real-time system?

Here the choice is huge and the standardization efforts still poor (but this is changing quickly):
PROFIBUS, BITBUS, WorldFIP, FIELDBUS, CAN, MIL-STD-1553, ATM, Reflective Memory, ...
The choice will depend on the price (MIL-STD-1553 is quite expensive, ATM also), on the availability of controllers, drivers, PLC, ...

------------------------------
Dedicated Systems and year 2000: what are the risks?

Martin Timmerman from Dedicated Systems Experts explained:
There can be two causes of year 2000 problems: hardware and software

Software:
--------------

The problem is there if you use somewhere only a two-digit system in the software. Therefore you should check all time data structures the software uses.
Most real-time systems do not use the absolute time for decision making, they work with relative time. However if a time delay is computed starting from 2 absolute times then you have a problem.
If the time is used only for time stamping then there is no problem. The year 2000 will be 00.
The message here is: for each individual system one should look if absolute time is used. If yes, wherefore it is used - are computations based on it?

Hardware:
--------------

- Most Time Of Day (TOD) ICs have only two digit year codes.
In this case the software, should use something like:

if TODyear < 60 add 2000
else add 1900

Problem could be with leap years (1900 is not a leap year, but 2000 is. you need to check IC spec)

- GPS can have problems (not at 2000, but on another date: this was error in GPS specifications)
- Other HW: See TOD (remark that some Time interfaces do not have any year information at all: IRIG-B for instance)

Special note for black boxes:

- Check the Interfaces with your system (if time is used) (use the ISD: Interface Specification Document) or check with the Black box manufacturer (if he is still alive.....)

Rule: everywhere time is used by the system, there is a potential year 2000 problem.

Remark: this may generate a lot of work. Subsystem by subsystem should be examined. You need good documentation for the subsystems, which might not be available. Having the design documentation is almost imperative and this may also be a fundamental problem for older systems.

======================================================

V- MARKET

------------------------------
Where can I find information related to real-time products?

Product directories :
VME Products Directory http://www.vita.com/vmeprod/prodir.html
Industry Pack and PMC Products Directory http://www.groupipc.com/products/products.htm
Chips Products Directory http://www.xs4all.nl/~ganswijk/chipdir/
ULC Buyer's Guide http://www.cera2.com/ulc.htm

RTOS :
http://www.cs.arizona.edu/people/bridges/oses.html
http://www.dedicated-systems.com/encyc/buyersguide/rtos/Dir228.html

New Products :
http://www.vita.com/npgallery/npgallery.html
http://www.embedded.com/prod.htm

------------------------------
Where can I find information about real-time Conferences, Workshops and Tradeshows?

http://www.realtime-os.com/rtresour.html#olconf
http://www.dedicated-systems.com/encyc/calendar/calendar.htm

------------------------------
International organization for standards?

List from Dedicated Systems Encyclopaedia http://www.dedicated-systems.com/encyc/techno/asso/standard/standard.htm

ANSI: American National Standards Institute (http://www.dedicated-systems.com/encyc/techno/asso/standard/organ's/ansi.htm)
IEC: International Electrotechnical Commission (http://www.dedicated-systems.com/encyc/techno/asso/standard/organ's/iec.htm)
IEEE: Institute of Electrical and Electronics Engineers (http://www.dedicated-systems.com/encyc/techno/asso/standard/organ's/ieee.htm)
ISO: International Organization for Standardization (http://www.dedicated-systems.com/encyc/techno/asso/standard/organ's/iso.htm)
OMG: Object Management Group (http://www.dedicated-systems.com/encyc/techno/asso/standard/organ's/omg.htm)
OSF: Open Software Foundation (http://www.dedicated-systems.com/encyc/techno/asso/standard/organ's/osf.htm)
X/Open (http://www.dedicated-systems.com/encyc/techno/asso/standard/organ's/xopen.htm)

------------------------------
International User and Manufacturer Groups?

List from Dedicated Systems Encyclopaedia: http://www.dedicated-systems.com/encyc/techno/asso/user/user.htm

VITA VMEbus International Trade Association (http://www.vita.com)
Mezzanines International (ex-GRoupIPC) Association promoting Mezzanines Solutions (http://www.mezzanines.org)
PICMG Association promoting the Compact PCI bus (http://www.picmg.com)
Profibus (http://www.profibus.com)

------------------------------
RTOS Market Study (Mainly Japan Market)

Because we could received only a few responses from outside Japan in regret, only the responses from Japan have been tabulated and analyzed. You can access the result from the following URL:

http://tron.um.u-tokyo.ac.jp/TRON/ITRON/survey97/result-e.html

Other Market studies are available (not for free) from some companies. One of them is http://www.dedicated-systems.com

======================================================

VI- RESEARCH AND FREE PRODUCTS

------------------------------
Which Research Institute and Universities are involved in the Real-Time field?

