BOLT BERANEK AND NEWMAN i K c CONSUITING DEVEIOPMENT RESEARCH THE STRUCTURE OP A LISP SYSTEM USING TWO-LEVEL STORAGE

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1 BOLT BERANEK AND NEWMAN i K c CONSUITING DEVEIOPMENT RESEARCH ^ APCRL » 0 G o / o ö O THE STRUCTURE OP A LISP SYSTEM USING TWO-LEVEL STORAGE Daniel G. Bobow Daniel L. Muphy Bolt Beanek and Newman Inc 50 Moulton Steet Cambidge, Massachusetts Contact No. AP19(628)-5065 Poject No. S668 Scientific Repot No. 6 k Novembe 1966 (The wok epoted was suppoted by the Advanced Reseach Pojects Agency, ARPA Ode No. 627, Amendment No. 2, dated 9 Mach 1965) Pepaed Fo: AIR FORCE CAMBRIDGE RESEARCH LABORATORIES OFFICE OF AEROSPACE RESEARCH UNITEP STATES AIR FORCE BEDFORD, MASSACHUSETTS Distibution of this document is unlimited CAMBRIDGE NEWYOR< CHICAGO I. OJANCEIES

2 AFCRL THE STRUCTURE OF A LISP SYSTEM USING TWO-LEVEL STORAGE Daniel G. Bobow Daniel L. Muphy Bolt Beanek and Newman Inc 50 Moulton Steet Cambidge, Massachusetts Contact No. AF19(628)-5065 Poject No Scientific Repot No. 6 4 Novembe 1966 (The wok epoted was suppoted by the Advanced Reseach Pojects Agency, ARPA Ode No. 627, Amendment No. 2, dated 9 Mach 1965) Pepaed Fo: AIR FORCE CAMBRIDGE RESEARCH LABORATORIES OFFICE OF AEROSPACE RESEARCH UNITED STATES AIR FORCE BEDFORD, MASSACHUSETTS Distibution of this document is unlimited

3 TABLE OF CONTENTS ABSTRACT - 1 SECTION I 1 INTRODUCTION SECTION IT 3 ORGANIZATION OP CORE MEMORY Segmentation of System Code Compiled LISP Functions. T 5 SECTION III 7 ORGANIZATION OP THE DRUM MEMORY JType Detemination of Pointes 8 Lltegl Acons., 8 Numeical Atoms 10 Constuction of Lists 11 I SECTION IV 14 VARIABLE BINDINGS AND THE PUSHDOWN LIST SECTION V 16 PERFORMANCE I SECTION VI 18 SUMMARY I ACKNOWLEDGEMENTS 19 REFERENCES 20 [ FIGURES I Fig. 1. Oganization of the Vitual Memoy 9 f

4 ABSTRACT In an Ideal list-pocessing system thee would be enough coe memoy to contain,a11 the data and pogams. This pape descibes a numbe of techniques used to build a LISP system which utilizes a dum fo its pincipal stoage medium, with a supisingly low time-penalty fo use of this slow stoage device. The techniques include caeful segmentation of system pogams, allocation of vitual memoy to allow addess aithmetic fo type detemination, and a special algoithm fo building easonably lineaized lists, A scheme Is descibed fo binding vaiables which is good in this envionment and allows fo complete compatibility between compiled and Intepeted pogams with no special declaations. -1-

5 i * [ SECTION I INTRODUCTION i ^- LISP is a list-pocessing language which Is being used extensively In eseach In atificial Intelligence. In the Ideal list-pocessing system, thee would be enough coe memoy to contain all the data that wee to be efeenced ove a long time peiod. In this case, a data efeence would take on the octe of one o two micoseconds, a speed typical of pesent coe memoies. When lage systems ae constucted, equiing upwads of one hunded o two hunded thousand wods, the cost of coe memoy usually becomes pohibitive. Memoy size equiements of this ode of magnitude ae, howeve, not the exception buy athe the ule fo many pojects planning eseach on natual language, speech pocessing, and a host of othe aeas. j i L Thus it becomes necessay to conside the use of bulk stoage memoy devices such as magnetic dums and discs. The poblem in using such devices is that, while thei data tansfe ate is sufficiently high (5-10 M-sec/wd), one must wait fo the data on the otating medium to come aound to the ead position. The aveage time to access data is onehalf the otation time, o typically about 17 to 33 milliseconds. To use a dum, the, as if it wee coe memoy, i.e. to ead single wods as they ae equied, would incease data efeence time by a facto of aound 20,000 making any list-pocessing system uselessly slow fo pactical poblems. -1-

