Editing PDP-11

{{Short description|Series of 16-bit minicomputers}}
{{About|the PDP–11 series of minicomputers|the PDP–11 processor architecture|PDP-11 architecture}}
{{Infobox computing device
| name = PDP–11
| title = 
| aka = 
| logo = [[File:PDP-11 wordmark.svg|frameless|class=skin-invert]]
| logo caption = 
| image = Pdp-11-40.jpg
| image_size = 
| caption = A PDP–11/40 CPU is at the bottom, with a TU56 dual [[DECtape]] drive installed above it.
| developer = [[Digital Equipment Corporation]]
| manufacturer = 
| family = [[Programmed Data Processor]]
| type = [[Minicomputer]]
| generation = 
| release date = {{Start date and age|1970}}
| lifespan = 1970–1997
| price = 
| discontinued = {{End date and age|1997}}
| units sold = around 600,000
| unitsshipped = 
| media = 
| os = [[DEC BATCH-11/DOS-11|BATCH-11/DOS-11]], [[MUMPS|DSM-11]], [[RSX-11|IAS]], [[RSX-11|P/OS]], [[RSTS/E]], [[RSX-11]], [[RT-11]], [[Ultrix]]-11, [[Seventh Edition Unix]], [[SVR1]], [[2BSD]]
| power = 
| soc = 
| cpu = 
| memory = 
| storage = 
| memory card = 
| display = 
| graphics = 
| sound = 
| input = 
| controllers = 
| camera = 
| touchpad = 
| connectivity = 
| platform = DEC [[16-bit computing|16-bit]]
| service = 
| dimensions = 
| weight = 
| topgame = 
| compatibility = 
| predecessor = 
| successor = [[VAX-11]]
| related = 
| website = 
}}
The '''PDP–11''' is a series of [[16-bit computing|16-bit]] [[minicomputer]]s originally sold by [[Digital Equipment Corporation]] (DEC) from 1970 into the late 1990s, one of a set of products in the [[Programmed Data Processor]] (PDP) series. In total, around 600,000 PDP-11s of all models were sold, making it one of DEC's most successful product lines. The PDP-11 is considered by some experts to be the most popular minicomputer.<ref name="bob">{{cite journal |last=Supnik |first=Bob |date=August 31, 2004 |title=Simulators: Virtual Machines of the Past (and Future) |journal=ACM Queue |volume=2 |issue=5|pages=52–58 |doi=10.1145/1016998.1017002 |s2cid=20078751 |doi-access=free }}</ref><ref>{{cite book |last=Rose |first=Frank |date=1985 |url=https://books.google.com/books?id=taKkaFzv8BsC&q=PDP-11 |title=Into the Heart of the Mind: An American Quest for Artificial Intelligence |page=37 |isbn=9780394741031 |access-date=2020-07-19 |archive-date=2024-07-01 |archive-url=https://web.archive.org/web/20240701172348/https://books.google.com/books?id=taKkaFzv8BsC&q=PDP-11#v=snippet&q=PDP-11&f=false |url-status=live }}</ref>

The PDP–11 included a number of innovative features in its [[instruction set]] and additional general-purpose [[Hardware register|registers]] that made it easier to program than earlier models in the PDP series. Further, the innovative [[Unibus]] system allowed external devices to be more easily interfaced to the system using [[direct memory access]], opening the system to a wide variety of [[peripheral]]s. The PDP–11 replaced the [[PDP-8|PDP–8]] in many [[real-time computing]] applications, although both product lines lived in parallel for more than 10 years. The ease of programming of the PDP–11 made it popular for general-purpose computing.

