Glossary of Multics acronyms and terms. Entries byTom Van Vleck ([THVV]) unless noted.

packed pointer

[BSG] Hardware feature on the6180, and a much rarer software format on the645, that encodes a virtual memory pointer (segment and word number, plusEIS bit offset on the 6180) in one 36-bit word. While 50% more storage-efficient than the two-wordITS pointer, a packed pointer cannot be used for indirection (seeeppbp) or the storage of a value where ring number had to be saved (seeTRR). Also, only twelve bits are available for the segment number (instead of 15 in an ITS pointer), and this is the tightest limit on the maximum number of segments in a process, far too small. ThePL/I compiler made heavy internal use of packed pointers (which have to be loaded with a special instruction, again, seeeppbp), but because of the above, their use in general system code is quite uncommon.

page

Fixed size piece of memory. The hardware supports both paged and unpaged segments. TheSDW for a paged segment points to apage table, an array ofPTWs (stored in theAST). Each PTW points to a page incore, or has a fault tag and an indication of where on secondary storage the page's contents can be found. The645 hardware supported two page sizes, 64 words and 1024 words. Bookkeeping and storage pool management was so complex that small pages were eliminated in 1968.

page control

[BSG]Ring 0,wired software responsible for managing the transfer and mapping of pages in and out of memory. Only a small amount of this is the famous "page replacement algorithm" (seeclock algorithm) due toCorbató; most of this code, almost all complex and highly-optimized assembler language, is the management of I/O and state, themultilevel paging hierarchy management, and interaction with and services provided forsegment control and other areas of the system. Page control is perhaps the most asynchronous, difficult, and critical component of the whole system. Successive gurus who worked there and left their marks wereStan Dunten,Steve Webber, andBernie Greenberg.

page fault

Still called that, or "missing page exception." When an instruction references a page whosePTW says it's not present, the hardware takes a pagefault. The hardwarefault vectors point to theFIM, (orwired_fim) which saves the machine state and invokespage control.

page multilevel

[BSG] Scheme implemented inMSS 16.0 (1971? 72?) bySteve Webber andRick Gumpertz to dynamically optimize the use of storage of three (or conceivably more) technologies and size/cost/speed tradeoffs by migrating data (in Multics, pages) "down" to successively slower, larger finite stores as more recently used data is moved "up", replacing it in the faster, smaller levels, as the system runs. An extension of the notion of demand-paged, least-recently-used (LRU)-managedcore to many levels, PML was similar to schemes being innovated on other operating systems at the time. [documented in Webber &Greenberg, "The Multics Multilevel Paging Hierarchy", Proc 1975 IEEE Intercon, 1975].

Multics PML arose (on the645) as the culmination of various schemes utilizing the small, fastfirehose drum for "the most used"segments, temporaryprocess directory segments, and other ultimately unsatisfactory formulae broadly classified as "static migration." PML, in contrast, by operating on pages and not segments as the system runs, is in essence the ultimate dynamic migration scheme. Multics PML had only one intermediate level, generically known as the "paging device", which was at first the firehose drum and then (on the6180) thebulk store.

PML occasioned the largest and most radical rewrite ofpage control in its history, a quantum escalation of the even theretofore unwieldy complexity of thisALM-codedsupervisor subsystem. PML's interaction withsegment control (activation and deactivation) was particularly complex (and a source of bugs), and the support of pages of non-active segments on the paging device was abandoned withNSS. Although PML was wholly excised from Multics in the early 80s when the economics of memories rendered the bulk store obsolete, PML remains IMHO one ofSteve Webber's titanic masterpieces.SeeRead-Write Sequence.

page table

[BSG] Array of hardware control words (Page Table Words, orPTWs) that define the actual location incore of thepages of asegment (orpage control data to locate the page on secondary storage if it is not in core.) The essence of the Multicsvirtual memory is that there is one page table per segment, as opposed to other schemes (e.g., Intel) where one or more page tables define some larger linear address space of which segments are subsetted. Page tables (like the segments they define) are per-system, not per-process. TheSegment Descriptor Word for a segment in theDSEG of each process in which the segment isknown (see alsoactive) contains the (same)absolute address of its page table.

