A TIME-SHARING DEBUGGING SYSTEM FOR A SMALL COMPUTER
Contents
1. McCARTHY Time Sharing Computer Systems in Management and the Computer of the Future edited by Martin Greenberger M I T Press 1962 PDP 1 manual Digital Equipment Corpo ration Maynard Mass S BoILEN User s Manual for the BBN Time Sharing System Bolt Beranek and Newman 50 Moulton St Cambridge Mass From the collection of the Computer History Museum www computerhistory org From the collection of the Computer History Museum www computerhistory org2. character w The pro gram knows whether the character is addressed to the user s program or to TYC On the other hand if the interrupt comes from the completion of the type out of a char acter the program types the next character from the buffer if any After transferring the information the break is ended Channels 6 7 11 14 and 15 are allocated to typewriters 2 The channel 16 dispatcher The computer is in restricted mode during the operation of the time sharing system As we stated earlier this means that zo instruc tions and instructions that halt the machine lead to sequence breaks on channel 16 The user programs his input output just as if there were no time sharing system Therefore when a channel 16 break occurs the program first looks at the instruction that caused the break Sup pose the instruction is a type out instruction If the type out buffer is not full the character that the user program wants to type is added to the buffer and if necessary a sequence break on the typewriter channel is instigated to start typing If the type out buffer is full the pro gram must be dismissed from run status If the instruction is a type in instruction a character is given to the user program if there is one in the buffer otherwise the program must be dis missed from run status If the instruction is discovered to be one that halts the machine the program is dismissed from run status and a note is left for TYC to t
3. formation from TYC using the same typewriter as his program uses for input and output Therefore it is important that no matter what program the user is attempting to debug he shall be able to regain control if it goes astray This is accomplished as follows When the user starts a program running he can either retain the typewriter for control purposes or else give it up to the program If he gives the typewriter to the program then characters it types appear on his typewriter and characters he types are given to his program if it asks for them Sup pose his bad program is taking characters from the typewriter but ignoring them He can then type the character center dot which is a non spacing character on the PDP 1 If he follows this by a carriage return the typewriter is then in the control of TYC and subsequent charac ters are interpreted by TYC If he actually wants center dot to be transmitted to his pro gram he must type it twice Suppose now that his program is in an output loop and refuses to stop typing Then he turns the power off on the electric typewriter The result of this is that the computer fails to get a done pulse from the next character typed within a second Control then goes to TYC which tells the channel 17 program to dismiss the user s program from core and returns the typewriter to the control of TYC as soon as the power switch is turned on again APPLICATIONS The most obvious application
4. of the BBN Time Sharing system is to speed up debugging by allowing each user more console time and good debugging languages In our opinion the reduction in debugging time made possible by good typewriter debugging languages and ade quate access to the machine is comparable to that provided by the use of ALGOL type lan guages for numerical calculation Naturally one would like to have both but this has not yet been accomplished on any machine We can now mention some other applications that our system makes possible by providing inexpensive console time 1 Small calculations At present there is a large gulf between desk calculators and compu ters One can start getting results 10 seconds after sitting down at a desk calculator but extensive calculations are very tedious The BBN Time Sharing System makes possible and economically reasonable providing a continuous transition from using the computer as a desk calculator at one extreme to writing ALGOL programs at the other An intermediate step is a system that allows the user to define functions by statements like flx a 72 3 0 x 4 3 awn g mn if m gt n then g n m else if rem n m 0 then m else g rem n m m aw UL and then be able to use these functions in arith metic calculations by writing something like g 3 21 x f 38 19 and have the computer print the answer by interpreting the formulas for the functions To some extent this has been ach
5. the Computer History Museum www computerhistory org 52 PROCEEDINGS SPRING JOINT COMPUTER CONFERENCE 1963 pected to be rare because when a user s program types out a multicharacter message successive characters go into a buffer area in the system core when the buffer is full the user s program is removed from run status until the buffer is nearly empty Therefore users with extensive output spend very little time in run status and the other users get correspondingly quicker service In fact the condition in which no users are in run status is expected to be so common that a provision for a background pro gram to be run only when no time sharer is in run status is contemplated THE TYC CONTROL LANGUAGE The language used to control the debugging is adapted from the DDT language devised by the TX 0 and PDP 1 group at M I T directed by Jack Dennis for the TX 0 and PDP 1 com puters The use of typewriters rather than the console switches for on line debugging has been developed at M I T and M I T Lincoln Labor atory since 1957 and at BBN since 1961 These languages have greatly increased the effective ness of the TX 0 TX 2 and PDP 1 computers and are now being developed for the IBM 7090 time sharing system by F J Corbato of the M I T Computation Center Unfortunately ex cept for a recent paper by Corbato the work has not been published Our only bow at history is to credit John Gilmore with the first such system