Here is a link to a page describing activities of Universities and Research Institutes:
http://www.dedicated-systems.com/encyc/techno/research/research.htm

This list includes the following Universities and Research Institutes:

Carnegie Mellon University, Pittsburgh, USA
Computer Science Department at Boston University
Cornell University, USA
DIRECT-Distributed Real-Time Control of the Research Division of Responsive System, GMD,
National Research Center for Computer Science in Germany.
DIstributed and Real-Time systems group at University of North Carolina, Chapel Hill, USA
Florida State University, Florida, USA
Information Systems Engineering at University of Western Australia
Kansas State University, USA
Real-Time and Distributed Systems Group at Carleton University in Ottawa, Canada
Real-Time Systems Group at University of Pennsylvania, USA
Real-Time Systems Laboratory (RTSL) at University of Illinois, Urbana Champaign, USA
Real-Time Systems Research Group at University of York, England
Tenet Group at University of California, Berkeley, USA
The Centre for Autonomous, Real-Time Systems (CARTS) at University of Massachusetts, Amherst
The Experimental Real Time Group at Uppsala University, Sweden
The Real-Time Systems research group at University of Texas, Austin, USA
University of Maryland, USA
University of Michigan, Ann Arbor, USA
University of Pittsburgh, USA
University of Virginia, USA
VTT Electronics
your research group could also be added to this list.

List of links to research centers http://www.cs.umd.edu/~fwmiller/etc/realtime/research.html

RT-Linux at http://www.rtlinux.org
your free product could also be added to this list.

------------------------------
Free Real-Time Product lists

Here is a collection of links towards free products http://www.eg3.com/realxrto.htm

======================================================

VII- CONTRIBUTIONS AND FAQ LOCATION

------------------------------
Where can I get the current copy of the FAQs?

The FAQs are posted every 4 weeks to comp.realtime, comp.answers, and news.answers.
It is available in html format at :
http://www.dedicated-systems.com/encyc/publications/faq/rtfaq.htm
http://www.groupipc.com/rtfaq.htm

http://www.faqs.org/faqs/realtime-computing/faq/
http://www.cis.ohio-state.edu/hypertext/faq/usenet/realtime-computing/top.html

They are also available for anonymous FTP on rtfm.mit.edu in pub/usenet/comp.realtime:
ftp://rtfm.mit.edu/pub/usenet/comp.realtime/
Comp.realtime:_A_list_of_real-time_operating_systems_and_tools_(LONG)
Comp.realtime:_Frequently_Asked_Questions_(FAQs)
Comp.realtime:_Welcome_to_comp.realtime

For those without direct FTP access, there is also a mail-server. Address a message to mail-server@rtfm.mit.edu; leave the subject blank and include in the body: send help. It will return the instructions for proper use.

------------------------------
Contributions to comp.realtime FAQs.

The following net.folks, among others, have contributed to this posting:

Martin Timmerman <mailto:mtimmerman@dedicated-systems.com>
Luc Perneel <mailto:l.perneel@dedicated-systems.com>
Christian Ebner <mailto:ebner@vmars.tuwien.ac.at>
Thomas M. Breuel <mailto:tmb@best.com >
Tim Chambers <mailto:tbc@col.hp.com>
Chuck Cox <mailto:chuck@synchro.com>
Peter Desnoyers <mailto:pjd@midnight.com>
Kevin Driscoll <mailto:driscoll@src.honeywell.com>
Kurt Fuchs <mailto:fs_fuchs@rcsw56.rcvie.co.at>
Milt Fulghum <mailto:fulghum@vss.fsi.com>
Donald Gillies <mailto:gillies@ee.ubc.ca>
Dan Hildebrand <mailto:danh@qnx.com>
Jean-Christophe Monfret <mailto:jeanchristophe.monfret@barco.com>
Marcelo C Mourier <mailto:mmourier@atmsys.com>
David L. Oseas <mailto:doseas@americasttv.com>
Alan F. Perry <mailto:alanp@eng.sun.com>
David B. Stewart <mailto:dstewart@cmu.edu>
John Theus <mailto:john@theus.rain.com>
Alexander Vrchoticky <mailto:alex@vmars.tuwien.ac.at>
Christopher Vickery
Lee Brown
A. Lester Buck
David Hansen
Russ Hersch
Rob Lesieur
Dave Lunger

Special thanks to Jean-Christophe Monfret who maintained and updated the FAQ during the last years.