A segment's page table is part of itsAST entry, and thus all page tables reside in onewired, unpaged segment, theSST (System Segment Table). Page tables come in four sizes, corresponding to segments of 4K words, 16K, 64K, and 256K, the sizes of these pools of each being system tuning parameters (seeconfig deck), and are kept in lists by pool for purposes of multiplexing. The limitation on page tables residing in one segment (built into theALM code of page control everywhere) ultimately proved to be a major bottleneck [latter-day Guru: was this ever addressed?]. SeeAST trickle.

page trace

Ring-buffer in the firstpage of aprocess'sPDS in which an entry is made for every page paged in since a process gainedeligibility. This is used to drive thepre-paging algorithm.

paging

(1) The division (by the645 /6180appending hardware) of asegment into fixed-size pieces, called pages, which can be present incore or not. If a page is not in core, a reference to it causes the CPU to fault, and the software that handles this fault (page control) finds a free core frame and fetches the page's contents from secondary storage (i.e., disk).

(2) The system's activity of handling page faults, fetching requested pages, and writing out other pages to make room. See"The Multics Virtual Memory: Concepts and Design."

paging device

Device used as the intermediate level store in a multilevel paging hierarchy ("page multilevel"), at first thefirehose drum, then later the bulk store.

PAL

(Pedagogic Algorithmic Language) Programming linguistics language created by MIT Prof.Art Evans, using ideas fromMartin Richards. Used in the first MIT course taught using Multics, 6.231, in the summer of 1969.

Palyn Report

Report prepared in 1978 by Palyn Associates forHoneywell corporate, recommending long range large systems plans. Story:"The Palyn Report".

PARC

[Earl Boebert] (1) GE-speak acronym for "Purchased and Rented Computers." Not used in Honeywell until after themerger in 1970, then common amongst beancounters and marketing weasels. "Churning the PARC" was the term for getting your rented machines back before you had squeezed every dime out of the customer. Used frequently in arguments against a strong Multics marketing effort.

[THVV] (2) Xerox Palo Alto Research Center, source of many innovations.

partition

Contiguous region of a disk volume. TheBOS PART configuration card defined partitions.

Pascal

Grenoble wrote a Pascal compiler for Multics in the 1980s, and James Gosling wrote a Pascal at Calgary for Multics.

[Ian G Batten] The Multics Pascal compiler had a flag to accept French keywords. SI, ALORS, FIN, all that.

password

[JHS] Secret character string used to authenticate the claimed identity of aperson.

[THVV] Multics passwords are stored in thePNT encrypted by a one-way function. They are eight characters long, and thelogin command has optional arguments to generate a pronounceable password for you. (Code written byMorrie Gasser of MITRE in connection with ProjectGuardian.) This facility can be invoked automatically after a password has expired. Story:"Multics Security".

TheDOCKMASTER site implemented one-time passwords using theWATCHWORD device.

path name

Multics segments are named by a sequence of entry names, separated by> characters, e.g.>udd>m>vv>go.ec. This path name provides a path from theroot directory to the segment. Pathnames areabsolute (descend from the root) orrelative to the currentworking directory.

PC support

Support for file transfer and terminal emulation between a PC and Multics. Feature in MR11.0? SeeMOWSE.

pdd

(or PDD,process_dir_dir). Alternate names of thedirectory>process_dir_dir, whose subdirectories are users'process directories, created and destroyed as processes are created and destroyed. CompareUDD.

PDS

[BSG] Process Data Segment. A per-process,ring 0 segment, containing per-processsupervisor data (other than theKST anddescriptor segment) as well as the ring 0stack of the process. The PDS has the same segment number in every process, and contains many symbolically-referenced data objects. When a process is madeeligible by thescheduler, the first page of the PDS, which contains data needed bywired code, including places forfaults to storeSCU data, is wired. ComparePRDS.