The design of the PDP–11 inspired the design of late-1970s [[microprocessor]]s including the [[Intel]] [[x86]]<ref name="bob"/> and the [[Motorola 68000]]. The design features of PDP–11 operating systems, and other operating systems from Digital Equipment, influenced the design of operating systems such as [[CP/M]] and hence also [[MS-DOS]]. The first officially named version of [[Unix]] ran on the PDP–11/20 in 1970. It is commonly stated that the [[C (programming language)|C programming language]] took advantage of several low-level PDP–11–dependent programming features,<ref>{{Cite web |at=Part I: DEC PDP-11, benchmark for the first 16/32 bit generation. (1970) |title=Section Three: The Great Dark Cloud Falls: IBM's Choice. |work=Great Microprocessors of the Past and Present (V 13.4.0) |last=Bakyo |first=John |url=https://www.cpushack.com/CPU/cpu3.html#Sec3Part1 |access-date=2023-04-30 |archive-date=2023-04-30 |archive-url=https://web.archive.org/web/20230430004127/https://www.cpushack.com/CPU/cpu3.html#Sec3Part1 |url-status=live }}</ref> albeit not originally by design.<ref>{{Cite conference |title=The Development of the C Language |first=Dennis M. |last=Ritchie |author-link=Dennis Ritchie |url=https://www.bell-labs.com/usr/dmr/www/chist.html |conference=Second History of Programming Languages conference |location=Cambridge, MA |date=April 1993 |book-title=History of Programming Languages-II |editor1=Thomas J. Bergin, Jr. |editor2=Richard G. Gibson, Jr. |access-date=2023-04-30 |publisher=ACM Press (New York) and Addison-Wesley (Reading, Mass) |isbn=0-201-89502-1 |archive-date=2015-06-11 |archive-url=https://web.archive.org/web/20150611114355/https://www.bell-labs.com/usr/dmr/www/chist.html |url-status=live }}</ref>

An effort to expand the PDP–11 from 16- to 32-bit addressing led to the [[VAX-11]] design, which took part of its name from the PDP–11.

== History ==
===Previous machines===
In 1963, DEC introduced what is considered to be the first commercial minicomputer in the form of the [[PDP-5|PDP–5]]. This was a 12-bit design adapted from the 1962 [[LINC]] machine that was intended to be used in a lab setting. DEC slightly simplified the LINC system and instruction set, aiming the PDP-5 at smaller settings that did not need the power of their larger 18-bit [[PDP-4]]. The PDP-5 was a success, ultimately selling about 1,000 machines. This led to the [[PDP-8|PDP–8]], a further cost-reduced 12-bit model that sold about 50,000 units.

During this period, the computer market was moving from [[computer word]] lengths based on units of 6 bits to units of 8 bits, following the introduction of the 7-bit [[ASCII]] standard. In 1967–1968, DEC engineers designed a 16-bit machine, the PDP–X,<ref>{{cite web|url=http://www.bitsavers.org/pdf/dec/pdp-x/|title=PDP-X memoranda|website=bitsavers.org|access-date=2017-07-13|archive-date=2017-09-23|archive-url=https://web.archive.org/web/20170923042101/http://bitsavers.org/pdf/dec/pdp-x/|url-status=live}}</ref> but management ultimately canceled the project as it did not appear to offer a significant advantage over their existing 12- and 18-bit platforms.

This prompted several of the engineers from the PDP-X program to leave DEC and form [[Data General]]. The next year they introduced the 16-bit [[Data General Nova]].<ref>{{cite web|url=https://archive.computerhistory.org/resources/access/text/2012/07/102702207-05-01-acc.pdf|title=Oral History of Edson (Ed) D. de Castro|access-date=April 28, 2020|archive-date=2016-03-05|archive-url=https://web.archive.org/web/20160305040632/http://archive.computerhistory.org/resources/access/text/2012/07/102702207-05-01-acc.pdf|url-status=live}}</ref> The Nova sold tens of thousands of units and launched what would become one of DEC's major competitors through the 1970s and 1980s.

===Release===
[[Ken Olsen]], president and founder of DEC, was more interested in a small 8-bit machine than the larger 16-bit system. This became the "Desk Calculator" project. Not long after, [[Datamation]] published a note about a [[desk calculator]] being developed at DEC, which caused concern at [[Wang Laboratories]], who were heavily invested in that market. Before long, it became clear that the entire market was moving to 16-bit, and the Desk Calculator began a 16-bit design as well.<ref name=McGowan98>{{cite web |last=McGowan |first=Larry |url=http://hampage.hu/pdp-11/birth.html |title=How the PDP-11 Was Born |date=19 August 1998 |access-date=2015-01-22 |archive-date=2015-06-17 |archive-url=https://web.archive.org/web/20150617022503/http://www.hampage.hu/pdp-11/birth.html |url-status=live }}</ref>

