Towards A Software Museum - Institut für Informatik
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1. not only would the old system fail to meet passengers demands for fast response it would actually be infeasible In short Lufthansa would not be able to maintain its flight operations scheduled air traffic would be impossible In order to solve this problem Lufthansa introduced an ERS in the late 1960s and early 1970s With the ERS reservations are processed in a central database This database is integrated into a worldwide network which can now be accessed not only by Lufthansa employees but by everyone using the Internet based World Wide Web Moreover it is connected to a network of reservation systems shared with many other airlines the SITA network so that their flights can be booked directly as well The exhibition intends to present this example of application software by contrasting the former manual procedure with today s electronic reservation system Of course the central hall cannot be rebuilt in its original size but it 12 Ernst Denert amp Klaus Peter L hr could be presented in the form of a diorama The main aim is to show the former tools as faithfully as possible Visitors will then be able to see how costly slow and error prone the manual procedure was The diorama will be contrasted with the fundamental workings of an ERS This will allow important software aspects to be demonstrated for instance e What a complex application is and the fact that an ERS is even linked to other applications including the t2. complexity of the task of establishing a software exhibition obviously requires a sizeable team of computer scientists science historians and committed custodians Such a team does not yet exist One object of this paper and not the least is to get people to help with this undertaking Success is not guaranteed as the subject certainly resists straightforward treatment But this very fact makes it such an exciting endeavour Acknowledgements We enjoyed our discussions with Friedrich Bauer who helped shape many of the ideas presented here His enthusiasm for the vision of a software exhibition confirmed our hope that the idea might not be too bizarre after all Stefan Freyer contributed to the file access example by practically exploring the required animation techniques in his Master s thesis Thanks also to Christian Zick for providing the picture of the music machine 14 Ernst Denert amp Klaus Peter Lohr Appendix Design of a Simple Plotter 1 There are 8 registers 4 integer counters and 4 Boolean switches counters Cl x position of pen C2 y position of pen C3 C4 switches S1 pen up down S2 colour black red S3 S4 2 There are 6 elementary commands inc increments given counter s by 1 dec decrements given counter s by 1 clr sets given counters to 0 init resets given counters to their initial values on turns given switches on off turns given switches off 3 There are 8 control commands if evaluate
3. installed software would simulate a simplified file management system and visualize its operation down to the handling of file descriptors block buffers and disk I O 3 Famous operating systems are presented say THE Multics Unix OS 360 together with details of the people and organizations involved prominent features of the systems are highlighted Connections to the visualized functionalities just mentioned are established What kind of media support would be most helpful here remains to be seen We aim not only to visualize software but also to make it tangible This would be extremely helpful in the software basics section mentioned earlier for explaining to children and others what software is all about 4 Ernst Denert amp Klaus Peter Lohr Software Exhibits Three Examples In order to make the software museum concept more concrete we will pick three examples of what might be called a software exhibit the first is concerned with the very notion of software the second is a visualization example and the third is a real world system We hope that this eclectic approach will help the reader to understand our intentions better Coming to Grips with Software T hear and I forget I see and I remember I do and I understand Chinese proverb Software is immaterial it consists of programs which are plans for sequences of actions to be executed automatically by a computer A definition like this d
4. point in time the values of the counters and switches are shown in the displays The elementary commands can be executed by hand by pushing the appropriate buttons Initial values for a program can be entered in this way 7 The above design allows for many variations and may not yet be optimal for our purposes Some experimental programming should lead to a good compromise between expressive power complexity and ease of use the latter certainly being the overriding concern Tt is open to debate whether or not the machine presented here is a stored program computer In any case the program store is different from the data store 16 Ernst Denert amp Klaus Peter Lohr