PDT

Project Definition Table. The per-project segment that defines whichusers maylog in on a project toUser Control. Aproject administrator can update this segment online by editing an ASCIIPMF file, compiling it withcv_pmf, and sending theanswering service a request to install it withup_pmf. The project administrator could specify optional per-user resource andspending limits here.

Perigon

[David Schroth] Multics site: A software engineering solutions company in Calgary Alberta, Canada, formerly known as ACTC Inc. Spinout (1983) from the University of Calgary, partly owned byBull. Did contract maintenance for all Multics sites. Acquired by CGI Group Inc. subsidiary CGI Information Systems and Management Consultants Inc. in September 1998. At that time CGI operated a dual 8/70M in Calgary. Formal support is no longer offered to the Multics community.

[THVV] The Perigon machine was shut down in July 2000. See theUC site history.

person

Multics registers individual people in thePerson Name Table (PNT). Each person has a name, apassword, a default project, and some historical information. This registration is done by the system administrators.Project administrators can then register persons on theirprojects, creating auser entry in the project'sPDT. One person can be registered as more than one user.

Phase One

Integration milestone during Multics development. The first time that Multics booted from tape and came up to type on theoperator's TTY. Accomplished December 1967 byNoel Morris andTom Van Vleck. Story:"Phase One".

phcs_

PrivilegedHardCore Supervisor gate. Agate which allows semi-privileged operations, such as examining the contents ofring 0, but doesn't let the user do things that might crash the system. A user with access tophcs_ is trusted, since this gate can be used to examinepasswords in the input buffers. Compare withhphcs_.

PHM

Page Has been Modified. Flag set by the CPU in aPTW when a page is stored into, used by thepage control replacement algorithm. (Seeclock algorithm.)

Phoenician

[EAR] AMultician who worked inPhoenix. SeeCISLian.

Phoenix

City in Arizona, whereLISD built large computers. Used metonymically to refer to LISD management.

photocomposition

Feature ofcompose.

PHU

Page Has been Used. Flag set by the CPU in aPTW when a page is read, used by thepage control replacement algorithm. (Seeclock algorithm.)

PHU1

[BSG] "Page used in quantum (time since madeeligible)". A softwarePTW flag set whenever theclock algorithm passed over aPHU and turned it off. Thus, PHU means "used since last pass of the clock hand" and PHU1 "used in quantum."

physical volume (PV)

[BSG]NSS term meaning "a disk pack". The contrast is withlogical volumes, groups of PV's: users can specify on which LV a segment is to reside, but not on which PV - that is up to the system alone. PV's, however, are the unit of data integrity and recovery, and this non-transparency in the illusion is apparent.

pipes

Unix feature added to the Multicsshell in the mid 80s. One could get the effect of pipes using io_call and stream manipulation from the beginning of the system, but the vertical bar pipe syntax was added to make this more convenient.

PIT

Process Initialization Table. Per-process segment in theprocess directory where the system stores constants determined at process creation time. Accessed only through theuser_info_ subroutine.

PL/I

[BSG] Programming Language #1, invented by George Radin of IBM in 1964. Combining scientific features from FORTRAN, business features like records from COBOL, and block structuring from Algol,PL/I's future looked rosy in 1965 when the team chose it for the Multics systems programming language and the idea of coding an operating system in a "higher level language" was radical. Multics's storage models, procedure models, data types, and run-time architecture were all designed around those of the PL/I language, and all system APIs were given in PL/I. Use of any other language for systems programming tasks was actively and officially discouraged. Nonetheless, over the years certain critical subsystems of substantive complexity and bulk (notablypage control and thescheduler) were recast into assembler language to optimize performance. Theversion 2 Multics PL/I implementation (Bob Freiburghouse andBarry Wolman, chief architects), was arguably the preeminent PL/I implementation of record. Paper:"The Multics PL/I Compiler" by Bob Freiburghouse.