6. words f 25 and would get ry 15 see Thus on account of its much larger memory the 7090 would have about the same relation between number of users and maximum re sponse time as the PDP 1 This is less satis factory because the expense of the larger ma chine requires it to serve many more users Nevertheless such a system would still be use ful If we make the more optimistic but reason able assumptions that only of the users sitting at typewriters will be in run status at a given time and that a 3 second response time is tolerable then the system could accommodate 100 typewriters which is economically quite reasonable This would require a better drum system than is available connected so as to allow memory swaps at core speed REFERENCES 1 J GILMORE Lincoln Lab memo out of print 2 C STRACHEY Time Sharing in Large Fast Computers in Proceedings of the Inter national Conference on Information Proc essing UNESCO Paris 15 20 June 1959 UNESCO Paris 1960 pp 336 341 3 J C R LICKLIDER Man Computer Sym biosis IRE Transactions on Human Fac tors In Electronics March 1960 From the collection of the Computer History Museum www computerhistory org A TIME SHARING DEBUGGING SYSTEM FOT A SMALL COMPUTER 57 4 F J CorBato 1962 WJCC An Experi mental Time Sharing System Fernando J Corbato Majorie Merwin Daggett Robert C Daley 1962 Spring Joint Computer Con ference J
7. A OMPUTER J McCarthy Computation Center Stanford University Stanford California S Boilen Bolt Beranek and Newman Inc Cambridge Mass E Fredkin Information International Inc Maynard Mass J C R Licklider Advanced Research Projects Agency Department of Defense The purpose of the BBN time sharing system is to increase the effectiveness of the PDP 1 computer for those applications involving man machine interaction by allowing each of the five users each at his own typewriter to interact with the computer just as if he had a computer all to himself The effectiveness of this inter action is further enhanced by the use of the TYC language for controlling the operation and modification of programs First the computer The PDP 1 is a single address binary computer with an 18 bit word and five microsecond memory cycle most in structions require ten microseconds to execute The basic memory size is 4096 words but up to 65 536 words may be addressed indirectly The machine we used has 8192 words 4096 of which are reserved for tha time sharing aratam Panah waa rh VAAN VALLEN MJLAUUA AD WYovellis LIGA LI user sees a 4096 word memory We shall de scribe further relevant features of the computer later The PDP 1 computer is manufactured by Digital Equipment Corporation of Maynard Massachusetts All the equipment described in this paper was made by D E C and their cooperation in developing and
8. Versions of the utility programs especially adapted to time sharing are desired EXTENSION TO LARGER COMPUTERS It is worthwhile to ask to what extent the time sharing technique described in this paper is of more general use As a computer the PDP 1 is characterized by high speed and relatively small memory Its low cost means that it will not ordinarily have to be shared by a very large number of users Suppose we wanted a time sharing system based on core drum swapping on another computer Suppose that n number of simultaneous users t time for a memory cycle m number of words in user s memory that have to be swapped r response time f fraction of time taken by swaps then we have r ntm l1 under the assumption that the drum keeps up with the core memory and that the read and write haives of the core memory cycle are used separately In the present case if we put f 25 n 5 t 5 x 10 sec m 4000 we get r 5 sec The difference between this result and the actual maximum response time of 85 sec comes mainly from the fact that the present drum system swaps a word about every 8 micro seconds instead of every five microseconds which in turn comes from using a standard 1800 rpm motor on a drum on which each track has 4096 bits around If we were to make a similar system for the IBM 7090 computer we could have n 5 t 2 x 10 5 m 16 000 the memory of this machine really consists of 16384 72 bit
9. ap in as he goes out A user may be removed from core for any of several reasons 1 His quantum of time is up and he should be put on the tail of the queue 2 He has filled an output buffer 3 He has asked for a character and there is none in the input buffer 4 He has tried to execute an illegal instruc tion or to use input output equipment not avail able to him 5 The typewriter control program has filled a buffer or has finished a request concerning his program If the channel 17 routine decides to remove the current user it will swap into core the next user in the round robin who is in run status A user not in run status can become so for any of the following reasons 1 An output buffer is almost empty 2 A character requested by his program has arrived 3 The typewriter control program wants him in core to interrogate registers to change them or to run the program 4 The typewriter control program TYC The typewriter control program is in core 0 and it interprets and obeys requests from the user to give him information about his pro gram to change it to run it and to stop it The same program must work for all users and whenever a user is put in core TYC is modified so that it refers to the current user s program From the collection of the Computer History Museum www computerhistory org A TIME SHARING DEBUGGING SYSTEM FOT A SMALL COMPUTER 55 The user makes his requests and receives in