The team decided that the best approach to a new architecture would be to minimize the memory bandwidth needed to execute the instructions. Larry McGowan coded a series of [[assembly language]] programs using the instruction sets of various existing platforms and examined how much memory would be exchanged to execute them. Harold McFarland joined the effort and had already written a very complex instruction set that the team rejected, but a second one was simpler and would ultimately form the basis for the PDP–11.<ref name=McGowan98/>

When they first presented the new architecture, the managers were dismayed. It lacked single instruction-word immediate data and short addresses, both of which were considered essential to improving memory performance. McGowan and McFarland were eventually able to convince them that the system would work as expected, and suddenly "the Desk Calculator project got hot".<ref name=McGowan98/> Much of the system was developed using a [[PDP-10]] where the SIM-11 simulated what would become the PDP–11/20 and Bob Bowers wrote an assembler for it.<ref name=McGowan98/>

At a late stage, the marketing team wanted to ship the system with 2K of memory{{efn|It is not clear in the document whether this is 2k bytes or 2k words – 4k in modern terms.}} as the minimal configuration. When McGowan stated this would mean an assembler could not run on the system, the minimum was expanded to 4K. The marketing team also wanted to use the forward slash character for comments in the assembler code, as was the case in the PDP–8 assembler. McGowan stated that he would then have to use semicolon to indicate division, and the idea was dropped.<ref name=McGowan98/>

The PDP–11 family was announced in January 1970 and shipments began early that year. DEC sold over 170,000 PDP–11s in the 1970s.<ref name=Ceruzzi03>Paul Cerruzi, ''A History of Modern Computing'', MIT Press, 2003, {{ISBN|0-262-53203-4}}, page 199</ref>

Initially manufactured of small-scale [[transistor–transistor logic]], a single-board [[Integrated circuit#LSI|large-scale integration]] version of the processor was developed in 1975. A two- or three-chip processor, the [[DEC J-11|J-11]] was developed in 1979.

The last models of the PDP–11 line were the single board PDP–11/94 and PDP–11/93 introduced in 1990.<ref name="16-bit Timeline">{{cite web |url=http://research.microsoft.com/~gbell/digital/timeline/16-bit.htm |title=16-bit Timeline |work=microsoft.com |access-date=November 8, 2016 |archive-date=December 8, 2008 |archive-url=https://web.archive.org/web/20081208063850/http://research.microsoft.com/~gbell/digital/timeline/16-bit.htm |url-status=live }}</ref>

== Innovative features ==

=== Instruction set orthogonality ===
{{See also|PDP-11 architecture}}

The PDP–11 processor architecture has a mostly [[orthogonal instruction set]]. For example, instead of instructions such as ''load'' and ''store'', the PDP–11 has a ''move'' instruction for which either operand (source and destination) can be memory or register. There are no specific ''input'' or ''output'' instructions; the PDP–11 uses [[memory-mapped I/O]] and so the same ''move'' instruction is used; orthogonality even enables moving data directly from an input device to an output device. More complex instructions such as ''add'' likewise can have memory, register, input, or output as source or destination.

Most operands can apply any of eight addressing modes to eight registers. The addressing modes provide register, immediate, absolute, relative, deferred (indirect), and indexed addressing, and can specify autoincrementation and autodecrementation of a register by one (byte instructions) or two (word instructions). Use of relative addressing lets a machine-language program be [[position-independent code|position-independent]].

=== No dedicated I/O instructions ===
Early models of the PDP–11 had no dedicated [[bus (computing)|bus]] for [[input/output]], but only a [[system bus]] called the [[Unibus]], as input and output devices were mapped to memory addresses.

An input/output device determined the memory addresses to which it would respond, and specified its own [[interrupt vector]] and [[interrupt priority level|interrupt priority]]. This flexible framework provided by the processor architecture made it unusually easy to invent new bus devices, including devices to control hardware that had not been contemplated when the processor was originally designed. DEC openly published the basic Unibus specifications, even offering prototyping bus interface circuit boards, and encouraging customers to develop their own Unibus-compatible hardware.
[[File:PDP-11-70.JPG|thumb|340x340px|A PDP–11/70 system that included two nine-track tape drives, two disk drives, a high speed line printer, a DECwriter dot-matrix keyboard printing terminal and a cathode ray tube terminal installed in a climate-controlled machine room]]
The Unibus made the PDP–11 suitable for custom peripherals. One of the predecessors of [[Alcatel-Lucent]], the [[Bell Telephone Manufacturing Company]], developed the BTMC DPS-1500 packet-switching ([[X.25]]) network and used PDP–11s in the regional and national network management system, with the Unibus directly connected to the DPS-1500 hardware.