5. program that simulates a flat file system using conventional syntax There are four standard windows There are a few additional statements for assignments conditionals and loops Stefan Freyer Visualisierung von Dateisystem Mechanismen Diplomarbeit FB Mathematik und Informatik Freie Universitat Berlin April 2000 Towards A Software Museum Challenges and Opportunities 9 st Frio Peet Proz Pras ppenditle data length ftelete rename aohia porition ize uzo Ma streams file descriptors variables block buffers al PASM Basic wa soma genen van puwa ga tone frau RER teas Fig 3 Startup view of windows for File Access visualization e Control allows users to step through their program in different sized steps forwards and backwards at different speeds To avoid information overload ticks indicate the files up to four whose relevant data are shown in the visualization window e Commands is the interface window it contains the file system interface The user can choose commands insert actual parameters and place the resulting statements in the user code window e User code is the program window it contains the program to be executed e Output displays any output produced by the printIn command e Visualization is the data window it contains the visualized data of the file system As the program is being executed the data flow is animated text and numbers fly across th
6. Towards A Software Museum Challenges and Opportunities Ernst Denert Klaus Peter Lohr sd amp m AG Institut fiir Informatik Thomas Dehler Str 27 Freie Universitat Berlin 81737 Miinchen amp Takustr 9 E mail denert sdm de 14195 Berlin E mail lohr inf fu berlin de Introduction Although the history of automatic computation is quite young the rapid development of electronic computers has awoken the interest of the historians who fear important facts about early computing might get lost The most visible result of the combined efforts of historians and computer veterans are computer museums which have been founded in several countries and have become quite popular Computer museums belong to the category of science and engineering museums Their exhibits tend to do more than just present some fancy machinery they try to educate visitors i e convey an understanding of how technical artifacts work and how they were perfected over generations of engineers A prominent example of this kind of museum is the Deutsches Museum in Munich one of the world s most comprehensive science and engineering museums Presenting computers in a museum has been done very successfully in the past but has focused on only one part of the computing business actually a small part hardware The really important part and one of the key technologies at the turn of the century and for a long time to come is software So why are software systems not prese
7. are and how have they solved the problems By way of an example let us consider database systems 1 Problems The ACID properties have to be guaranteed and efficient queries must be supported for large amounts of data 2 Approaches Some typical techniques are indexing locking logging etc and we have seen more refined manifestations of these techniques in the evolution from indexed sequential files to relational to object oriented database systems 3 Examples We may want to present say VSAM IMS and Oracle as prominent representatives We will revisit database systems in Section 3 3 below gt ACID is the acronym for atomicity consistency isolation durability Towards A Software Museum Challenges and Opportunities 3 There is one additional theme a software exhibition has to cover software engineering the very craft of building software systems Representing the history of software engineering adequately with its victories and defeats may be impossible to achieve in a museum We must admit that we have not given it much thought so far Arrangement As a museum defines itself through its exhibits we have to answer the question What is a software exhibit The simplistic answer is 1 A condensed educational documentation of the system is presented on wall charts including a requirements document a user s manual a design sketch and some code fragments 2 The program is actually installed on a compu
8. e the language is given But to illustrate typical techniques that occur in many systems we should use an extremely readable albeit formal language designed for the very purpose of explanation An in depth discussion of language issues is beyond the scope of this paper the reader will get a glimpse of a typical language in the example in the next section An Example Operating System Support for File Access We tested our ideas using an example from the domain of operating systems the exhibit File Access should convey to the visitor an understanding of an important piece of systems software A file system can be viewed as an abstract data object exporting operations such as open read etc The functionality of these operations is quite straightforward but the implementation is not Hands on experience of the exhibit File Access would start by opening a window that presents the interface shown in Fig 2 A kind of mixfix syntax is used here For example READ FROM RETURN is an operation name the parameters are interspersed with parts of this name FROM merely enhancing readability and RETURN having the obvious special meaning it separates by value parameters from by result parameters Parameter types are given below the parameter names Possible exceptions are not indicated in the specification so as to avoid information overload They can of course occur at runtime example 8 Ernst Denert amp Klaus Peter Lohr reading from a clos