pl1_operators_

Segment referred to by every compiledPL/I program, containing open code (threaded code) cliches for common constructs, includingcall/save/return. Normally every process shares the same (wired) copy of the segment, but a user can modify the operator pointer in thestack root to switch operators to, for example, test a new version or to turn on call tracing.

pln

[BSG] Print with Line Numbers. A private command byMax Smith that was used heavily in the development community, very similar to UNIXgrep. Although the Multics product had nothing like it, widespread feeling that it was notMulticious, as it had no obvious place in an organized, extant or proposed suite of tools, and an immensely useful feature of "knowing" about the insides ofarchives that raised questions about other commands, prevented it from ever being made part of the product. While pln's many fans were often frustrated by the double-edgedness of the at-times conservative sword of Multiciousness, and could not understand why the world at large could not benefit from this incredibly useful tool, precisely such a profusion of diverse, unrelated tools is in fact one ofUnix's most well-known flaws.

PMDC

Phoenix Multics Development Center. The group ofMulticians located in Phoenix at the HoneywellCamelback Road Facility. Worked on MRDS, compose, system release, trouble report handling, and communications.

PMF

Project Master File. A per-project ASCII file that can be compiled into aPDT withcv_pmf by aproject administrator.

PML

MSS 16.0 andpage control jargon forPage Multilevel.

PNT

Person Name Table.User Control'spassword anduser name registration database.

pointer register

[BSG] Register on the6180 (or "pointer pair" on the645) containing asegment number and a word number within that segment (and on the 6180, aring number and a bit number forEIS), i.e., a generic pointer into the segmented address space. Other than constant and code references, Multics instructions usually make references relative to pointer registers, for the same reasons of relocatability that obtain in any architecture. On the 6180, the pointer registers visible to the Multics instructions are actually fabricated illusions constructed of "address registers" shared with the non-Multics (GCOS) CPUs and segment and ring registers managed by theappending unit, which lock-step mimics everyCU operation on the "address registers" to create the software illusion ofpointer registers. On the 645, the "pointer pairs" were wholly within thebase frame, being built of 8ABRs which could function either as segment or word pointers and be associated into pairs. During normal MulticsPL/I code, pointer registers are reserved for the stack frame, the entry vector ofpl1_operators_, and less strictly, the procedure's actual parameter list, itslinkage section, and theoperator table at the base of the currentstack. In addition to numbers 0, 1 ... 7, the pointer registers also hadnames,ap,ab,bp,bb,lp,lb,sp,sb, based upon their conventional use in Multics PL/I object code. .

POLTS

Peripheral OnLine Test System run byTOLTS. SeeAU77,Multics Online Test and Diagnostics Reference Manual.

Port Logic

[BSG] That portion of the6180 processor that takesabsolute addresses and stores or returns data for them. Thecache is an integral part of the port logic, whose main database is the (single, global) CPU 72-bit memory data bus. The port logic also has to deal with many non-memory operations that are implemented in theSCU, such as clock reading and interrupt generation, and is responsible for selecting ports based upon address. As main memory (i.e., theSCU) is a fully asynchronous external component, the port logic can manage simultaneous, overlapped operations on different SCUs, and route the completion notifications back to the right module of the CPU (seeControl Unit,Appending Unit). This facilitatesinterlace, a port logic featurette whereby alternate word-pairs are addressed to alternate SCUs for maximum overlap. The 6180-proper port logic has a four-element hardware ring buffer in which it queues and from which it processes requests and notifications.

post-purging

To hasten theeviction of "a process's pages" fromcore at the time it losteligibility. Seepre-paging

PPR

[BSG] Procedure Pointer Register - a tripartite processor register defining thesegment,ring, and location of the next instruction to be executed, the Multics equivalent of the "instruction counter", which is the "location" component of the above. The processor actually implemented the three components on three different boards in three different ways, but Multics chose to conceptualize it as one. Thering component ("PRR") defines the current ring of execution. CompareTPR,TRR.

