THE UNIVERSITY 0 F M I CH I G A Technical Report 12 FLEXIBLE "HIGH-SPEED INTERFACE BETWEEN IBM 1800 AND DEC PDP-7 COMPUTERS R. F. Brender J.L. Foy, Jr. CONCOMP: Research in Conversational Use of Computers F.H. Westervelt, Project Director ORA Project 07449 supported by: ADVANCED RESEARCH PROJECTS AGENCY DEPARTMENT OF DEFENSE WASHINGTON, D.C. CONTRACT NO. DA-49-083 OSA-3050 ARPA ORDER NO. 716 administered through: OFFICE OF RESEARCH ADMINISTRATION ANN ARBOR November 1968

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ABSTRACT The principal function of the interface is to transfer blocks of data from the memory of either computer to the memory of the other. Once initialized, the interface transfers data via cycle-steal with respect to each of the two computers' memories. Either computer can detect the status of the interface at any time during a data transfer, and can halt or modify the operation as desired. When a transfer operation has been completed, the interface can signal either or both computers. Special circuitry permits resolution of conflicts resulting from simultaneous attempts by both computers to use the interface. Provision is also included for direct signaling of either computer by the other, via an "interruption." The operation of the interface during data-transfers is completely controlled by registers internal to itself which can be loaded by either computer; and the condition of the data-handling circuits is indicated at all times by these same registers, which can also be read by either computer. This allows the interface to be commanded fully by either computer or by both. The use of addressable registers to control the interface permits considerable and unusual flexibility in its operation. Independent counters govern a parallel-to-serial-to-parallel data path via a shift register. This allows flexible control of iii

data density and format. This feature permits efficient use of data storage devices (including the computers' own memories), and is especially desirable when the two computers differ in memory word size as in this case. V

TABLE OF CONTENTS Page ABSTRACT......................................... iii I. INTRODUCTION.............................. 1 II. FUNCTIONAL ORGANIZATION..................... 7 7 2.1 Control Registers...................... 10 2.2 Data Movement.......................... 11 2.3 Control Functions....................... 13 III. INTERACTIVE COMPUTER PROTOCOLS............. 16 BIBLIOGRAPHY..................................... 19 vii

I. INTRODUCTION This is the third of three related reports describing work performed by members of the Logic of Computers Group, a research unit of the Department of Computer and Communication Sciences at The University of Michigan. The Logic of Computers Group computer facility consists of two, small, general-purpose computers and related peripheral equipment. It is intended to provide a vehicle for heuristic investigation of problems involving large-scale simulations of generalized adaptive systems, including a large class of biologically oriented models. The first two reports* document those portions of the system software that are largely or completely finished, and that are not likely to undergo further substantial development. They are intended to 1. serve as a progress and research reports describing the capabilities of the current software, 2. serve as a user's manuals, and 3. provide enough system information to allow later users *Brender, R.F., Frantz, D.R., Foy, J.L. Jr., and Schunior, T.W., Specialized System Software for Interacting DEC PDP-7 and IBM 1800,. Technical Report 11, Concomp Project, University of Michigan, Ann Arbor,. October 1968. Frantz, D.R., Brender, R.F., Foy, J.L. Jr., LOCOSS: A Multiprogramming Monitor for the DEC PDP-7, Technical Report 10, Concomp Project, University of Michigan, Ann Arbor, October 1968. -1 -

-2 - to modify or maintain the system. The LOCOSS system for the PDP-7 and the 1800 file system are basic and flexible tools. Descriptions of several other systems components are included for completeness. The hardware configuration is summarized at the end of this section. In general, the TSX system provided by IBM is the basic software nucleus for the 1800. LOCOSS is the basic software nucleus on the PDP-7. LOCOSS, the Logic of Computers Operating System for the PDP-Seven, was developed to provide a suitable run-time environment in which to run application programs. It provides buffered, overlapped, and essentially device-independent input/ output. A keyboard Command Interpreter provides a number of real-time control services and simple debugging aids. Multiprogramming capabilities are an essential part of the system organization and allow flexible organization of application programs. LOCOSS, in our estimation, provides unusually flexible capabilities and services on a machine of this size, and requires less than 2K (decimal) of core. The availability of bulk storage on the 1800 disk via the "minor" 1800-PDP-7 interface (in use since April 1968) made it feasible to provide system programs and, perhaps more importantly, user source files, "on-line." To implement this, a disk file system was developed for the 1800. This system provides variable-length, serial-bycharacter data files to both 1800 and PDP-7 users. Both