9. e screen and arrows are drawn and redrawn Fig 4 shows the screen after the program shown in the User code window has been executed 10 Ernst Denert amp Klaus Peter L hr Eaa E a Froo Pear Fro Fros jin opendfilename ciosem road te variabieJengm rile Me data engh lappendifie datalengr idelete filensene sookiiie cosition o3 opanCropom heme Towards Jappenditie a Sofware Museum maagtmea te 1 block buffers isin YASS Sonea nee ACn Sven cUe Comm force soma Fig 4 Snapshot of File Access visualization Towards a Toolkit for Program Animation The exhibit File Access is a one of a kind hand crafted item Now the museum will certainly display a multitude of exhibits from different kinds of both systems and applications software We may want to provide insight into many of these programs so there is an obvious need for tools that support the animated visualization of programs Several such tools exist but program animation is still an active research subject We do not know of any system that would support the animation sketched above Existing systems tend to address the design and analysis of large systems they are meant to support the expert and try to abstract from details The level of detail we need is found e g in the VCC system for C program animation John T Stasko John Domingue Marc H Brown Blaine A Price eds Software Visualization T
10. ed stream aborting the execution and producing an error message INTERFACE FileAccess Allows reading writing from to text files A text file is identified using a file name Opening a file creates a stream through which characters can be read written sequentially OPEN filename RETURN streamhandle Text Pointer to Stream Creates stream between program and file CLOSE streamhandle Pointer to Stream Discards a stream READ howmany FROM streamhandle RETURN data Number Pointer to Stream Text Reads specified number of characters from stream to data WRITE howmany data TO streamhandle Number Text Pointer to Stream Writes specified number of characters from data to stream SEEK position IN streamhandle Number Pointer to Stream Adjust stream to continue reading writing at given position in file Fig 2 File system specification Clicking on an operation is done in either of two modes interactive or non interactive In the former case the operation is executed immediately In the latter the operation is appended to the program under construction In both cases the user is prompted to give the actual parameters first the operation call is then added to the program window Variables are introduced on the fly The relevant data structures for File Access are streams file descriptors and block buffers They can be visualized as shown in Fig 3 this is a screen shot from a visualization
11. es will be addressed Basic Principles for a Software Exhibition Goals The raison d tre of any museum whatever its theme is to save characteristic items from oblivion to preserve masterpieces to present them as exhibits and thus to contribute to an understanding of the history of a specific field The educational efforts range from almost non existent as in art galleries to rather elaborate as in ethnological museums These three facets exhibits history education what do they mean for a software exhibition While everybody knows what a painting is few people know what a computer program is So the educational aspect will be essential for any software exhibition Only if people understand general software principles concepts and techniques will they be able to appreciate specific software exhibits and understand historical developments This implies that first and foremost there has to be a software basics section which explains the basic notions of algorithm program software and how all this relates to hardware Then different kinds of software have to be covered both systems software operating systems compilers and application software For each kind it is necessary to answer three questions 1 What are the problems to be solved 2 What are the typical approaches to solving these problems and how have these approaches evolved over time 3 What are the prominent historical examples of that kind of softw
12. he MIT Press 1998 2 John J Shilling John T Stasko Using animation to design object oriented systems Object Oriented Systems 1 1994 5 19 Wim De Pauw et al Jinsight Visualizing the Execution of Java Programs http www research ibm com jinsight Ricardo A Baeza Yates Gast n Quezada Gast n Valmadre Visual debugging and automatic animation of C programs In P Eades K Zhang eds Software Visualization World Scientific 1996 46 58 Towards A Software Museum Challenges and Opportunities 11 VCC also suggests the ideal solution a tool that would automatically generate an animated exhibit given a certain program module While aiming at complete automation seems unrealistic there is a fair chance of finding a semi automatic solution The static visualization part will not be too hard But attractive animation will require some non trivial manual intervention Semi automatic generation of animated programs should certainly not be confined to one specific programming language We should have a generic tool that comes in three parts a front end that would compile the program into an intermediate language version a middle part that would generate a vanilla visualization and animation and a back end that would allow the curator to produce a polished version interactively Lufthansa s Reservation System As a third example of an exhibit we would like to present an application that is very vivid and interesting fro