Thanks to Mark Linimon who did also maintain the FAQ.

======================================================

Welcome reactions, additions, and corrections to this posting via email at l.uhres@dedicated-systems.com

FAQ

What's new in the FAQ?

Table of contents

I- INTRODUCTION

II- DEFINITIONS

III- PUBLICATIONS COVERING REAL-TIME TOPICS

IV- POLEMIC TOPICS

V- MARKET

VI- RESEARCH AND FREE PRODUCTS

VII- CONTRIBUTIONS AND FAQ LOCATION

I- INTRODUCTION

------------------------------ What is the purpose of this FAQ ?

------------------------------ What is the charter of comp.realtime?

------------------------------ Where should I ask questions about real-time systems?

------------------------------ What is considered good net.etiquette on comp.realtime?

II- DEFINITIONS

------------------------------ What exactly is meant by real-time?

------------------------------ What is POSIX 1003.1b (formerly 1003.4)? Where is it available?

------------------------------ What makes an OS a RTOS?

------------------------------ What is a good RTOS?

------------------------------ What is RMA?

III- PUBLICATIONS COVERING REAL-TIME TOPICS

------------------------- Here are some references to the theory and practice

------------------------- Real-Time and Embedded Systems related magazines

------------------------------ What other net.resources are available on real-time systems?

------------------------------ Which Web Sites give information about real-time?

IV- POLEMIC TOPICS

------------------------------ Is Windows NT (or windows 95, or even Windows CE now) a Real-Time Operating System?

------------------------------ Is Windows CE 2.0 a real-time operating system?

------------------------------ Which methodology should I use to design a Real-Time System?

------------------------------ Which programming language should I use to develop a Real-Time System?

------------------------------ What kind of processor should I use for my Real-Time System ?

------------------------------ What kind of bus should I use for my Real-Time System ?

------------------------------ What Mezzanine technology should I use ?

------------------------------ What real-time network should I use for my real-time system?

------------------------------ Dedicated Systems and year 2000: what are the risks?

V- MARKET

------------------------------ Where can I find information related to real-time products?

------------------------------ Where can I find information about real-time Conferences, Workshops and Tradeshows?

------------------------------ International organization for standards?

------------------------------ International User and Manufacturer Groups?

------------------------------ RTOS Market Study (Mainly Japan Market)

VI- RESEARCH AND FREE PRODUCTS

------------------------------ Which Research Institute and Universities are involved in the Real-Time field?

------------------------------ Free Real-Time Product lists

VII- CONTRIBUTIONS AND FAQ LOCATION

------------------------------ Where can I get the current copy of the FAQs?

------------------------------ Contributions to comp.realtime FAQs.

------------------------------
What is the purpose of this FAQ ?

------------------------------
What is the charter of comp.realtime?

------------------------------
Where should I ask questions about real-time systems?

------------------------------
What is considered good net.etiquette on comp.realtime?

------------------------------
What exactly is meant by real-time?

------------------------------
What is POSIX 1003.1b (formerly 1003.4)? Where is it available?

------------------------------
What makes an OS a RTOS?

------------------------------
What is a good RTOS?

------------------------------
What is RMA?

-------------------------
Here are some references to the theory and practice

-------------------------
Real-Time and Embedded Systems related magazines

------------------------------
What other net.resources are available on real-time systems?

------------------------------
Which Web Sites give information about real-time?

------------------------------
Is Windows NT (or windows 95, or even Windows CE now) a Real-Time Operating System?

------------------------------
Is Windows CE 2.0 a real-time operating system?

------------------------------
Which methodology should I use to design a Real-Time System?

------------------------------
Which programming language should I use to develop a Real-Time System?

------------------------------
What kind of processor should I use for my Real-Time System ?

------------------------------
What kind of bus should I use for my Real-Time System ?

------------------------------
What Mezzanine technology should I use ?

------------------------------
What real-time network should I use for my real-time system?

------------------------------
Dedicated Systems and year 2000: what are the risks?

------------------------------
Where can I find information related to real-time products?

------------------------------
Where can I find information about real-time Conferences, Workshops and Tradeshows?

------------------------------
International organization for standards?

------------------------------
International User and Manufacturer Groups?

------------------------------
RTOS Market Study (Mainly Japan Market)

------------------------------
Which Research Institute and Universities are involved in the Real-Time field?

------------------------------
Free Real-Time Product lists

------------------------------
Where can I get the current copy of the FAQs?

------------------------------
Contributions to comp.realtime FAQs.