-3 - symbolic and binary data are kept on-line in this manner. A keyboard utility routine on the 1800 provides simple means to load, dump, list, or copy from or to all 1800 I/O devices and the disk files. Even the approximately tripled listing rate possible with the 1050 printer (15 characters per second, hardware tabs) has been very useful. The pace of program development accelerated greatly as it became possible to be more and more disk-dependent. It was necessary to provide a new text editor because modification of the available one proved impossible. In addition to taking advantage of the deviceindependent I/O of LOCOSS, the editor provides a couple of string search and replacement commands that are quite useful, The PDP-7 Assembler was adapted to accept disk file input, although it still punches object code on tape. DDT was made available on-line, and may be loaded and called from LOCOSS. A very powerful macro language, ML-I, was adapted to the disk I/O and made a part of the system. Thus program creation, editing, assembly, debugging, and execution all take place on-line under control of LOCOSS with a minimum of superfluous hard-copy generation. While the current interface is sufficiently fast for these human-oriented tasks, it will not suffice for the kind of interactive processing desired for the problem-oriented system. Therefore, the authors designed the high-speed general-purpose interface described in this report. It offers flexibility and control substantially beyond current inter

-4 -facing practice as we know it. The general ideas employed in it should be very useful in other multiple computer systems (as opposed to multiple CPU systems with common memory). Implementation of this interface should be completed by the end of 1968; more complete reports on it will be issued later.

PDP-7 System Summary CPU 8K of 1.75 sec core 18 bits/word hardware interrupt Teleprinter (33KSR) 10 chars per second Paper Tape Reader 8-channel 300 char per second Paper Tape Punch 8-channel 63 char per second Dataphone (201A) synchronous 2000 bits per second connected to switched network CRT Display (Modified 338) A display consisting of a DEC 338, less the PDP-8 portion of the 338, is interfaced to the PDP-7. This is locally known as a 337 and is the prototype for the DEC 339. The display operates asynchronously from instruction files in the PDP-7 memory. It provides point, increment, short vector, vector, and character plotting modes and is capable of branches and subroutining as well as conditional branches, depending on the state of user-controlled switches. -5 -

1800 System Summary CPU(1801C2) 16K of 2psec core 16 bits/word + parity and storage protection priority interrupt system (12 levels) 3 index registers 1- and 2-word instruction formats 4 data channels Keyboard-Printer (1816) 15 chars/sec Card Read-Punch (1442) read 300 cards per minute punch 60 cards per minute Disk (2310A1) 1 drive movable head interchangeable cartridges (2315) 512,000 words per cartridge -6 -

II. FUNCTIONAL ORGANIZATION This report describes the functional design of a highspeed interface between a Digital Equipment Corporation PDP-7 and an IBM 1800. The need for such an interface arose from a desire to integrate both computers into a single system for performing highly interactive, or "conversational," simulations of certain classes of processes. The PDP-7 is equipped with a cathode-ray-tube display for interactive purposes, and also with a medium-speed (AT&T 201A) dataphone interface for communication with the University's timesharing system. In addition, it has the standard inputoutput devices: high-speed paper tape reader and punch, and Model 33 KSR teletype. The 1800 is equipped with a one-millionbyte disk in addition to the usual I/O devices: card reader and punch, and printer-keyboard. Effective use of both machines in the simulation system requires that the PDP-7 have access to the 1800's disk, and that the 1800 have access to the PDP-7's display; both of these functions require rapid transfers of data between the two machines. A relatively low-speed, single-character-at-a-time interface has been in constant use on the above system for the past five months, providing useful experience in the problem of interface design while allowing the development of software for the interactive system to proceed. The new inter-7 -

-8 - face will greatly increase the speed of operations currently being performed, and will allow many other functions which are currently impossible or unwieldy. The principal function of the interface is to transfer blocks of data directly from the core storage of either computer to the core storage of the other. The amount of data to be transferred, and its origin and destination, is specified by registers within the interface. These registers can be loaded by either computer, with or without the active cooperation of the other. Once initialized, the interface transfers data under the control of its own circuitry in conjunction with timing signals generated by the two computers' cycle-steal/data-break facilities; no attention to the transfer is required of either central processing unit. Either computer can, however, detect the status of the interface at any time during a data transfer, and can halt or modify the operation as desired. When a transfer operation has been completed, the interface can interrupt either or both computers, allowing them to process the newly moved data, set up another transmission, or take any other desired action, Either computer can initiate a data transfer without interfering in any way with the program running in the other machine, and special circuitry has been included to permit reliable resolution of any conficts resulting from simultaneous attempts by both computers to use the interface. Provision is also included for direct interruption of either computer by the other.