13. icketing and the check in systems e How worldwide access to a central application is made possible i e how Lufthansa s employees travel agents and ultimately everybody can book flights through the ERS e How such a system must be operated so that it works reliably around the clock e What a database accomplishes being the core component of the ERS e How data access is synchronized so that a seat on a flight is actually given to only one passenger It would be a special attraction if the visitors were able to access the real Lufthansa system from within the museum This might even be realized taking stock of Lufthansa s old booking procedure has been the subject of a recent Master s thesis Lufthansa s archives were searched and witnesses of that period were interviewed Technical and Organizational Issues As mentioned earlier a software exhibition cannot possibly be conceived independently of a computer exhibition Explaining what a program is will invariably be tied up with explaining the functionality of computer hardware It remains to be explored how the hardware presented in Section 3 1 can be related to the real hardware found in the computer exhibition It is obvious that extensive computer support hardware and meta software will be indispensable for the documentation visualization and maintenance of software exhibits Given the shortage of resources museums commonly face free software and cheap hardware
14. layed in a separate program window The data structures involved are visualized in a data window using boxes tables and arrows representing pointers With object oriented designs encapsulated objects would first appear as blank boxes clicking on them would reveal their inner structure Both code and data should be animated As the user steps through the code the current statement is highlighted A chosen step can be the evaluation of a condition an elementary statement assignment or operation call or a complete loop For expanded operation calls the system will step through the individual statements of the operations The data structures are animated by changing box contents and arrows and by visualizing how data moves i e is copied from one box to the other Depending on the specific exhibit data flow between the program and the environment could be visualized as well Remember that to specify or understand the behaviour of an abstract data object we usually refer to an abstract model that is independent of the representation chosen by the implementer Note that what is shown in the data window cannot clearly be classified as either a model or representation it is more concrete than a model but it may be more abstract than data declarations in program code Speaking of program code which programming language should be used here The answer depends on the nature of the exhibit Of course if we have a piece of original softwar
15. m a user point of view and very fruitful from a technical point of view seat reservation It is only for the last 30 years that Lufthansa LH flights have been booked using an electronic reservation system ERS Before that booking was done in a huge hall at Frankfurt Airport with boards mounted to the walls showing the individual flights e g LH 400 Frankfurt New York with a space for each day on which they operated for a period of several months in advance Employees were in charge of processing all reservation requests which they received via telephone via telex or in writing By attaching a pincard containing the passenger s reservation data to the board an employee visualized the reservation and ensured that no seat was assigned twice The hall was so big that binoculars were used for checking the boards for vacant seats on a specific flight In addition to this central reservation hall there were offices in about six cities where in a similar way a certain contingent of flights could be booked which were assigned to these offices by the headquarters The city offices communicated with the Frankfurt headquarters via telephone or telex Thus the essential tools of this booking procedure were boards notes record cards and card file boxes telephone and telex Most importantly a huge number of employees were needed in spite of the relatively small volume of air traffic Today Lufthansa s bookings could not be handled this way
16. must be used wherever possible A network of PCs running Linux would certainly represent an adequate infrastructure Large format screens and electronic whiteboards might be desirable for some exhibits and may become less expensive in the future As regards hardware items we hope to find industrial sponsors who are interested in supporting an undertaking as foolhardy as a software exhibition The main effort will of course be procuring adapting and developing the meta software mentioned above This would strain the resources of even the wealthiest museum We expect however to find enough enthusiasts in the computer science community to help develop software and prepare specific exhibits S J A Haidn Hardware und Software Computertechnik im Einsatz in den 1960er und 70er Jahren Magisterarbeit Historisches Seminar Ludwig Maximilians Universit t M nchen M rz 2000 Towards A Software Museum Challenges and Opportunities 13 Still a software exhibition may experience funding problems in the long run Unlike other exhibitions of say cars machines household items etc maintenance does not just involve dusting or repairing or procuring of an item once in a while Even if the exhibits do not change the infrastructure has to be maintained and will require permanent renewal merely because of the fast pace of hardware development In this respect the museum s situation is no different from that of any other computer user The
