Design considerations for an educational time-sharing system

Transcription

1 Design consideations fo an educational time-shaing system by ROBERT F. HARGRAVES, JR. Datmouth College Hanove, New Hampshie and ANDREWG. STEPHENSON Time-Shae Copoation Hanove, New Hampshie INTRODUCTION In 1963, Datmouth College developed a time-shaing system fo the GE-235 and Datanet-30 computes. This was a geneal-pupose multilingual computing system in which the language BASIC achieved geat populaity. This system had a geat impact on the Datmouth campus, and its successes led to its adoption by G E as a commeical time-shaing system. But at Datmouth, the success of this system led to its own demise; the demand fo the use of the compute by students, faculty membes, and a substantial numbe of outside uses meant that the system always opeated nea its peak capacity-just unde 40 uses. Thus this system which initially was judged to povide a convenient and poweful computing sevice gew (in the view of those gown accustomed) to have cetain unsatisfactoy chaacteistics. Nonetheless, the system continued to povide good sevice fo the functions fo which it was specifically tailoedpoviding good edit-compile-and-go sevice fo shot BASIC pogams. In fact, it was this sevice which dew moe and moe people to use the compute. Some of the less satisfactoy points of the system could not easily be emedied, howeve. Random access stoage fo etention of uses' saved pogams was in geat demand. In ode to keep a cuent pool of available disc stoage, it became necessay to puge pogams fom seconday stoage as they lapsed into disuse. By the time the 265 system (235 + D-30) left the campus, the puge peiod had dopped fom 30 days to 48 hous. (It was a fact that although puged pogams wee always witten out onto mag- netic tape, no one eve felt stongly enough about the loss of a pogam to wite the code necessay to etieve a puged pogam fom the tape.) In ode to potect thei pogams, uses fell to the countemeasue of conscientiously calling up all of the othewise unused pogams in thei catalogs evey day in ode to avoid having them tagged as unused and puged fom the stoage. Consequently the stoage situation wosened hoibly. Development of new computing languages was a difficult and hazadous task. Since the 235 hadwae had no potection facilities, it was necessay fo the executive system to tust the compiles not to ovewite stoage assigned to othe puposes. In spite of this, BASIC, AGO, and ISP systems wee available and easonably foolpoof (fom the point of view of the executive system). FORTRAN was neve tustwothy enough to enable the system at Datmouth to suvive the lack of index-ange checking, due to the capicious natue of students faced with an oppotunity to alte the executive. A watchful intepete fo the assembly language code, TSAP, made it possible to wite and debug some small suboutines, but the installation and testing of a compile was necessaily an on-line opeation on a dedicated machine. Pogams could not easonably be allowed to access data in seconday stoage. Only one use pogam could eside in the coe stoage of the 235 at one time, and the I/O wait time fo a athe slow disc meant that othe use pogams would be faced with additional delays. 657

2 658 Sping Joint Compute Confeence, 1969 Some uses felt a desie to incease the speed of computation. This was especially impotant fo such simulation and modeling studies as wee caied out by t.he enginees and business students in two of the pofessional schools at Datmouth. Many of the potential uses of the system wee unable to gain access to the system because all of the datasets and communications pots to the compute wee busy. It was hoped that the numbe of uses who could simultaneously gain sevice fom the system could be inceased. By a coopeativ(3 aangement with the Geneal Electic Copoation; Datmouth was able to eplace the old 265 system with a GE-635 system. A softwae dystem had to be designed fo this new machine which would satisfy the demands ceated by the olde 265 system. Design objectives In addition to coecting the defects of the old 265 system, it was especially impotant to be able to eplicate the good featues of that system-namely, the ease of use of the command language, the simplicity of modifying and editing pogams, and the caefee edit-compile-and-go sevice automatically povided fo the use. The activities of uses of the 265 system wee sampled ove a peiod of a few days in ode to detemine the fequency of activities such as etieving pogams fom seconday stoage, ceating pogams at the teletype, listing such pogams, executing use-ceated pogams, outputting to the teletype, waiting fo input fom the teletype, etc. These figues based upon