-9 - The interface performs a parallel-to-serial-to-parallel conversion of the data being transferred, which allows efficient use of the memories of the two computers for storing each other's data since the word size differs for the two machines. This technique also makes it possible to transmit only part of each memory word of the PDP-7 to the 1800, a commonly desired operation. The interface consists of a Data Register, several control registers, and associated timing and control circuitry. The 34 -bit Data Register is composed of a 16-bit (A) and an 18-bit (B) section; the 16-bit section can be loaded in parallel from the 16-bit output bus of the 1800, and the 18-bit section can be loaded in parallel from the 18-bit I/O bus of the PDP-7. Similarly, the 16-bit section of the Data Register can be gated to the input bus of the 1800 while its 18-bit section can be gated to the I/O bus of the PDP-7. The two sections of the Data Register, then, are capable of independent, parallel transfers of data to and from their associated computers, while the Data Register as a whole can be shifted left or right. Serial and parallel operations are performed alternately: data are parallel-transferred from the sending computer to its section of the Data Register, the latter then shifts right or left as necessary to move the data into the other section, and then the data are parallel-transferred from that section to the receiving computer.

-10 - 2.1 Control Registers Six control registers are essential to the operation of the interface: these are the A Address Register, the B Address Register, the Unit Count Register, the Status Register, the A Shift Counter, and the B Shift Counter. The first four of these registers are individually addressable. They are capable of being read by either computer, and may also be loaded by either computer (except for certain bits in the Status Register which reflect conditions of the interface not directly controllable by the external machines). The two Address Registers control the address in memory (for their respective computers) to which or from which data will be written or read by the A and B sections of the Data Register. Thus, during an operation in which data are being transferred from the PDP-7 to the 1800, the B Address Register will initially contain the starting address of the block of data in the PDP-7's memory and will be incremented once each time a word is transferred to the interface; similarly for the A Address Register and the 1800. The Unit Count Register initially contains the number of "units" (see later discussion) to transfer and is decremented by one each time a unit transfer is completed. When the Unit Count Register reaches zero, the data transfer operation is considered finished. The effective word size for each computer is determined by the values loaded into the Shift Counters. The frame size for the 1800 is always 16. For the PDP-7, the frame size is

-11 - 8, 12, 16, or 18, as specified by a two-bit code in the Status Register. Thus the A Shift Counter is always reloaded with 16 when it counts down to zero, and the B Shift Counter is reloaded as determined by the Status Register frame-size code. 2.2 Data Movement The following is a detailed description of the events that take place during a transfer from the PDP-7 to the 1800 in the "18/16" mode, and will illustrate the functions of the Data Register and the two Shift counters. When the "start" command is given, the A Shift Counter is set to 16 and the B Shift Counter to 18. A word is loaded from the PDP-7 into the B portion of the Data Register. Shifting is begun, and both shift count registers are decremented for each bit position shifted. Shifting continues until one of the shift count registers reaches zero. In this case, this will occur after 16 shifts, which have moved the low-order 16 bits of the PDP-7 word into the A portion of the data register. Shifting is suspended until the A part of the data register can be stored in the 1800 memory; then the A Shift Counter is reloaded with 16, and shifting continues. After two more shifts, the B Shift Counter will go to zero. At this point, the two high-order bits of the PDP-7 data word are in the A portion of the data register. Shifting is suspended while the next PDP-7 memory word is loaded into the B portion of the Data Register. Then the B