6 As a fist step towad utilizing dum stoage efficiently, we oganize the dum into blocks of wods, o pages, and bing into coe an entie block of wods wheneve one fom that block was equied. As may be seen fom the timings above, the exta time equied to tansfe 200 to 300 wods instead of one, is negligible compaed to the access time. If multiple efeences ae made to a block once it has been moved into coe, then the speed of data efeences is inceased by a facto equal to the numbe of efeences made to a given page befoe anothe must be bought in. In this pape we descibe a numbe of techniques we have used in the BBN LISP system to maximize the numbe of Incoe efeences pe dum efeence. These techniques should incease the speed of opeation of any list-pocessing system embedded in a time-shaing system which uses paging to nup a lage vitual memoy stoed on a dum into a smalle coe memoy, e.g. the MULTICS system (3). -2-

7 SECTION II ORGANIZATION OF CORE MEMORY t Ou LISP system has been Implemented on a Digital Equipment Copoation PDP-1. This machine has a coe memoy of 16K (K = 1024) and a dum memoy of 8&C. Access time to one 18 bit wod of coe Is 5 micoseconds, and aveage access time to a wod on the dum Is 1? milliseconds. The PDP-1 has no Index egistes, floating point instuctions, special push-down-11st instuctions, o paging hadwae. It has an unusually lage collection of I/O devices, includig pape tape eade and punch, teletypes with eade and punch, mag-tape dives, high speed display and light pen, and a \ Gaphicon (RAND) tablet. The LISP system can communicate with all these i/o devices. f I I The BEN LISP system contains both an Intepete and a compile. The opeation of the compiled code is completely compatible with the Intepetation by the intepete of S-expession definitions of functions. One may un mixed sets consisting of functions which ae intepeted and othes which ae un afte being compiled. The scheme fo binding vaiables which allows this complete compatibility is discussed in detail below. The 16K of coe memoy must be allocated among pemanent code, list stuctue stoage, and compiled code. To this end we have dedicated 4K to compiled code, 4K to the -3-

8 supeviso and pemanent code, and 8K to list stuctue and pushdown list stoage. Since a list element consists of two 18 bit wods;, this 8K of memoy is equivalent to at most 4K LISP wods. Segmentation of System Code The pemanent code fo the system is well ove 10K in length itself. In ode to stay within the allotted ^K of memoy we have segmented the system code into 6 ovelays which have minimal inteaction oetween them, minimizing the numbe of swaps necessay between ovelays. We keep pemanently in coe the inteupt outines fo sevicing use on-line inteaction, and an elementay time-shaing supeviso (ou system allows a small numbe of uses - usually 2 o 3 - to use the machine as a LISP dedicated time-shaing system), The emainde of the 4K is used as swapping aea fo the feixowing ovelays: 1. Intepete and compiled code unne 2. Some non-citical special machine coded suboutines fo manipulating list stuctue 3. Input-Output and fomatting 4. A special package fo manipulating the athe stupid magnetic tape dives on the machine 5. The gabage collecto 6. An initialization package Since all segmentation and paging is done by softwae on this machine, we felt that explicit segmentation and complete swapping of ovelays was pefeable to calling suboutines off the dum as needed. This is in contadistinction to ou philosophy on unning compiled LISP code. Fo the system code we know what easonable segments ae; in the latte case the system could not know (and we did not want -4-