the numbe of actual uses on the 265 system at the time that the measuements wee made wee extapolated to 200 simultaneous uses which wad the design taget fo the 635 system The following ough genealizations wee obtained: Evey 10 seconds a log-in activity would occu. Evey thee seconds thee would be an editcompile-and-go activity initiated by the use with a RUN command.. Evey one second thee would be a SAVE o OD command equiing updating a catalog esiding in the seconday stoage. A few snapshots of 265 usage esulted in the following appoximate beakdown of activities: 40 pecent of the uses wee building files by typing fom a teletype; 20 pecent of the uses wee unning pogams which wee oadblocked fo output to the teletype; 10 pecent of the uses wee unning pogams which wee competing fo cental pocesso time; 10 pecent of the uses wee simply listing files on the teletype; 10 pecent of the uses wee idle; and 10 pecent of the uses wee pefoming othe activities. These figues gave a vey Ol}gh idea of the type of computing activity whjch was to be expected on the 635 system. Poposed techniques of system implementation wee measued up against this imagined activity. To some extent, these activity pojections ae unealistic because uses expect a good deal moe sevice fom a machine such as the 635 on a pe use basis. Also, the activities attibuted to the use can in pat be attibuted to the chaacteistics of the 265 computing system instead; we do not eally have a measue of what the use would like to be doing. Also, the chaacte of use to which the system is put may change. If many uses wee to shift to highly inteactive pogams, the natue of the system load could change dastically. Nonetheless, it is impotant to have a quantitative pefomance goal in mind when designing a timeshaing system and to pefom gedenken expeiments giving a O-ode appoximation to the estimated pefomance unde the most fequent types of activity. In ou expeiences, the majoity of all design decisions wee easily esolved by compaing esults of such thought expeiments to peviously established goals. The moe difficult design poblems can be attacked by building simulation models and implementing these models with a compute pogam using a time-shaing system; the availability of a timeshaing system was a geat asset in handling the hade poblems of system design. This technique was vey useful in appoaching the coe stoage memoy management poblem, in designing an algoithm fo keeping tack of the available space in seconday stoage, and in designing a swap scheduling algoithm. Thus, although the completed system was designed to povide 200 uses with the same sevice as they eceived on the 265 system, the uses' appetites fo computing have gown to the point whee it would be vey difficult to find 200 who would be satisfied with the estained sot of sevice they peviously found so valuable. Vaious classes of potential uses of the computing system wee ecognized, and an attempt was made to pedict thei peculia equiements. It was deemed impotant that the computing system be able to povide education in the udiments of compute pogamming fo the majoity of undegaduates. The impotant function of humanizing the compute is accomplished at Datmouth by making the compute available in the undegaduate libeal ats cuiculum. 90 pecent of the undegaduates avail themselves of this oppo-

3 Design Consideations fo Educational Time-Shaing System 659 tunity. It is extemely impotant that a simple enough command language and pogamming language exist so that a student is able to accomplish something in his vey fist session at a teminal; in this way, he is encouaged to make futhe use of the compute. This is not to peclude advanced featues in a command language o pogamming language; the equiement is only that thee be a small subset of these languages which may be used by the student when he is leaning. Afte a student has leaned to use the compute, the computing system must emain a useful tool thoughout the student's educational expeience. Cuently, thee ae nealy 100 undegaduate couses given at Datmouth which make significant use of the compute. Applications in classics involve analysis of Geek and atin text; these analysis pogams ely on the existence of sting-pocessing featues in the pogamming language. Pesons involved in the analysis of expeimental data equie the pogamming language to be able to access files in seconday stoage in which this data is kept. The use of teaching pogams is a blossoming field at Datmouth, and this activity einfoces the equiement fo a libay in which the standad teaching pogams can be stoed and be easily accessible to the leaning student, who is moe inteested in Spanish than computing. This highly