17. nds on experience of different devices is considered essential for a thorough grasp and proper understanding of the notion of program Grasping Graphics Computers have memories and peripheral devices Realistic computer programs have branches and loops and they cause effects in memory and in their environment To bridge the gap between computers and these simple music boxes we suggest a simple electromechanical plotting device e The plotter is controlled by a program on a punched tape which is almost identical in appearance e g has the same number of lines to the music tape Each column contains one command rather than a chord Unlike the music gt This part of the exhibition would of course be organized jointly with the hardware division Remember that early plotters were operated off line in this way controlled by a punched tape that had been produced by a computer 6 Ernst Denert amp Klaus Peter Lohr tape this tape should remain fixed a control unit moves along the tape in both directions e There are a few registers and both integer counters and Boolean switches The values of two of these registers correspond to the coordinates of the pen e The content of the registers can also be observed on displays on a panel and it is possible to modify the content from the panel Thus the plotter can be operated both manually and by means of a program A fairly limited set of commands suffices for quite a variety of
18. nted in computer museums The answer seems obvious software is immaterial and invisible so there is nothing to be presented Or so we think The authors set out to question this dogma We are convinced that it is possible to make software an exciting exhibit in a museum a software museum Our opinion was confirmed when we visited the newly created Mathematical Cabinet in the Deutsches Museum Here Friedrich L Bauer has managed to present the most elusive subject imaginable mathematics to the visitors or at least to wet their mathematical appetite by allowing a playful encounter with strange objects It occurred to us that software too could be made visible tangible and playful and thus reach people Friedrich L Bauer Mathematisches Kabinett Deutsches Museum Miinchen 1999 2 Ernst Denert amp Klaus Peter Lohr The immaterial nature of software is not the only problem a software exhibition has to solve The sheer mass of past and existing software the disappearance of the supporting hardware platforms and the virtually unlimited range of applications make it hard to see how a software exhibition could ever cover its subject in an adequate way In exploring the issues we will first develop a few principles that we consider crucial to any successful attempt to create a software exhibition Then to make things more concrete we will present three case studies of exhibits Finally several technical and organizational issu
19. oes not really make sense to those who have never written a program A software museum has to reach out to ordinary people not only to the initiated so visitors should be led to an understanding of what these immaterial programs are all about Material Programs for Playing Music If a program is an executable plan then how does a programmer plan the execution steps and how can a machine execute those steps In our view grasping the essentials of programming should be facilitated by allowing visitors to physically grasp programs as well as the executing machines and the embedding environments the machines interact with Visitors should even be able to build and run simple programs by themselves A program is of course just a piece of information independent of the physical representation that may be used for communication among humans e g handwriting on paper or machines e g bits in electronic memory Still there are program representations for mechanical or electromechanical devices that are much more material than either handwriting or electronic bits This is how we want the term material programs to be understood consider e g a music box that is driven by a program represented as pins on a revolving cylinder touching the tuned teeth of a metal comb or take the classical mechanical loom where the program is punched into a cardboard tape or take a hand organ Actually there is a programmable toy that resemble
20. programs A sample design is given in the appendix The number of commands does not exceed the number of stave lines we used for the music box Understanding Programs Through Visualization Software is invisible and unvisualizable No matter what specific exhibits are chosen for a software museum we will encounter all kinds of algorithms concrete programs fragments of systems and complete systems Exhibiting software items in any representation will only make sense if visitors are enabled to understand their important properties both static structural and dynamic This is not possible by mere code inspection In recent years however software visualization has made significant advances in terms of program comprehension and debugging There will be more progress in this direction and we believe that visualization can play a pivotal role in a software museum After all any museum relies heavily on visual exhibits Program Visualization and Animation Programs want to be executed It is obvious that not even the most ingenious visualization of a program as a static item could allow us to dispense with an animated execution Program animation comes in different forms from simple highlighting of statements to elaborate visualization of the dynamic effects on data peripherals and networks We will use a mixture of different techniques as described below Both code and data animation techniques will be applied In many ca