PRDS

[BSG] Processor Data segment. A per-processor, fully-wiredring 0 segment, containing not only symbolically-referenced per-processor data but astack used when runningfault andinterrupt handlers that cannot takepage faults orinterrupts, such aspage control itself, and thescheduler. The processor carries around theSDW for the PRDS from process to process like a carpet-bag, dropping it in the same slot in thedescriptor segment of each process as the processor moves from process to process, literally holding it in the accumulator during every process-switching instruction (LDBR). ComparePDS.

pre-paging

[BSG] Feature of thepage control/traffic control interaction which tries to hasten theeviction of a "process's pages" from memory at the time it loseseligibility, and to read those pages in in parallel at the time a process regains eligibility, before letting that process run. The notion of "a process's pages", of course, does not flow naturally from thevirtual memory concept, and the choice of which pages that a process had referenced since becoming eligible (seepage trace) were to be considered for pre-paging and/orpost-purging is controlled by a bit map that maps various criteria, such as being aprocess-directory page, being used since the last sweep of the page-replacementclock algorithm (seePHU,PHU1), being modified (seePHM), being on thedrum orpaging device into the decisions to pre-page, post-purge and "turn off the used bit". If my memory serves me well, as it were, this mechanism sported more controls than anyone knew how to use. Seeswapping.

prelinker

[BSG] (1) The Multicssupervisor is pre-linked statically at systembootload time by a pair of assembly-language programs known as the "prelinker".

(2) In the late 70s, the capability of pre-linking select users' address spaces as a performance boon was added. This function is done by a separate "prelinker."

PRHA

Multics site: Puerto Rican Highway Authority. San Juan, Puerto Rico. Installed 1977. SeePRHA site history.

Primos

Prime Computer's operating system. Heavily influenced by Multics and Multicians.

printer

[BSG] Multics supported the standard-issueGE andHoneywell line printers of the era of fanfold paper, fitted with special type-bars and chains to support theASCII character set. As these printers were designed for a 6-bitBCD character set (and were thus incredibly fast at printing foot-thick dumps and the like), printing upper-and-lower-case characters required odd modes and complex escape sequences. Although softwarily possible, users never attach printers or direct output to them -- the only mode of printer use is to queue ASCII files to be printed by the Printerdaemon, using thedprint (laterenter_output_request) command.

Before the era of GUIs and WYSIWYG editing, fanfold listings were an invaluable aid in developing and debugging programs. Story:"Multics Printer Software".Info segment forenter_output_request command

privileged mode

[BSG] TheGCOS versions of the Multics processors implemented a traditional one-bit hardware user/supervisor distinction: the CPUprivileged mode ("master mode" on the645) enabled system-control instructions, which were otherwiseillegal. In Multics,privileged became a bit (seeREWPUG) in theSDW, effectively ANDed withring of execution beingring 0. Very fewsupervisor segments had this bit. Today, modern processors only have rings.

probe

Multics high-levelPL/I source language debugger. Seedebug.Info segment forprobe command

process

An address space and a locus of control. A Multics process is a complex object, because it has a complicated address space.

Article:Multics Execution Environment.

process directory

[BSG]Directory created when a process is created for exclusive use of that process, namely per-process and scratchsegments, includingstacks (including thePDS) andcombined linkage segment.Disk quota in the process directory is not charged against the user. All process directories are subdirectories of>PDD

process overseer

Procedure called at process initialization time. It sets up the process environment and then calls the process'slistener.load Control has a table of process overseers and their load control weights.

process preservation

Multics can save a process across a communications interruption. A user can later reestablish communications and connect to or destroy the preserved process.

profile

(1) Facility to count the number of times each source statement in a program is executed. Compile a program with the-profile option, run it, and then useprint_profile to print out per-statement counters in itsstatic section. SeeMultics Execution Environment.