inteactive use budens the swapping mechanism. Undegaduates ae being intoduced to the methodology of analysis of sociological data which is amassed in seconday stoage. Paallel accesses to lage, shaed, but elatively static data bases ae equied. Thoughout these applications, the ease of use of the computing system should be maintained in ode to facilitate the intoduction of computing techniques into a boad spectum of couses. The compute should aid faculty and gaduate student eseach in the social sciences, the physical sciences, engineeing, humanities, and the pofessions. These eseach demands often tax the compute at two extemes. On the one hand, thee is the demand fo aw compute powe to handle such poblems as occu in solid-state physics whee eigenvalues must be sought fo a poposed Hamiltonian descibing the chaacteistics of a cystal. A :Monte Calo technique can be used in modeling a business activity. On the othe hand, at the gaduate level, both the sociological and business applications of the compute also can make use of lage data bases, and the softwae opeating system should povide the capability fo andomly accessing these data. A highly impotant equiement is that systems pogammes should be able to pefom thei maintenance and system development functions in paallel with nomal time-shaing activities. In ode to povide as much sevice as possible to a boad spectunl of uses, the time-shaing system should opeate in its nomal mode nealy aound the clock. Thee ae moe benefits to this than simply elieving systems pogammes fom the necessity fo keeping midnight hous. The computing system must be able to be developed moe fully duing its expected lifetime than is possible with only a few evening hous fo system development wok. A stagnant computing system dies; the system must be extendable in ode to take on new poblems. If not, the system development pesonnel become disinteested in the system, and it decays apidly if thee is no one left who can make even tivial changes in ode to cope with the eve-occuing, new ly-pesented, unfoeseeable cicumstances. This' chaacteistic may be unique to the Datmouth system because thee ae no full-time systems pogamming people to develop and maintain the com~ puting system. Undegaduates ae the pincipal souce of pogamming talent and ideas, and since to them this is moe a labo of love than of money, they do not make stong contibutions to pojects in which they have lost inteest. Moe specifically, this capability equies that editos, assembly pogams, and debugging pogams be made available to the systems pogammes. The ability to test machinelanguage code without endangeing the system is a must. Compiles and utility pogams must not enjoy any pivileged status, but must adhee to a standad set of systems inteface conventions so that they may be debugged and installed without heoic sugey on the executive system. Related to the question of system development is the necessity to be able to use softwae developed fo othe opeating systems fo the 635 so that unnecessay effot is not wasted in duplicating such lage and useful tools as maco assembly pogams. It should also be pointed out that the design of the time-shaing system was not budened by the necessity to do standad gade ecoding, class scheduling, payoll, and accounting-such tasks equie lage amounts of pape and cad handling. Of couse, these tasks ae necessay to the functioning of the college, but they ae accomplished economically by an aleady existing 1401 system opeating independently. Final design The hadwae povided by Geneal Electic was to be based upon a 635 pocesso. The 635 has a maste mode and a slave mode. An executive pogam unning in maste mode can contol all input-output facilities and supevise the use of such featues as the time egiste and the base addess egiste (BAR). Memoy

4 660 Sping Joint Compute Confeence, 1969 potection and addess elocation is pefomed on the 63 ) fo pogams unning in slave mode. The BAR eontains an uppe bound against which all slave mode efeences to memoy ae checked, and it contains an incement which is added to the addesses of all efeences to memoy. The existence of only one BAR means that slave pogams un in only addess space. The configuation of the essential pieces of the hadwae is shown in Figue 1. The dum povides a mechanism fo swapping slave mode jobs in and out of coe memoy. The GE Datanet-30 computes povide the inteface fom the 635 system to telephone company datasets. Each of these D-30 computes accepts data fom ove.50 teletypes on a bit-by-bit basis,and tansmits only complete lines of infomation to the I/O contolle. The discs each povide 16 X 10 6 ASCII chaactes of stoage which ae available to the softwae system and povide geneal utility stoage fo uses. The achitectue of the softwae system was influenced by thee things: The expeiences with the 265 system. The published concepts of the MUTICS system. A ealization of the limitations of the capabilities of a pat-time staff of Datmouth students and faculty membes. Fom the expeiences on the 265 system, a patten of use to be expected at Datmouth was pedicted. These expeiences had also familiaized students with the pinciples of a time-shaing system. The :MUTICS system povided examples of ways to genealize file stuctues so that a file system would be convenient to use and yet. would anticipate futue needs. 