9 to put the onus on the use fo keeping ack of) segments that would fit in 4K of stoage. All ovelays ae absolute code and can theefoe un only in fixed locations. Compiled LISP Functions Pogams compiled fom S-expession definitions of LISP ^ functions ae stoed on ehe dum in elocatable fom. When unning, these functions ae contained in a ing buffe in coe tj about 3^00 wods, popely elocated. Let us conside what happens when a compiled function is called. The call contains a pointe to the atom which names the function (essentially the symbolic name of the function). This name is used to seach an in-coe tansfe vecto containing stating locations --f all outines in coe. The seach is done by "hashing" the name, ad seaching the tansfe vecto until the name o an empty space is found. We have found that with the ing buffe full, the tansfe vecto of 128 wods (64 name-stating location pais) is only 1 about l/4 full. Theefoe the aveage numbe of checks to detemine the pesence of a function in coe is only slightly 1 lagei than 1. If the function called is in coe, a tansfe is made to the stating addess given. Thus, in this case, 1 we have made no dum efeences to link functions. This pocedue could be impoved if we wanted to modify the code of the calling function to contain a Jump indiect though the tansfe vecto. Howeve, we pefe that the only addess binding Jone on compiled code be simple elocation. 1 If the function called is not in coe, we obtain its dum addess fom the function cell of its atom. We then ead into the ing buffe the fist page of the compiled code fo thlg function (i.e. fom its initial position to the end of the 256 wod block which constitutes a page on the dum). [ -5-

10 The fist wod of the pogam contains the length of the pogam. Successive pages ae ead In until the entie pogam Is In coe. If at any time we ovelap the end of the ing buff-, we stat again fom the beginning of the ing buffe with the beginning of the pogam. The tansfe vecto is updated by emoving enties fo functions which have been wholly o even patially ovewitten, (taking c»e to mak them popely fo the hash lookup) and a new enty is added fo the pogam. Retuns fom calls ae also made though the tansfe vecto. Thus, pogams which have called suboutinea may be ovewitten, In tnis case, the pogam will be ecalled fom the dum befoe the etun Is effected. Sets of functions seem to stabilize unde this system, and if all the pogams to be used fo a easonable peiod can fit in coe, they soon eside thee. -6-

11 SECTION III ORGANIZATION OF THE DRUM MEMORY LISP assumes that it Is opeating in an envionment containing 128K wods, that is fom 0 to 400,000 octal. Only 88K actually exist on the dum. The emaining potion of the addess space is used fo epesentation jf small integes between -32,76? and 32,767 (offset by 300,000 octal), as descibed below. All data stoage is contained within this vitual memoy, including list stuctue, compiled code, atom value cells, popety cells and function cells, pint name stoage and pushdown list stoage. This vitual memoy is divided into pages of 256 wods. Refeence to the vitual stoage ae made via an in-coe map which supplies «/he addess of the equied page if it is in coe, o taps to a supevisoy outine if the page is not in coe. This dum supevisoy outine selects an in-coe page, wites it back on the dum if it has been changed, and eads the equied page fom the dum. Closed suboutine efeences to an in-coe wod though the map takes appoximately 170 micoseconds (be- cause of the poo set of opeation codes on the machine, and the lack of an index egiste). A efeence to a wod not in coe, which must be obtained fom the dum, takes between 17 and 33 milliseconds. It takes the longe time if a page must be witten out on th> dum befoe the efeenced page can be ead in. Thus, it ally pays to minimize dum efeences. -7-