-12 - Shift Counter is reloaded with 18 and shifting resumed. After 14 more shifts, the A Shift Counter will go to zero. The A portion of the Data Register, which now contains two bits of data from one PDP-7 word and 14 bits from the next, will be transferred into the 1800's memory. This process continues in the same manner until both Shift Count Registers go to zero simultaneously. This will occur when an integral number of words has been moved in each machine, and is considered one Unit Operation. The Unit Count Register is then decremented. The entire sequence is started over unless the Unit Count Register has become zero, which signals the end of the transfer operation. When the user wants to transmit-data from the PDP-7 to the 1800 for processing, he uses the "16/16" mode. This mode is very similar to that described above, except that the B Shift Counter is loaded with 16, so that after every rightshift of 16 bits the B section of the Data Register is reloaded with another 18-bit word while the 16-bit A section is being written into the 1800. Reloading the B section after a shift of only 16 places destroys the two high-order bits which remain there; this information is simply lost. However, it is the purpose of this mode to transfer the loworder 16 bits of each word of the PDP-7 to corresponding words in the 1800, preserving the "word" relationships necessary for efficient processing. (Often the data will fit naturally into a 16-bit frame anyway, as when two 8-bit characters are packed

-13 - together, so that no useful data at all are lost by transmitting only 16 bits.) The above two functions, and their inverses, are simply achieved by having the shifting and parallel-transferring of the Data Register controlled by the two Shift Counter registers. (In more general applications, the frame sizes could be given by two additional loadable and readable Frame Size Registers which could be set as desired at the beginning of each transfer. The A Frame Size Register would be used to load the A Shift Counter each time the latter counts down to zero; similarly for the B Frame Size Register.) 2.3 Control Functions The control circuits of the interface are able to accept a number of commands, which may be issued by either computer under program contrd. Most of these commands affect the control registers or other control circuitry; only two of them, Start and Stop, affect the data-handling circuits directly. A Start command causes the interface to begin data transfers as indicated by the control registers, while Stop command halts such an operation at the end of the next unit transfer. The following is a list of the commands which the interface recognizes, with a short description of each: a. Start-begins memory-to-memory data transfers as specified by the control registers; b. Attention-generates an "attention" interruption in the other computer;

-14 - c. Test and Seize-examines the Status Register and sets or does not set an indicator bit depending on whether the interface is available or is in use by the other computer (includes provisions for reliable resolution of contentionsimultaneous attempts by both computers to seize an available interface); d. Blast-resets all control circuits and registers, halting immediately any data transmission operation; e. Sense-makes the contents of the Status Register available for examination to determine the state of the interface; f. Load-allows any of the control registers to be loaded with data from the computer issuing the command; g. Store-allows the computer issuing the command to read any of the control registers (into the accumulator of the PDP7, directly into memory on the 1800); h. Stop-halts current data transfer at the next unit (integral word) boundary. The Sense command always brings data into the computer's accumulator, while the Store command specifying the Status Register brings data directly into memory in the case of the 1800. The Status Register provides detailed control information to both computers and to the interface; the following is a summary thereof:

-15 - a. Interface seized by 1800 b. Interface seized by PDP-7 c. Interface busy (data transfer in progress) d. Operation complete (Unit Count Register = 0) e. Enable 1800 interruption upon operation-complete f. Enable PDP-7 interruption upon operation-complete g. Direction of transmission h. Exception conditions i. Attention code Each of these is indicated by one or more bits in the Status Register. In order to set up a data transfer, a computer first seizes the interface when it is (becomes) free, then loads the A and B Address Registers with appropriate addresses, loads the Unit Count Register with the length of the data block, and sets the bits of the Status Register to indicate direction of transfer and handling of the operation-complete interruption. It then issues a Start command, and the transfer commences. Note that the "seized" bits are merely indicators for use by the two computers' software, their main function being the resolution of contention situations; they do not affect either machine's ability to control the interface.

III. INTERACTIVE COMPUTER PROTOCOLS This interface has simple and unusually flexible control characteristics and allows for a wide range of computer interaction. In cases where one computer has prior knowledge of the desired source or destination of information in the other computer, it can completely initiate the data transfer without interrupting the activity of the other computer. This allows simpler control programs than those where cooperation is required, and requires less time by both machines in servicing the needs of the interface. The design provides for maximum density of information strage when different sized words are involved in the two computers. The following example will illustrate the flexibility and advantage of the variable frame-size control on each parallel data path. The PDP-7 has a word consisting of 18 bits. Attached to this computer is a CRT display (modified DEC 338), which uses instructions stored in the memory of the PDP-7. These instructions always occupy the low-order 12 bits of the 18-bit word. Certain other data frequently handled by the PDP-7 consist of only 8 bits per computer word ("character" data) and also 16 bits per computer word ("packed character" data and 1800 instructions). The 1800 has a 16-bit memory word, and also a disk storage device. The interface described here allows any of the 8-, 12-, 16-, or 18-bit data types to be transferred efficiently to or -16 -