21. s a music box punched holes on different stave lines of a cardboard tape represent notes The tape is fed into a mechanical device similar to a music box and the program is executed by turning a crank see Fig 1 Prepare your own tape and play your favourite tune Towards A Software Museum Challenges and Opportunities 5 Fig 1 Primitive music box using cardboard tape Music is in fact a prime example of the abstract concept of program Admittedly these programs are rather simple as there is no memory no parametrization and no branching though simple repetitions do occur But the steps to be executed have a well defined meaning and a piece can be executed by both humans and automatic devices We plan to use music as the introductory analogue to computer programs Visitors especially children are encouraged to punch their favourite songs or those that are presented by the museum into tapes and feed them into music boxes There will be both mechanical and electromechanical boxes and there will be traditional boxes with fixed or removable cylinders Barrel organs should be on display as well It should be emphasized that the particular designs of the different devices are less important than the fact that the differences between them do not really matter Greensleeves is independent of how its notes are represented and played and you can have an automaton play it if this satisfies your demands The ha
22. s logical OR of given switches if true proceeds with next command else continues after next matching cont or retn whatever is next ifno evaluates logical OR of given switches if false proceeds with next command else continues after next matching cont or retn whatever is next ifeq checks for equality of given counters then proceeds like if ifls checks Ci lt Ck i lt k for given counters then proceeds like if if 0 checks given counters for 0 then proceeds like if cont continues with next command i e no op retn returns to last if command exit continues after next retn or else stops program 4 Note that the if command and its variants works both as the traditional if and as a while depending on whether the matching command is cont or retn Also note that the control commands are designed as nearly as possible to control syntax Executing a control command requires scanning the program the Towards A Software Museum Challenges and Opportunities 15 benefit is that jump instructions and the ensuing notion of label or address are avoided The nesting of control structures however is very limited 5 The program tape has 18 lines one line for each command and four lines to specify the counter or switch numbers 1 4 The possible commands could of course be encoded in a more compact fashion but only at the price of worsening the look and feel 6 A possible layout of the panel is shown in Fig 5 At any
23. ses a software exhibit may lend itself to being modelled as an abstract data object such as a Modula module The object has a procedural interface comprising several operations The available operations are presented in u Instead of a punched tape a metal rod with adjustable pins would be preferable where each pin can be in one of the two positions Up Down This would make it extremely easy to build and modify a program by hand without the hassle of punching or sealing holes Frederick P Brooks No silver bullet essence and accidents of software engineering IEEE Computer 20 4 April 1987 10 19 John T Stasko John Domingue Marc H Brown Blaine A Price eds Software Visualization The MIT Press 1998 Note that we talk about program animation here Algorithm animation works on a higher level of abstraction possibly not even showing code but only an animated model a x oo Towards A Software Museum Challenges and Opportunities 7 an interface window on the screen Visitors can trigger operations interactively in arbitrary sequence Or they can construct a program in advance using the given operations and then step through the program The chosen operations are composed in a program window in either case Browsing through the software behind the interface is made possible by hypertext functionality a call statement can be expanded recursively by clicking on it causing the code of the operation to be disp
24. ter it can be run perhaps even be used interactively by visitors It is obvious that this approach would never work The possibilities for running legacy code are very limited Emulation is rarely supported and exact simulation of obsolete hardware including peripherals is an unrealistic task Moreover even if the real system were running would this be helpful to the visitor who cannot check the code anyway And even if we could check it who would go through the documentation and try to understand what is going on Not even the typical computer scientist This is not to say that exhibits should not use computers Actually the museum should rely heavily on computers not for running original software masterpieces but for running special educational software mainly for documentation and visualization purposes Take the subject of operating systems for instance A section on operating systems would be arranged as follows 1 An introduction to the purpose of operating systems is given possibly drawing on the visitor s knowledge as a computer user and the typical jobs done by an operating system are sketched using a wall chart In addition online access to two or three contemporary operating systems is given 2 Typical operating system areas e g file management are chosen for an in depth study They are first explained again using a wall chart and then presented on a computer to allow an interactive playful encounter The
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