10. building the modifications and additions to the basic computer required for time sharing was essential to the success of the project l Attached to the computer is a high speed magnetic drum memory divided into 22 fields each of 4096 words A basic operation of the drum system is the memory swap accomplished in 33 milliseconds In this operation 4096 words are transferred from the core memory to a drum field and simultaneously the core memory is loaded from a different drum field This per mits the following time sharing mode of opera tion A 4096 word drum field is allocated for sav ing the core image of each user when his pro gram is not running A user s program in run status is run for 140 milliseconds then if there is another user also in run status the state of core memory is stored in the first user s core image on drum and simultaneously the second LIC Pn SM a CUIU allu Aa Ver naawa CULC user s image is loaded into second user s program is started in the appro priate place In the worst case of all five users in run status the system makes its rounds in 5 x 140 33 865 milliseconds For man machine interaction this means that if the user types a character calling for a response from his program that requires less than 140 milli seconds 14 000 machine instructions he will get the first character of the response in less than 865 seconds This worst case is ex From the collection of
11. ell the user what happened Paper tape input and output is handled in a similar way except that the dispatcher must check whether the user has the punch or reader assigned to him If not the user s program is dismissed and a complaint is made to him As soon as the reader or punch has been relin quished he can continue the program from where it left off The typewriter and paper tape instructions are interpreted and simulated by the channel 16 dispatcher so precisely that programs written before the time sharing system was developed can be run without change in the system provided they do not themselves use the sequence break system This means that almost all the previously used symbolic assembiers typewriter input output routines text editors etc can be used without change and that the user can use the TYC language to debug rou tines that are to be used outside it 3 The channel 17 clock routine Every 20 milliseconds or so a sequence break signal is given on channel 17 Since channel 17 is the lowest priority channel this break can take effect only when no typewriter paper tape or channel 16 dispatcher break is in progress Moreover except when the channel 17 program turns off the sequence break system it can be interrupted by typewriter or paper tape se quence breaks The basic task of the channel 17 clock routine is to decide whether to remove the current user from core and if so to decide which user pro gram to sw
12. es 3145G This gives the type writer up to the program but he can get it back for control purposes by typing center dot If he does so he can interrogate and change registers without stopping his program but if he wants to stop it he types H Once it is stopped he can start it where it left off by typ ing G without a numerical argument He can also interrogate the arithmetic registers either while the program is running or while it is stopped Of course the contents of a register interrogated when the program is interrupted at a random moment may not be significant this depends on the program Except at the beginning and the end of his session the user does not ordinarily use the paper tape apparatus Instead he designates a position on the drum for the punch and a position for the reader using TYC An instruc tion in the user s program to punch a character onto paper tape actually results in entering the character into a buffer for transmission to the drum when the buffer is full or no punch in structions have been given for a while This feature allowed us to make available in the time sharing system symbolic assembly pro grams and other utility programs that were developed previously The TYC language is described in detail in reference 7 From the collection of the Computer History Museum www computerhistory org A TIME SHARING DEBUGGING SYSTEM FOT A SMALL COMPUTER 53 RELEVANT FEATURES OF THE PDP 1 COMPUTER In order
13. for the TX 0 in 1957 First we shall consider the facilities for ex amining and changing registers Suppose the user wants to examine register 344 He types 844 and the computer types back the contents of this register interpreted as a computer instruc tion if possible Thus it might type back add 4072 The user then has several options 1 He can carriage return and close the reg ister if he is satisfied with its contents 2 He can ask to see the next register by hitting the backspace key The computer might then type back 345 sub 4075 3 He can ask to see the previous register 4 He can ask to see register 4072 by hitting the tab key 5 Hecan ask for the contents of the register as an octal or decimal number 6 He can change the contents of the register by typing new contents such as add 4073 and then hitting one of the delimiter characters The computer user who does not have ex perience with systems of this kind may not immediately realize the increase in effectiveness that these facilities alone give over console de bugging The advantages are that typing is easier than flipping switches that a record is obtained of what was done and that useful features can be added to the control language As a further feature the user can define sym bolic names of addresses so that his typeouts can take the form a addb 3 a 1 subb 6 In order to start a program say at register 3145 the user typ
14. his mode and if there is no break started in the sequence break system then any of the following events lead to a sequence break on channel 16 1 An attempt to obey an zo instruction 2 An instruction that would normally stop the computer such as a halt instruction or an illegal instruction 3 An attempt to refer to core 0 The instructions that enter and leave extend mode and restricted mode are considered to instructions The time sharing system operates only when a sequence break has occurred and hence is not subject to the restrictions 6 The Channel 17 Clock Every 20 milliseconds a signal for a sequence break on channel 17 occurs Programs can turn off channel 17 or any other channel by an 20 instruction if they don t want breaks to occur Note that channel 17 is the lowest priority channel The clock is a multivibrator whose period is controlled by a potentiometer HOW THE TIME SHARING WORKS The time sharing executive program is not readily described by a single flow chart because its different parts act asynchronously as de termined by sequence breaks It includes the following programs 1 The typewriter to program Associated with each typewriter is an input output program and a buffer area These pro From the collection of the Computer History Museum www computerhistory org 54 PROCEEDINGS SPRING JOINT COMPUTER CONFERENCE 1963 grams are entered when sequence breaks occur Suppose a user types a