Higher-performance members of the PDP–11 family departed from the single-bus approach. The PDP–11/45 had a dedicated data path within the [[Central processing unit|CPU]], connecting semiconductor memory to the processor, with core memory and I/O devices connected via the Unibus.<ref>{{cite book|url=http://bitsavers.org/pdf/dec/pdp11/handbooks/PDP1145_Handbook_1973.pdf|title=PDP-11/45 Processor Handbook|publisher=[[Digital Equipment Corporation]]|date=1973|page=15|access-date=2022-10-20|archive-date=2022-10-09|archive-url=https://ghostarchive.org/archive/20221009/http://bitsavers.org/pdf/dec/pdp11/handbooks/PDP1145_Handbook_1973.pdf|url-status=live}}</ref> In the PDP–11/70, this was taken a step further, with the addition of a dedicated interface between disks and tapes and memory, via the [[Massbus]]. Although input/output devices continued to be mapped into memory addresses, some additional programming was necessary to set up the added bus interfaces.

=== Interrupts ===
{{further|PDP-11 architecture#Interrupts}}
The PDP–11 supports hardware [[interrupt]]s at four priority levels. Interrupts are serviced by software service routines, which could specify whether they themselves could be interrupted (achieving interrupt [[Nesting (computing)|nesting]]). The event that causes the interrupt is indicated by the device itself, as it informs the processor of the address of its own interrupt vector.

Interrupt vectors are blocks of two 16-bit words in low kernel address space (which normally corresponded to low physical memory) between 0 and 776. The first word of the interrupt vector contains the address of the interrupt service routine and the second word the value to be loaded into the PSW (priority level) on entry to the service routine.

=== Designed for mass production ===
The PDP–11 was designed for ease of manufacture by semiskilled labor. The dimensions of its pieces were relatively non-critical. It used a [[Wire wrap|wire-wrapped]] [[backplane]].

== LSI-11 ==
[[File:PDP-11 (459312210).jpg|thumb|PDP–11/03 (top right)|289x289px]]
The LSI–11 (PDP–11/03), introduced in February 1975<ref name="16-bit Timeline" /> is the first PDP–11 model produced using [[large-scale integration]]; the entire CPU is contained on four LSI chips made by [[Western Digital]] (the [[MCP-1600]] chip set; a fifth chip can be added to extend the instruction set). It uses a bus which is a close variant of the Unibus called the LSI Bus or [[Q-Bus]]; it differs from the Unibus primarily in that addresses and data are multiplexed onto a shared set of wires rather than having separate sets of wires. It also differs slightly in how it addresses I/O devices and it eventually allowed a 22-bit physical address (whereas the Unibus only allows an 18-bit physical address) and block-mode operations for significantly improved bandwidth (which the Unibus does not support).

The CPU [[microcode]] includes a [[debugger]]: firmware with a direct serial interface ([[RS-232]] or [[current loop]]) to a [[Computer terminal|terminal]]. This lets the operator do [[debugging]] by typing commands and reading [[octal]] numbers, rather than operating switches and reading lights, the typical debugging method at the time. The operator can thus examine and modify the computer's registers, memory, and input/output devices, diagnosing and perhaps correcting failures in software and peripherals (unless a failure disables the microcode itself). The operator can also specify which disk to [[Booting|boot]] from. Both innovations increased the reliability and decreased the cost of the LSI-11.