12 Type Detemination of Pointes In standad wholly-in-coe LISP systems the type of element a pointe Is efeencing (fo example, an atom vesus an S-expesslon} can only be detemined by looking at the Item Itself, which might equie a dum efeence. We avoid this unnecessay dum efeence by dividing the vitual memoy space into a numbe of aeas as shown In Pig. 1. As can be seen fom this map of stoage, simple aithmetic on the addess of a pointe will detemine its type. We chose to allocate stoage athe than povide an in-coe map of stoage aeas, because the map would take up valuable in-coe space, and evey aldlvlonal page of stoage that we can squeeze into coe educes he nutbe of dum efeences. Liteal Atoms When a sting of chaactes epesenting a liteal atom is ead in, a seach is made to detemine if an atom with the same pintname has been seen befoe. If so, a pointe to that atom is used fo the cuent atom. If not, a new atom is ceated. Thus, as in all LISP systems, a liteal atom has a unfoue epesentation. Pou cells (wods) ae associated with each liteal atom. These cells contain pointes tc the pint-name of the atom, the function which it identifies, its top level o global value, and its popety list. A pointe to an atom points to its value cell. Since these value cells occu in only one pat of the addess space, one can tell fom a pointe (addess) whethe o not it is pointing to a liteal atom. Instead of having the othe cells on the sa^.e page with the value cell, each is put in a sepaate space in a nosition computable fom the addess of the value cell. Sepaating value -8-

13 t f VIRTUAL MEMORY 400,000Q i HASH TABLE 3oo,ooo 8 m PRINT-NAMES 270,000 8 C H 03 PRINT-NAME POINTERS 230,0008 I E CO E C I FUNCTION CELLS 220,00C 8 210,000g PUSHDOWN LIST FULL WORDS 200,000 8 JL 170,000 8 cm PROPERTY LIST CELLS VALUE CELLS I LIST 160, ,000 8 STRUCTURE 1 COMPILED CODE io,ooo 8 Pig. 1. Ocn'.zatlon of the Vitual Memoy.

14 cells and function cells, fo example. Is useful because most uses will not use the same name fo a global vaiable as they will fo a function, and, theefoe. If the fou cells ae bought In wheneve any one was asked fo. It Is likely that the othe thee cells would neve be efeenced. Yet, they use up oom In coe which could be used fo othe stoage. Similaly, the pint-name pointes associated with atoms ae needed duing input and output, but aely duing a computation. Theefoe, duing computation these cells ae neve in coe. Numeical Atoms In 709^ LISP, numeical atoms (numbes) do not have a unique epesentation; that is, a numbe of diffeent pointes may be efeencing numbes with the same value. This implies thzt fo compaison of numbes, o fo aithmetic opeations, the value of the numbes mut be obtained, and compaison of numeical atoms cannot be Just a compaison of pointes. The values of numbes ae stoed in "full wod" space. In 7094 LISP, pointes to numbes in list stuctue do not point diectly to the values of the numbes in full wod space. Fo each numeical atom, a special wod fom fee stoage is acquied with bits set in this wod to indicate type, and a pointe to the value of the numbe in full wod space. A pointe in list stuctue which efeences a numbe points to this "heade" wod. Because type infomation is implicit in pointes (addesses) in ou LISP system, we do not need this exta level of indiection, and pointes to numbe values diectly addess fee wod space. This obviously saves possible dum efeences in aithmetic opeations and compaisons. In addition, we utilize the fact that not all addesses in the 17 bit addess space of the dum can legitimately appea as -10-

15 -s pointes In list stuctue. Pointes between 200,000 (octal) and ^00,000 (octal) ae, theefoe, used In the context of list stuctue to epesent diectly "small" integes between and (offset by 300,000 octal). Thus, a pointe of 300,003 (octal) occuing In a list Is the numbe 3. Again, eliminating a level of indiectness educes the numbe of dum efeences equied. Anothe advantage of using this fom fo 1 small Integes is that numbes in this ange ae epesented uniquely; thus, aithmetic compaisons can be done diectly on the pointes. In addition, no additional stoage is equied, and this educes the numbe of gabage collections that must be invoked. Taditionally, almost all the numeical opeations done in LISP ae on these small integes. Constuction of Lists I Caeful allocation of the addess space of vitual memoy alleviates only some of the poblems of list pocessing in a paging envionment. List stuctue, unless specifically and puposefully oganized, tends to become andom and thus defeats the advantages of the paging scheme. If only one-tenth of the existing list stuctue can eside in coe, and it is efeenced andomly, then nine out of ten data efeences, on the aveage, will equie a dum opeation. An examination of list-handling pocesses indicates that lists ae usually pocessed sequetially; that is, pogams geneally m poceed down the elements of a list by taking the CDR of the I successive tails. A pocess may be handling seveal lists at onoe, but will typically make numeous efeences to each. One of the best means of speeding up a paged LISP appeaed to be lineaizing lists and concentating them on as few pages as -11-