1 The pagmatics of getting an opeating time-shaing system Figue I-Hadwae configuation The D-30 computes act as communications contolles fo up to 200 teletype lines. The dum is used pimaily fo swapping unning on a 6::t\ using a pat-time staff and having only limited debugging time avai~able, empharized the need fo simplieity of design. An impotant featue of the time-shaing systen is the concept of a file. The system was designed to be file-oiented. All conununications and contol functions ae accomplished though the medium of the file constuct. The hadwae of the 635 is such that the sm!1llest convenient element fo a file is a 36-bit wod. In its most fundamental fom, a file consists of a contiguous sting of N wods esiding in an unspecified place with wods numbeed 0, 1,..., N-l. Pogams can be allowed to alte o fetch any wod in a file, accessing these wods diectly; the file appeas to be homogeneous to the use. Since the fixed ecod lengths of vaious andom access seconday stoage devices fo the 635 ae 40 wods fo the discs, 64 wods fo the dum, and 108 wods fo the RACE, it becomes clea that it is necessay to isolate uses' pogams fom such awkwad ecod lengths. This also leaves to the executive the flexibility to select files fo stoage on the dum, discs, o RACE unit depending upon the amount of stoage available on these devices, the histoy of past activity of the file, and the use's pediction of activity expected. Since all ecod boundaies have effectively been eased in the softwae, the poblems of the use-'witten sla.ve mode pogams have been geatly simplified in this time-shaing system. The effect of this design pinciple is that the executive system is made esponsible fo all peculiaities of andom access stoage devices. Fo example, the executive takes upon itself the esponsibility of handling ead-alte-ewite cycles when it is necessay to update wods which fom only pat of a physical ecod. Othe files in this system ae simple vaiants of this concept. Some files, such as magnetic tape files, ae natually seial and cannot be andomly accessed. A file containing a coe image of an executable 635 pogam can have a RUN conunand diected at it. This spawns a new job in the time-shaing system, and the file is then efeed to as a job file. The fundamental computational stuctue of this time-shaing opeating system is a job. A job consists of a 635 compute pogam opeating on data contained in one addess space along with the pogam. Illustated in Figue 2 ae a numbe of files though which the job conununicates. Each job in the system is an independently evolving computational pocess, and in pinciple, each can be consideed to be unning on its own independent compute. Jobs can only inteact though files; howeve, this does not necessaily equie the use of seconday stoage facilities. The job constuct has some of the flavo of the vitual

5 Design Consideations fo Educational Thlle-Shaing System 66i Figue 2-Job envionment A single job uns with no knowledge of the job which spawned it. It can only communicate to files. The fault vecto is an exception-handling mechanism machine techniques. Each job uns in a machine vey much like a 635 compute. Wods 0 to 31 of a job ae eseved as a fault vecto. Fo example, if a efeence to memoy is out of bounds, the addess of the next instuction to be executed is stoed in location 2, and contol is tansfeed to location 3. A basic pemise of this type of oganization- is that all unusual cicumstances should be communicated to the job itself athe than causing an abot o some othe activity out of the contol of the unning job. A unning job can initiate tansfeal of infomation to o fom a file by executing a coded maste mode enty (MME) instuction. Each such equest specifies the location of a tap to which contol will be tansfeed upon completion of the file opeation, inteupting the job at this point in ode to do so. This allows a job to supevise many file activities which ae taking place asynchonously. The impotant payoff of this type of oganization is that it allows the fundamental sevice modules of