(2) Per-user segment used by theabbrev command to store abbreviations.Info segment forprofile command

program_interrupt

After aQUIT signal orfault in an interactive program that establishes a newshell command level, a user can issue theprogram_interrupt, pi command. This command signals the PL/I condition "program_interrupt". If the interactive program has established a handler forprogram_interrupt, the handler will re-enter the interactive command loop. Theqedx editor, for example, has such a handler, so that if a user issues a very long-running command and then repents, he can QUIT and issue thepi command to resume editing, discarding the whole command execution safely.Info segment forprogram_interrupt command

project

[BSG] Group of Multicsusers working together, for the same firm or organizational division, for whom Multics can pool alldisk record quota and establish common access rights, and grant other group privileges. The system registers a name andpassword for eachperson; when a person logs in, he or she specifies aproject (possibly by default), and the combination is called auser. ThePDT lists a project's users and records resource consumption for each.

project administrator

[BSG] Person who administers a Multicsproject. Among the responsibility of the Project Administrator is registering and deregisteringusers on the project (a user can only log in if he or she has a project to log in to), and creatinghome directories under theproject directory.

[THVV] Decentralized management of the user community is an important feature ofUser Control; the system administrators establish the registration of persons, and their initialpasswords, but project administrators can register persons as users of their projects without consulting the system administration, and modify users'resource limits and get reports of current usage.

project directory

[BSG]Directory for a givenproject, always a subdirectory of>udd. The subdirectories of the project directory are users'home directories. In the directory pathname>udd>Multics>Morris,>udd>Multics is the project directory.

[THVV] Project administrators can movedisk quota from the project directory to individualusers' home directories.

Project MAC

MIT Project MAC, suggested by J.C.R.Licklider, was the organization that began the research that led to Multics. It is now called the MITLaboratory for Computer Science.

Story:Project MAC.

[Kenneth Flamm] Project MAC was a large and well-funded effort. Its initial grant from DARPA for a little over $2 million per year was quickly raised. Funding peaked at $4.3 million in 1969, slumped to under $3 million in 1973, and rose again in the late 1970s. Project MAC's research staff peaked in 1967 at 400.

PRR

Procedure Ring Register. Part of thePPR. Stored in theSCU data.

PTL

[BSG]Page Table Lock. More accurately, alock on all the data-bases ofpage control, that is,PTWs,core frame tables, andpaging device map. This lock, protecting a huge amount of performance- and integrity-critical data, is used only bywired code masked against interrupts, includinginterrupt andpage fault handlers, so it was designed as a "spin lock", which would only be found locked on multi-processor systems. As Multics grew, the necessity of alleviating this growing performance bottleneck resulted first in theCow's Stomach, and later in an extremely non-modular tryst oftraffic control and page control, also byGreenberg, allowing processes to wait (seenotify) on the Page Table lock as a highly special case.

PTW

Page Table Word. 36-bit word interpreted by the hardware, which contains theabsolute address of apage, or a fault tag and an indicator of where the page is on secondary storage.

PUMA

[RRR] Proposal to Boeing (Multics as a document processing system for Boeing's technical manuals for its commercial aircraft). Seems like 1978 or so? And what the heck did "PUMA" stand for again? "Publishing Utility for Manual Automation???"

pure procedure

Multics compilers all generate code that does not modify itself. This may seem like a no-brainer, but it was quite radical when introduced: OS/360 struggled with the concept of "reentrant" (same thing as pure procedure), "serially reusable," and another flavor, forget the name, of program.CTSS stored into its code in lots of places, but we realized that this would be very bad on a multiprocessor system.

PVT

Physical Volume Table.NSS table listing physical volumes.

pxss

Process exchange, switchstack, theALM-coded module which performed Multics processorscheduling and dispatching. Source code:pxss.alm.