16 possible. To do this, a special CONS, o list-wod constucting suboutine, was witten. This attempts to assign a new list-wod on a page aleady In use by the list of which this new wod Is a pat. The algoithm Is descibed below. In constucting a new list pointe, a fee wod pai must be obtained fom a fee stoage list. Instead of keeping one lage fee stoage list as Is done on the 7094 vesion of LISP, we have a sepaate fee stoage list on each page. Thus, the system can detemine If a new pointe pai can be placed on a paticula page. Using these fee stoage lists, we now constuct a new pointe pai (a LISP dotted pai consisting of two 18 bit PDP-1 wods) accoding to the following algoithm: To constuct Z CONS [XjY]: 1. If Y Is not an atom and thee Is oom on the page with Y then place Z on this page. 2. Othewise, If X Is not an atom and thee Is oom on the page with X put Z on that page. 3. Othewise, If thee Is a page In coe with oom place Z on this page. 4«Othewise, place Z on a page In vitual memoy with oom. This page Is found by seaching an In-coe table which gives the Initial location of the fee stoage list on each page. If stoage Is available. This algoithm tends to minimize coss linkages between pages and to limit any single stuctue to a vey few pages. This has been bon out by tacing though a numbe of faily lage -12-

17 stuc' es and computing the numbe of page cossings. When wokiüg with such a stuctue. It is unlikely that one will make efeences to moe than a few pages fo a elatively long peiod of time. Since these pages can eside ii- coe, no dum efeences ae needed. Fo example, in enteing the definition of a function, the entie definition tends to appea on a single page. Thus, duing the intepetation of a function multiple dum efeences ae usually unnecessay. When fee stoage is exhausted, gabage collection is necessay. A numbe of gabage collection schemes (4) have been Invented and Implemented in vaious vesions of LISP. In some of these schemes, fee stoage is compacted by a folding pocess in which empty cells in lowe stoage ae filled with the content of cells in highe stoage. Howeve, this is a vey bad type of scheme fo a paging envionment, because this tends to effectively shuffle pointes, and make lists extend ove moe pages than ae necessay. In ou system, we simply mak used cells and collect stoage on each page as it stands. In addition to this standad gabage collection algoithm, we also utilize anothe scheme fo dumping onto seconday stoage (magnetic tape) a compacted epesentation of tho list stuctue in use in the system. This scheme (descibed in (4)) based on an algoithm fist suggested by Mavin Minsky (5) has the desiable popety that lists ae, in geneal, lineaized in the CDR diection. i [ [ i i -13-

18 SECTION IV VARIABLE BINDINGS AND THE PUSHDOWN LIST A numbe o schemes have been used in diffeent vesions of LISP fo stoing the values of vaiables. These include: 1. Stoing values on an association list paied with the vaiable names. 2. Stoing values on the popety list of the atom which is the name of the vaiable. 3. Stoing values in a special value cell associated with the atom name, putting old values on the pushdown list, and estoing these values when exiting fom a function. 4. Stoing values on the pushdown list. The fist thee schemes all have the popety that values ae scatteed thoughout list stuctue space, and, in geneal, in a paging envionment would equie efeences to many pages to detemine the value of a vaiable. This would be vey undesiable in ou system. In ode to avoid this scatteing, and possible excessive dum efeences, we utilize a vaiation on the fouth standad scheme, usually only used fo tansmitting values of aguments to compiled functions; that is. -14-