A [[Writable control store|Writable Control Store]] (WCS) option (KUV11-AA) could be added to the LSI-11. This option allowed programming of the internal 8-bit micromachine to create application-specific extensions to the PDP–11 instruction set. The WCS is a quad Q-Bus board with a ribbon cable connecting to the third microcode ROM socket. The source code for EIS/FIS microcode was included so these instructions, normally located in the third MICROM, could be loaded in the WCS, if desired.<ref>{{cite book |title=LSI-11 WCS user's guide |date=June 1978 |publisher=Digital Equipment Corporation |edition=1st |url=http://www.bitsavers.org/pdf/dec/pdp11/1103/EK-KUV11-TM_LSI11_WCS.pdf |access-date=7 January 2023 |archive-date=23 February 2023 |archive-url=https://web.archive.org/web/20230223212303/http://www.bitsavers.org/pdf/dec/pdp11/1103/EK-KUV11-TM_LSI11_WCS.pdf |url-status=live }}</ref>

Later Q-Bus based systems such as the LSI–11/23, /73, and /83 are based upon chip sets designed in house by Digital Equipment Corporation. Later PDP–11 Unibus systems were designed to use similar Q-Bus processor cards, using a Unibus adapter to support existing Unibus [[peripheral]]s, sometimes with a special memory bus for improved speed.

There were other significant innovations in the Q-Bus lineup. For example, a system variant of the PDP–11/03 introduced full system [[power-on self-test]] (POST).
<gallery>
PDP-11-M7270.jpg|Q-Bus board with LSI-11/2 CPU
KL DEC F11.jpg|DEC "Fonz-11" (F11) Chipset
KL DEC J11.jpg|DEC "Jaws-11" (J11) Chipset
</gallery>

== Decline ==
The basic design of the PDP–11 was flexible, and was continually updated to use newer technologies. However, the limited [[throughput]] of the Unibus and [[Q-Bus]] started to become a system-performance [[wikt:bottleneck|bottleneck]], and the 16-bit logical address limitation hampered the development of larger software applications. The article on [[PDP-11 architecture#Memory expansion|PDP–11 architecture]] describes the hardware and software techniques used to work around address-space limitations.

DEC's 32-bit successor to the PDP–11, the [[VAX|VAX–11]] (for "Virtual Address eXtension") overcame the 16-bit limitation, but was initially a [[superminicomputer]] aimed at the high-end [[time-sharing]] market. The early VAX CPUs provided a PDP–11 [[compatibility mode]] under which much existing software could be immediately used, in parallel with newer 32-bit software, but this capability was dropped with the first [[MicroVAX]].

For a decade, the PDP–11 was the smallest system that could run [[Unix]],<ref name="fiedler198310">{{cite news | url=https://archive.org/stream/byte-magazine-1983-10/1983_10_BYTE_08-10_UNIX#page/n133/mode/2up | title=The Unix Tutorial / Part 3: Unix in the Microcomputer Marketplace | work=BYTE | date=October 1983 | access-date=30 January 2015 | author=Fiedler, Ryan | pages=132}}</ref> but in the 1980s, the [[IBM Personal Computer|IBM PC]] and its clones largely took over the small computer market; ''[[Byte (magazine)|BYTE]]'' in 1984 reported that the PC's [[Intel 8088]] microprocessor could outperform the PDP–11/23 when running Unix.{{r|hinnant198408}} Newer microprocessors such as the [[Motorola 68000]] (1979) and [[Intel 80386]] (1985) also included 32-bit logical addressing. The 68000 in particular facilitated the emergence of a market of increasingly powerful scientific and technical [[workstation]]s that would often run Unix variants. These included the [[HP 9000]] series 200 (starting with the HP 9826A in 1981) and 300/400, with the [[HP-UX]] system being ported to the 68000 in 1984; [[Sun Microsystems]] workstations running [[SunOS]], starting with the [[Sun-1]] in 1982; [[Apollo/Domain]] workstations starting with the DN100 in 1981 running [[Domain/OS]], which was proprietary but offered a degree of Unix compatibility; and the [[Silicon Graphics]] [[SGI IRIS|IRIS]] range, which developed into Unix-based workstations by 1985 (IRIS 2000).

Personal computers based on the 68000 such as the [[Apple Lisa]] and [[Mac (computer)|Macintosh]], the [[Atari ST]], and the [[Amiga|Commodore Amiga]] arguably constituted less of a threat to DEC's business, although technically these systems could also run Unix derivatives. In the early years, in particular, [[Microsoft]]'s [[Xenix]] was ported to systems like the [[TRS-80 Model II#Model 16|TRS-80 Model 16]] (with up to 1 MB of memory) in 1983, and to the Apple Lisa, with up to 2 MB of installed RAM, in 1984. The mass-production of those chips eliminated any cost advantage for the 16-bit PDP–11. A line of personal computers based on the PDP–11, the [[DEC Professional (computer)|DEC Professional]] series, failed commercially, along with other non-PDP–11 PC offerings from DEC.