the time-shaing system which handle file activities fo many uses to be stuctued as independently unning jobs communicating via a multitude of files. Since all such modules have the unifom job stuctue, they can be debugged in time-shaing using the standad debugging techniques applicable to any job. The homogeneous stuctue of files eases the physical ecod boundaies in files fo pogams accessing them. Theefoe, the function of allocating ecods on seconday stoage is necessaily pefomed by the executive system. In many opeating systems, the esponsibility fo allocating stoage fo a file has been an iksome nuisance left to the pogamme. The pogamme is often foced to peallocate stoage fo a file befoe a pogam begins execution. A pogamme witing in a highe-level language may vey well have little idea of how much stoage should be allocated. If too little is allocated, the pogam may be pematuely teminated because no moe stoage will be allocated, o the pogam may un inefficiently because the allocated stoage has been incementuly geneated accoding to pevious specifications and fagmented into too many pieces to allow the pogam to un efficiently. If the pogamme oveestimates his stoage equiements (the natual eaction), the pogam may un popely, but valuable peallocated stoage space will emain unused. In this opeating system, stoage is allocated by the executive as it is needed in esponse to commands given by a use pogam to copy data into a file. The numbe of ecods allocated on a device at anyone time is always a powe of 2; sufficient contiguous stoage space is always allocated to at least satisfy the data being appended by the cuent copy command. If it late occus that additional allocations ae necessay, the ajlocato is esticted to at least double the amount of stoage aleady allocated fo a given file. The efficiency of use of allocated seconday stoage is aound 70 pecent. This was felt to exceed the efficiencies actuajly achieved by othe allocation schemes in pactice. The device addesses of the segments of a file which have been allocated incementally ae ecoded in a file contol block fo the file; this allows diect andom access to any wod in the file with only one seek. The linked segment technique is not used since it is not suitable fo andomly accessible files. Sincet he total length of the segments gows at least exponentially, the numbe of device addesses equied to descibe a file is small, and the device addess list equies little coe stoage space. Nonetheless, if a file is felt to have been fagmented excessively, it can be eallocated contiguously in seconday stoage by the simple expedient of copying it ove with one copy COIhmand. This allocation scheme means that the management of fee stoage can be handled by a buddy technique. 2 A communications file allows two jobs to inteact diectly without the use of seconday stoage. A communications file has one end in each of two jobs. It is the softwae analog of a channel-to-channel adapto. This stuctue allows job-to-job inteactions using the same poceduef:) as fo moe conventional files. The two ends ae labeled maste end and slave end. A job at the slave end of a communications file cannot easily distinguish this file fom a conventional file. Since a job at the maste end of a communications file can contol and monito all data tansmitted on that file, a maste end job can simulate a data file, theeby poviding a useful debugging aid and also poviding a convenient mechanism fo intefacing unning jobs to unexpected data stuctues.

6 662 Sping Joint Compute Confeence, 1969 Scatch files ae associated only \\ith a unning job and disappea upon temination of that job. Catalog files ae associated with a file stuctue which is pat of the time-shaing opeating system. The stuctue of cataloged files has been chosen to be a tee stuctue, so that algoithms which deal with this stuctue can be simply implemented. The file stuctue includes both files and catalogs. Files contain any infomation whatsoeve and may be ead o witten by use pogams. Catalogs contain passwods, access pemission, attachment counts, and device addesses of othe files o catalogs. Such sensitive data as device addesses, attachment counts, and coded dates ae all citical to the pope pefomance of the executive system, theefoe this infomation cannot be ead o alteed diectly by a use job using standad file calls, but only though special catalog calls upon the executive. This flexible tee stuctue can be constucted o modified by use pogams and was judged to be sufficient to satisfy pojected needs. Coss links wee not intoduced into this tee stuctue because of the maintenance poblems these links would intoduce in a dynamically changing file stuctue. An