19 we place these values on the pushdown list. But since we use an intepete as well as a compile, the vaiable names must be kept. The pushdown list thus contains pais each consisting of a vaiable name and its value. The Intepete need only seach down the pushdown list fo the binding (value) of a vaiable. One advantage of this scheme is that the cuent top of the pushdown stack is usually in coe, and thus, dum efeences ae aely equied. Fee vaiables wok automatically in a way simila to the association list scheme. In fact, the pushdown list is a guaanteed linea vesion of the association list.» An additional advantage of this scheme is that it is completely compatible with compiled functions which pick up thei aguments on the pushdown list fom known positions, instead of doing a seach. To keep complete compatibility, ou compiled functions put the names of thei aganents on the pushdown list, although they do not use them to efeence vaiables. Thus, fee vaiables can be used between compiled and intepeted functions with no special declaations necessay. The names on the pushdown list ae also vey useful in debugging, fo they povide a complt symbolic backtace in case of eo. Thus, this technique, fo a small exta ovehead, minimizes dum efeences, povides symbolic debugging infonnation, and allows completely fee mixing of compiled and intepeted outines. With appopiate passing down of pushdown list pointes we can even achieve the same effect as the standad PUNARQ device on the 7094 fo functional aguments. This PUNARQ device peseves local context well enough to handle the Knuth's "Man and Boy" compile poblem (Algol Bulletin 18.1). -15-

20 SECTION V PERFORMANCE The poof of pogamming techniques is in the unning. We have made a numbe of measuements on ou system to test the validity of ou assumptions. To test the hypothecis that ou CONS algoithm was helping to minimize dum efeences, we an a computation fo about 35 minutes with a numbe of countes in vaious functions. The computation made was the compilation of appoximately 10 pages of LISP functions which define an elementay pogamming language called GHOST. We wee utilizing about 12,000 wods of fee stoage; coe can only hold about 3,500. Typical gabage collections eclaimed about 5,000 wods. In this time we pefomed 31,000 CONS's, and 150,000 CAR's and CDR's (not counting those done in gabage collection). Fo these CONS's and CAR's-CDR's only 5,500 dum efeences wee needed, o only about 2.5 pe cent of the cases. If stoage wee distibuted andomly, then the expected numbe of efeences should have anged between 50 pe cent and 70 pe cent depending on whethe the space in use fo list stuctue was neae 7,000 o 12,000 wods. With the 2.5 pe cent figue, we computed that the system spent 10 pe cent of its time waiting fo the dum. If the system h&d to go to the dum fo 50 pe cent of its data :^= 3^"'TMMiain '.-^^ -...

21 efeences. It would be spending well ove 90 pe cent of Its time waiting fo the dum. Extacting the same Infomation fom a numbe of othe uns fo diffeent uses of the system indicates that the dum is efeenced between 2 pe cent and 10 pe cent of the time. The types of Jobs being done by the use wee not ecoded. The highe pecentage was ealized when the use was involved in many small online inteactions with the system. Howeve, it is in Just this case that waiting fo the dum is least costly to the use, since most of his time is spent thinking, Anothe facet of the cost of using the dum should be mentioned. Even in poblems in which all the stoage needed can fit into coe, you. ae paying a pice fo the dum facility. Instead of diect efeences to coe, the system must go though a softwae map to detemine coe addesses. We compaed the unning time of one pogam on an in-coe LISP system,and a paged LISP system,on an SDS-9^0. Despite the fact that all the data fit in coe, this expeiment indicated that we ae losing a facto of about 2 in speed when we use the softwae map. Of couse, if a hadwae paging map wee available this poblem would vanish. -17-

22 SECTION VI SUMMARY In auminay, ou LISP system supassed ou expectations and Is poving a useful eseach tool fo a numbe of atificial intelligence pojects. Caeful segmentation of system code, aangement of data spaces by type, oganization of list stuctue, and special attention to the binding of vaiables all contibute to the success of the system. 18- i «msi

23 , ^ ä mmmm^ ACKNOWLEDOEKENTS The authos would like to thank L, Pete Deutsch of Bekeley, Califonia fo KTltlng (in LISP) the fist vesion of the compile used on this system, and fo a numbe of stimulating discussions. The wok epoted hee was suppoted by the Advanced Reseach Pojects Agency, ARPA Ode No. 627* Amendment No. 2- dated 9 Mach