15. ieved by the pro gram Expensive Desk Calculator written by Robert Wagner at M I T 2 Editing Texts Several programs have been written to use the PDP 1 computer to edit paper tape texts The user can originate text make insertions and deletions display the cor rected text to make sure it is correct find all occurrences of certain strings of symbols These editing programs are much more con venient for correcting programs than using flexowriters or than making changes in a card One such program Colossal Typewriter operates as follows There are two modes text mode and control mode When the program is in text mode each character typed by the user is added to a buffer held in the core memory of the computer There are four exceptions to this If the user types a backspace the program deletes the last char acter from the buffer and this operation can be repeated as many times as may be required to correct a local error Another character causes the program to type out the last 20 char acters in the buffer and a third returns the computer to control mode The fourth special character the single quote causes the cliche whose name is the following character to be entered in the text if there is such a cliche Thus typing a causes the cliche named a to be entered In addition the cliche Feature may be used to enter any of the four control characters into the text The user need only type followed by the characte
16. r in question From the collection of the Computer History Museum www computerhistory org 56 PROCEEDINGS SPRING JOINT COMPUTER CONFERENCE 1963 In control mode the user has the facilities to type out the buffer to as pseudo punch the buffer to read pseudo read from the drum into the buffer a number of lines or until the first occurrence of a given cliche to reset the ends of the buffer to give the current buffer a name as a cliche to kill the buffer and to go into text mode Other text editing programs allow the use of the CRT on the computer to display lines of text 3 Teaching Programs An experimental teaching laboratory using a system based on the principles of this paper is being installed at stanford University Additional applications of large time sharing systems are described in 2 3 and 5 OPERATING EXPERIENCE The BBN Time Sharing System has been in operation at Bolt Beranek and Newman Inc since September 1962 The computer is oper ated in the time sharing mode four hours per day Initially the number of typewriters was two but this has been increased to five The present system has been found to have the following weaknesses which we hope will be corrected There is no program library on the drum so that excessive use of the paper tape reader is required Magnetic tape files for user programs are not available in the system Five computer operated typewriters in one room make too much noise
17. to explain the detailed functioning of the BBN time sharing system it is necessary to understand the input output system of the PDP 1 computer the sequence break system and the restricted mode 1 Typewriters and paper tape Each tot instruction transfers a single char acter between the 18 bit zo register of the com puter and the external device All io instruc tions have the same left 5 bits the 6th bit is normally used to determine whether the wait before the next instruction is determined by the external device or whether the program keeps this responsibility The remaining 12 bits of the instruction are used to determine the device and the direction of transfer The char acters are 8 bits on paper tape and 6 bits for typewriters When the computer is not in se quence break mode a character typed by the user results in turning on a sense flag which can be interrogated by the program to determine when to pick up the character 2 Drum In transfers from the drum the number of words to be transferred and the locations in core and on the drum are specified During the transfer the arithmetic unit of the machine is tied up 3 The machine has other input output devices but their operation does not have to be described in this paper 4 The sequence break system Besides the simple form of input output de scribed above the machine has a sequence break system which permits input output and com putation to be carried out sim
18. ultaneously and asynchronously The BBN time sharing system makes full use of sequence break Tne sequence break system has 16 channels to the outside world arranged in a priority chain with channel 0 having highest priority and channel 17 lowest priority Associated with each channel are four registers in core 0 When a signal comes from an io device that the device has a character ready for the computer or is ready to receive a character from the computer an interrupt will occur if an interrupt is pres ently allowed on that channel When an inter rupt occurs the contents of the ac accumulator io input output and pe program counter registers are stored in three of the regis ters and control is transferred to the fourth which must therefore contain a jump to a pro gram for dealing with the interrupt When this program has finished its work it must execute an indirect jump through the register where the program counter was stored when the break started This returns control to the program s rm oy rad 7 a N lt that was interrupted and tells the sequence break system that the break is over Once a break is started on a channel a break cannot occur on a lower priority channel nor can another break occur on the same channel until the break is over Each channel can store one break until it is allowed to occur 5 Restricted Mode This mode was devised specifically for the time sharing system When the computer is in t
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