application of the tee stuctue of files and catalogs to timeshaing sevice is shown in Figue 3. To open a file o catalog, a supa catalog must peviously have been opened. Two distinct sets of accesses may be ganted by the file system depending upon whethe the job attempts to open the file with o without a passwod. Fo example, a job may be given ead and wite pemission on a file if it supplies the pope passwod and name, and only ead pemission if the job does not supply a passwod. This two-class potection scheme supplies most of the potection needs which aise in this opeating system. If a highe degee of potection i:::; equied, the catalog enty can specify that a tap pogam is to be un wheneve a job attempts to gain access to a paticula file. The esponsibility of poviding potection has been left up to the owne who must wite a tap pogam which decides what access to the file should be ganted. The amount of available coe stoage may not meet the total demand fo coe stoage fo all of the jobs which ae executing in the time-shaing opeating system at one time. In this opeating system, a unning job may occasionally be swapped out in its entil'ety fom coe onto a dum in ode to make oom fo othe jobs. The system multipogams those jobs which coexist in coe stoage; execution is altenated anong jobs as input/output opeations and swapping ae ovelapped with job execution. Since a unning job is also a file, the pimitive file opeation pefomed by the executive is a tansfe of data fom one file to anothe file. Theefoe peviously initiated file ope.j...j..j...j...j...j Figue 3-File stuctue This application of the hieachial tee stuctue ~how~ how files ae categoized at seveal levels ations can take place even if a job file has been swapped out and esides on the dum. The executive knows the instantaneous location of all job files, and if necessay it will povide buffes to continue the file tansfe opeation even if neithe of the files involved in the tansfe ae located in coe. This featue is especially impotant fo low-speed input/output devices such as the teletype. A snapshot of the job stuctue which exists in the time-shaing opeating system is illustated in Figue 4. The module D30IXT is at the oot of the job hieachy. This module communicates ove two files which ae eally D-30 communications contol computes. These computes communicate blocks of infomation containing entie teletype lines to the job D30INT in the 635 compute. Infom1ation exchanges between the D-30 and the 635 computes ae limited to once each half second in ode to minimize the numbe of inteuptions to the 635 and thus keep the ovehead acceptable. It is the fullction of D30INT to block and unblock these tansmissions and to povide communications fo those jobs opeating unde the exec uti ve system which equie teletype

7 Design Consideations fo Educational Time-Shaing System 663 Figue 4-Job stuctue This is a hieachial tee stuctue. The oot of the tee stuctue of the Datmouth time-shaing system is D:~OI~T input o output. This is done by establishing a communications file between D30IKT and, fo example, a BASIC compile opeating as a unning job in the system. Thus the job at the slave end of the communications file eads and wites an input/output device which is effectively a teletype. When D30IKT finds that a new teletype is on the system, D30INT sets up a communications file with OGIN. The effect is that a new teletype file is pesented to OGIN. This module had the esponsibility fo equesting the use numbe and validating it, and tacing down the catalog tee stuctue to locate the individual use's catalog of saved pogams. A potion of the tee stuctue with which OGIN deals is shown in Figue 3. OGIN then passes an end of the newly ceated communications file and the newly found catalog file to SIMON (simple monito). This monito has been especially witten to povide a basic coe of sevices fd many uses. SIMOK povides file etieval, editing and updating sevices, and calls in compiles and othe sevice outines whee necessay. At anyone instant in time, most of the uses on the system use only SI1\10~ and no lowe-level unning job, thus eseving a good deal of system capacity fo othe jobs in the opeating system. The thee modules D30INT, OGIN, and SIMON have all been witten to handle m'ultiple uses; they ae each one job in the time-shaing system. The compilation and execution of a use's pogam is accomplished by SIMON by spawning a job which accomplishes this in esponse,to a use's equest. Fo example, if a use wishes to execute a BASIC pogam, a file containing the BASIC compile is spawned in such a manne that the necessay file pe-exists when thls pogam goes into execution. The compile then eads the souce text, compiling