24 PEPERENCES 1. McCathy, J. et ai, LISP 1.3 Pogammes Manual, M. I. T. Pess, Cambidge, Massachusetts, Bekeley, E. G. and D. G. Bobow (eds.). The Pogamming Language LISP, Its Opeation and Applications, M. I. T. Pess, Cambidge, Massachusetts, Cobato, P., E. Glase et ai, "The MULTICS System," Poc. PJCC, Spatan Pess, Baltimoe, Mayland, Bobow, D. G., "Stoage Management in LISP," Poc. IPIP Conf. on Symbol Manipulation Languages, Noth Holland. (In pepaation) 5. Minsky, M. L., "A LISP Gabage Collecto Algoithm Using Seial Seconday Stoage (ev.)," Memo 38, Atificial Instelllgence Goup, M. I. T., Cambidge, Massachusetts, Cohen, J., "A Use of Past and Sic Memoies in List Poc< ssing Languages," Comm. ACM, Januay

25 Iln-lflHRi f1f>d Secuity Classification DOCUMENT CONTROL DATA - R&D (Secuity elatsification of title, body of ah*tact and indexing annotatici muil 6«enteed uihen the oveall epot it '.ossified) I. OftloiNATiNO ACTK.v (Copoal» auttjx) Bolt Beanek and Newman, Incopoated 50 Moulton Steet to. REPORT SECURITY. AJIIFIC ATION.. IlnJaasifled IK OR&UP l REPORT T The Stuctue of a LISP System U3lng Two-Level Stoage 4. DKJCR.fTive MOTES (Type of epot and inclusive dates) Inteim Scientific Repot %. AUTHORTS; (Laet none, fiisl nans, initial) Bobow, Daniel G. Muphy, Daniel L.». RtRORT DATE ^ Ko\imbe 1966 AP19(o28)-5065 «Si-i c SVxo T fl 0 C e cn T^c ARPA Ode No. 627 Amend> 2 k. PROJECT mtxmkwx * No C. OOO ELEMENT 615Ü501R d. OOO (UMELEMENT n/a AVAILASILITY/LIMITATION NOTICE» 7a TOTAL NO. OF PACES TS NO. OF RCFt 20 6 a ORlOIMATOR'* REPORT NUHiERTS; Scientific Repot No. 6»b. othin mamy HO(SI (Any olkt numbes Aot may be assigned this epot) APCRL-66-77^ DISTRIBUTION OP THIS DOCUMENT IS UNLIMTED II. SUPPLEMENTARY NOTES Hq. APCRL, OAR(ORB) United States Ai Poce L. fi. Hanscom Fid., Bed &ilj& > II AMTRAtT ' ' ti SPONSORING MILITARY ACTIVITY Advanced Reseach Pojects fta. Agency In an Ideal list-pocessing system thee would be enough coe memoy to contain all the data and pogams. This pape descibes a nunbe of techniques used to build a LISP system which utilizes a dum fo its pincipal stoage medium, with a supisingly low time-penalty fo use of this slow stoage device. The techniques include caeful segmentation of system pogams, allocation of vitual memoy to allow addess aithmetic fo type detemination, and a special algoithm fo building easonably lineaized lists. A scheme is descibed fo binding vaiables which is good in this envionment and allows fo complete compatibility between compiled and intepeted pogams with no special declaations. DD FO ', " 1473 '"' I JAN ««"" Unclassified Secuity Classiiicmtion