-17 - from the 1800 and stored by the 1800 on its disk. This interface also allows the 1800 to manipulate the display directly by modifying the display file in PDP-7 core storage without requiring the assistance of the PDP-7. Since all of the registers of the device can be loaded or read by both machines, they can be used to pass more than one word at a time when the interface is used in a simple "single word" mode. The following describes a possible computer interaction that can not be achieved with the usual methods: Suppose that the 1800 is sending a block of data to the PDP-7, and further that the PDP-7 has some information for the 1800 that is of a higher priority than the information currently being transferred. The PDP-7 may opt to stop the current transmission in midstream, load those registers with the control data needed for the desired high-priority transfer, and cause that transfer to take place. When completed, the PDP-7 reloads the interface registers with their contents at the time it was stopped and restarts the previous transfer. Since the completion signals provided by the interface are under the control of the registers, the transfer performed by the PDP-7 need not interrupt the 1800 computer. The 1800 machine will, however, be signaled when its own transfer is completed (if it so desired). Thus the 1800 need not even be aware that the PDP-7-initiated transfer took place except for the change in its memory that resulted. Indeed, the PDP7 computer could interrupt its own transfer in a similar manner if warranted.

I/O BUS I IBM 1800 "A" SHIFT COUNT n z 0 r-i C) > 3 C) m CD I 00 i I/O BUS Figure 1. Interface Block Diagram

BIBLIOGRAPHY IBM 1800: Functional Characteristics, Form A26-5918. IBM 1800 Timesharing Executive System: Concepts and Techniques, Form C26-3703. PDP-7 User's Handbook, DEC No. F-75, Digital Equipment Corporation, Maynard, Mass., 1965. Frantz, D.R., Brender, R.F., and Foy, J.L. Jr., LOCOSS: A Multiprogramming Monitor for the DEC PDP-7, Technical Report 10, Concomp Project, University of Michigan, Ann Arbor, October 1968. Brender, R.F., Frantz, D.R., Foy, J.L. Jr., and Schunior, T.W., Specialized System Software for Interacting DEC PDP-7 and IBM 1800, Technical Report 11, Concomp Project, University of Michigan, Ann Arbor, October 1968. -19 -

Unclassified Security Classification -20 - DOCUMENT CONTROL DATA R&D (Security claasification of title, body of abstract and indexing annotation muat be entered when the overall report Ti clasasfied) 1. ORIGINATING ACTIVITY (Corporate author) -a. REPORT SECU.RITY C ASSIFICATION Unclassified THE UNIVERSITY OF MICHIGAN 2b. GCROUP CONCOMP PROJECT 3. REPORT TITLE FLEXIBLE HIGH-SPEED INTERFACE BETWEEN IBM 1800 and DEC PDP-7 COMPUTERS. 4. DESCRIPTIVE NOTES (Type of report and incluieve date.) Technical Report 12 5. AUTHOR(S) (Laot name. first name, initial) R.F. Brender, and J.L. Foy, Jr. 6- REPORT DATE 7. TOTAL NO. OF PAGES 7b. NO. OF REFS November 1968 21 6a. CONTRACT OR GRANT NO. S9. ORIGINATOR'S REPORT NUMBER(S) DA-49-083 OSA-3050 Technical Report 12 b. PROJECT NO. c. 9b. OTHER RPORT NO(S) (Any other numbers that may be aesigned this report) d. 10. A VA IL ABILITY/LIMITATION NOTICES Qualified requesters may obtain copies of this report from DDC. 11. SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY Advanced Research Projects Agency 13. ABSTRACT The principal function of the interface is to transfer blocks of data from the memory of either computer to the memory of the other. Once initialized, the interface transfers data via cycle-steal with respect to each of the two computers' memories. Either computer can detect the status of the interface at any time during a data transfer, and can halt or modify the operation as desired. When a transfer operation has been completed, the interface can signal either or both computers. Special circuitry permits resolution of conflicts resulting from simultaneous attempts by both computers to use the interface. Provision is also included for direct signalling of either computer by the other, via an "interruption." I im The operation of the interface during data-transfers is completely controlled by registers internal to itself which can be loaded by either computer; and the condition of the data-handling circuits is indicated at all times by these same registers, which can also be read by either computer. This allows the interface to be commanded fully by either computer or by both. The use of addressable registers to control the interface permits considerable and unusual flexibility in its operation. I '' DD JAN 1473 Unclassified Security Classification