it diectly into coe and ovelays the compile with a package of un-time suboutines. The code in the compile and the un-time package is pue pocedue, but the existence of only one BAR fo this machine equies that each BASIC pogam in execution has along with it its own copy of the un-time package. Othe languages cuently available unde this system include FORTRAN, AGO, and AFFF. ISP and AGO-68 effots ae undeway. Backgound capabilities ae being intoduced to allow pesons with long-unning jobs o jobs which equie special peipheal equipment such as the cad eade o line pinte to initiate such sevice equests fom a teletype. The use ceates a file of infomation descibing the backgound equest whlle using a teletype in nomal time-shaing unde SIMON. The use types the command BACKGROUND in ode to have his job contol language statements checked fo validity by BACKSYS. If this backgound equest is found to be pope, it is witten into a queue of backgound equests. A wakeup signal is then sent to the backgound monito (BACKMON) which intoduces these jobs into the system. This monito opeates in such a way as to attempt to optimize the mix of unning jobs to achieve satisfactoy pefomance fo backgound jobs without undue loss of pefomance fo foegound jobs. The file and job oganizations pay dividends hee: A BASIC pogam unning unde SIMO:N conducts teletype input/output ove a communications file nomally attached to D30INT and ultimately a teletype. In backgound opeation this communications file is linked to BACKMON whlch supplies input fom a use-specified input file, and diects output to a use-specified output file, inteposing a copy of the input file fo claity. A capacity fo softwae simulation of othe opeating systems and fo efficient debugging of machine language pogams has been implemented by means of the SQUEEZE pimitive fo the executive system. This technique was vey successfully used in MOD, an ealie opeating system fo the 635 developed as a vehicle fo the constuction of futhe time-shaing opeating systems. The name SQUEEZE is taken fom the fact that the base addess egiste (BAR), which limits the ange of allowable memoy efeences, is set to a smalle ange (squeezed) by this call. Additionally, all actions of an executing pogam which would nomally cause some executive esponse, such as execution of a ::\time (maste mode enty) instuction, divide-check, etc., ae bought to the attention of the pogam whlch issued the SQUEEZE by esetting the BAR to its unsqueezed value and saving the pogam status appopiately. The effect of this simple technique is that a pogam may be embedded in an oute pogam, and that oute pogam

8 664 Sping Joint Compute Confeence, 1969 can be witten to intepet l\1~1e's, divide-checks, etc. of the inne pogam in any way deemed fit. An 'oute pogam called DDT has such a capability, and allows patching, dumping, and beakpointing capabilities fo debugging any of the jobs in the system. If the SQUEEZE technique" wee useful only fo debugging pogams, it would have been well woth the effot fo this cuts down checkout times fo assemblylanguage jobs by a facto of ten ove the load-un-medump technique. Howeve, this technique also eaps benefits in poviding the capability to wite oute pogams which can simulate the action of othe opeating systems fo the 635. Fo example, the GECOS opeating system is simu1ated using this technique. The ma"co assembly pogam, G MAP, developed to un un d e1' u 1E'!OS v,uns nomauy." unae this simulato until a l\1~fe o othe caust. fo executive action is encounteed. The simulating pogam detemines the action which GMAP expects GECOS to pefom, and it initiates a coesponding action by making an appopiate call in the time-shaing opeating system. This technique has meant that many useful pogams opeating unde othe 635 opeating systems have been taken ove diectly without modification and function usefully in the cuent time-shaing system. ACKNOWEDGl\1ENTS The wok descibed in this pape was pefomed by a goup of pesons unde the diection of John G. Kemeny, and the authos ae pleased to have been pivileged to wok with these people. Thomas E. Kutz, Stephen J. Galand, John S. McGeachie, and Godon Bull made significant contibutions. Sidney W. ~1ashall and Neal F. Weidenhofe wee Datmouth gaduate students who wee essential to the success of the poject. The Datmouth undegaduates who wee esponsible fo majo aspects of the system wee David Magill, Pete Nielsen, Steven Hobbs, Gegoy Dobbs. Anthony Dwye; Paul Velleman, and Andew Behe~. Ronald Foottit, William Fofa, and Rajenda Kanodia acceleated pogess duing six-month visits. REFERENCES 1 F J CORBATO V A VYSSOTSKY Intoduction and oveview of the multics system Poc F J C C K C KNOWTON CAe M Vol