26 Secuity Classification KEY WORDS ROLE WT LINK C List Pocessing LISP Paging Systems Dynamic Stoage Allocation Two Level Stoage Allocation INi/RUCTIONS I. ORIGINATING ACTIVITY: Ente the name and adde^a of tne contacto, aubconuacto, gantee. Depatment of Defenae activity o othe oganization (copoate autho) iaau.ng the epot. 2a. REPORT SECURITY CLASSIFICATION; Ente the oveall secuity claaaification of the epot. Indicate whethe "Resticted Data" ia included. Making ia to be in accodance with appopiate secuity egulations. 26. GROUP: Automatic downcadiig ia specified in DoD Diective and Amed Foces Indvstiel Manual. Ente the goup numbe. Alao, when applicable, ahow that optional makings have been uaed fo Coup 3 and Goup 4 aa authoized. 3. REPORT TITLE: Ente the complete epot title in all capital lettea. Titlea in all caaea ahould be unclassified. If a meaningful title cannot be»elected without clasaificalion, show title claaaification in all capitala in paenlheaia immediately following the title. 4. DESCRIPTIVE NOTES: If appopiate, ente the type of epot, e.g.. inteim, pogess, summay, annual, o, final. Give the inclusive dales when s specific epoting peiod ia coveed. 5. AUTHOR(S): Ente th- named) of autboha) aa ahown on o in the epot. Ente laat name, tumi name, middle initial. If militay, ahow ank and banch of sevice. The name of the pincipal autho ia anabaolute minimum equiement. 6. REPORT DATE: Eme the date oi the epot aa day, month, yea, o month, yea. If moe than one date appeaa on the epot, use date of publication. 7a. TOTAL NUMBER OF PACES: The total page count ahould follow nomal pagination pocedue», :.«., ente the numbe of pages containing infomation. 76. NUMBER OF REFERENCES: Ente the total numbe of efeences cited in the epot. 8a. CONTRACT OR GRANT NUMBER: If sppopiate, ente the applicable numb.' of the contact o gant unde which the epot waa witten c, I id. PROJECT NUMBER: Ente the appopiate militay depatment identification, auch aa poject numbe, aubpojfcl numbe, system numbe», taak numbe, etc. 9a. ORIGINATOR'S REPORT NUMBER(S): F..e the official epot numbe by which the document will be identified and contolled by the oiginating activity. This numbe must be unique to this epot. 96. OTHER REPORT NUMBERS): If the epot hea been aaaigned any othe epot numbes (eithe by the oiginato o by the spomo), alao ente thia numbefs). 10. AVAILABILITY/LIMITATION NOTICES: Ente any limitations on futhe dissemination of the epot, othe than those impoaed by secuity claaaification, using standad atatementa such aa: (1) "Qualified equestes may obtain copiea of this epot fom DDC." (2) "Foeign announcement and diaaemination of thia epot by DDC is not authoized." (3) "U S. Govenment agencies may obtain copiea of this epot diectly fom DDC. Othe qualified DDC uses ahall equest though (4) "U. S. militay agencies may obtain copies of thia epot diectly fom DDC. Othe qualified uses shall equest though (5) "All distibution of this epot is contolled. Qualified DDC uses isalt equest though If the epot haa been funished to the Office of Technical Sevices, Depatment of Comaece, fo sale to the public, indicate thia fact and ente the pice, if known. 11. SUPPLEMENTARY NOTES: Uae fo additional explanatoy notes. 12. SPONSORING MILITARY ACTIV.: Ente the name of the depatmental poject office o laboatoy sponsoing (paying fojihe eseach and development. Include addess. 13. ABSTRACT: Ente an abstact giving a bief and factual summay of the document indicative of the epot, even though it may also appea elsewhee in the body of the technical epot. If additional apace ia equied, a continuation aheet ahall be attached. It ia highly desiable that the abatact of classified epots he unclassified. Each paagaph of the abatact ahall end with an indication of the militay aecuity claaaification of the infomation in the paagaph, epesented ss (TS), (S), (C), o (V). Thee is no limitation on the length of the abatact. Howeve, the suggested length ia fom ISO to 225 wod». 14. KEY WORDS: Key wods ae technically meaningful tema o shot phaaea that chaacteize a epot and may be used aa index entiea fo cataloging the epot. Key woda must be selected so that no aecuity claaaification ia equied. Identifies, such aa equipment model deaianation, tade name, militay poject code name, ceogaphic localion, may be uaed as key wods but will be followed by an indication of technical context. The assignment of links, ules, and weight» is optional. Unclassified Secuity Clasaificacion