I1 n 1 cc' i fi A r -21 - Security Classification 14. LINK A LINK B LINK C _____________KEY WORDS__ _ROLE WT ROLE WT ROLE WT PDP-7 IBM 1800 Interface Data Transmission Cycle Steal INSTRUCTIONS 1. ORIGINATING ACTIVITY: Enter the name and address of the contractor, subcontractor, grantee, Department of Defense activity or other organization (corporate author) issuing the report. 2a. REPORT SECURITY CLASSIFICATION: Enter the overall security classification of the report. Indicate whether "Restricted Data" is included. Marking is to be in accordance with appropriate security regulations. 2b. GROUP: Automatic downgrading is specified in DoD Directive 5200. 10 and Armed Forces Industrial Manual. Enter the group number. Also, when applicable, show that optional markings have been used for Group 3 and Group 4 as authorized. 3. REPORT TITLE: Enter the complete report title in all capital letters. Titles in all cases should be unclassified. If a meaningful title cannot be selected without classification, show title classification in all capitals in parenthesis immediately following the title. 4. DESCRIPTIVE NOTES: If appropriate, enter the type of report, e.g., interim, progress, summary, annual, or final. Give the inclusive dates when a specific reporting period is covered. 5. AUTHOR(S): Enter the name(s) of author(s) as shown on or in the report. Enter last name, first name, middle initial. If military, show rank and branch of service. The name of the principal author is an absolute minimum requirement. 6. REPORT DATE- Enter the date of the report as day, month, year; or month, year. If more than one date appears on the report, use date of publication. 7a. TOTAL NUMBER OF PAGES: The total page count should follow normal pagination procedures, i.e., enter the number of pages containing information. 7b. NUMBER OF REFERENCES: Enter the total number of references cited in the report. 8a. CONTRACT OR GRANT NUMBER: If appropriate, enter the applicable number of the contract or grant under which the report was written. 8b, 8c, & 8d. PROJECT NUMBER: Enter the appropriate military department identification, such as project number, subproject number, system numbers, task number, etc. 9a. ORIGINATOR'S REPORT NUMBER(S): Enter the official report number by which the document will be identified and controlled by the originating activity. This number must be unique to this report. 9b. OTHER REPORT NUMBER(S): If the report has been assigned any other report numbers (either by the originator or by the sponsor), also enter this number(s). 10. AVAILABILITY/LIMITATION NOTICES: Enter any limitations on further dissemination of the report, other than those imposed by security classification, using standard statements such as: (1) "Qualified requesters may obtain copies of this report from DDC." (2) "Foreign announcement and dissemination of this report by DDC is not authorized." (3) "U. S. Government agencies may obtain copies of this report directly from DDC. Other qualified DDC users shall request through (4) "U. S. military agencies may obtain copies of this report directly from DDC Other qualified users shall request through,t (5) "All distribution of this report is controlled. Qualified DDC users shall request through If the report has been furnished tc the Office of Technical Services, Department of Commerce, for sale to the public, indicate this fact and enter the price, if known. 11. SUPPLEMENTARY NOTES: Use for additional explanatory notes. 12. SPONSORING MILITARY ACTIVITY: Enter the name of the departmental project office or laboratory sponsoring (paying for) the research and development. Include address. 13. ABSTRACT: Enter an abstract giving a brief and factual summary of the document indicative of the report, even though it may also appear elsewhere in the body of the technical report. If additional space is required, a continuation sheet shall be attached. It is highly desirable that the abstract of classified reports be unclassified. Each paragraph of the abstract shall end with an indication of the military security classification of the information in the paragraph, represented as (TS), (S), (C), or (U). There is no limitation cn the length of the abstract. However, the suggested length is from 150 to 225 words. 14. KEY WORDS: Key words are technically meaningful terms or short phrases that characterize a report and may be used as index entries for cataloging the report. Key words must be selected so that no security classification is required; Identifiers, such as equipment model designation, trade name, military project code name, geographic location, may be used as key words but will be followed by an indication of technical context. The assignment of links, rules, and weights is optional. I GPO 886-551 Unclassified Security Classification

UlIVSITY OF MICHIAN l 3 91111IIIIII02526 7561 3 9015 02526 7561