In 1994, DEC<ref>{{cite newsgroup|url=https://groups.google.com/d/msg/biz.digital.announce/mnNlxqdYTwo/Bh84UGO09kEJ|newsgroup=biz.digital.announce|title=Press/Digital and Mentec Announce PDP-11 Software Agreement|date=June 29, 1994|access-date=September 25, 2020|archive-date=July 1, 2024|archive-url=https://web.archive.org/web/20240701172334/https://groups.google.com/g/biz.digital.announce/c/mnNlxqdYTwo/m/Bh84UGO09kEJ|url-status=live}}</ref> sold the PDP–11 system-software rights to [[Mentec|Mentec Inc.]], an Irish producer of LSI-11 based boards for Q-Bus and ISA architecture personal computers, and in 1997 discontinued PDP–11 production. For several years, Mentec produced new PDP–11 processors. Other companies found a [[niche market]] for replacements for legacy PDP–11 processors, disk subsystems, etc. At the same time, free implementations of Unix for the PC based on [[BSD]] or [[Linux]] became available.

By the late 1990s, not only DEC but most of the New England computer industry which had been built around minicomputers similar to the PDP–11 collapsed in the face of microcomputer-based workstations and servers.

== Models ==
The PDP–11 processors tend to fall into several natural groups depending on the original design upon which they are based and which I/O [[Bus (computing)|bus]] they use. Within each group, most models were offered in two versions, one intended for [[Original equipment manufacturer|OEMs]] and one intended for end-users. Although all models share the same instruction set, later models added new instructions and interpreted certain instructions slightly differently. As the architecture evolved, there were also variations in handling of some processor status and control registers.

=== Unibus models ===
[[File:Digital PDP11-IMG 1498 cropped.jpg|thumb|Original PDP–11/20 front panel]]
[[File:Pdp-11-70-panel.jpg|thumb|Original PDP–11/70 front panel]]
[[File:PDP-11-70-DDS570.jpg|thumb|Later PDP–11/70 with disks and tape]]

The following models use the [[Unibus]] as their principal bus:
* PDP–11/20 and PDP–11/15 – 1970.<ref name=pdp-11-faq-models>{{cite web |url=http://www.village.org/pdp11/faq.pages/11model.html |title=The PDP-11 FAQ |publisher=Village.org |date=2000-04-18 |access-date=2014-04-14 |url-status=dead |archive-url=https://web.archive.org/web/20160618161413/http://www.village.org/pdp11/faq.pages/11model.html |archive-date=2016-06-18 }}</ref> The 11/20 sold for $11,800.<ref>{{cite web |url=http://vintagecomputer.net/digital/PDP11-20/PDP11_Price-List_19691215.pdf |title=PDP-11 Price List (1969) |access-date=2020-10-19 |archive-date=2024-07-01 |archive-url=https://web.archive.org/web/20240701172332/http://vintagecomputer.net/digital/PDP11-20/PDP11_Price-List_19691215.pdf |url-status=live }}</ref> The original, non-microprogrammed processor was designed by Jim O'Loughlin. [[Floating point]] is supported by [[peripheral]] options using various data formats. The 11/20 lacks any kind of [[memory protection]] hardware unless retrofitted with a KS-11 [[Memory-mapped I/O|memory mapping]] add-on.<ref name="Ritchie">{{cite web |first=Dennis M. |last=Ritchie |author-link=Dennis Ritchie |title=Odd Comments and Strange Doings in Unix |url=https://www.bell-labs.com/usr/dmr/www/odd.html |publisher=[[Bell Labs]] |date=22 June 2002 |access-date=18 October 2015 |archive-date=3 January 2016 |archive-url=https://web.archive.org/web/20160103171003/http://www.bell-labs.com/usr/dmr/www/odd.html |url-status=live }}</ref> There was also a very stripped-down 11/20 at first called the 11/10,{{citation needed|date=August 2023}} but this number was later re-used for a different model.
* PDP–11/45 (1972),<ref name=pdp-11-faq-models/> PDP–11/50 (1973),<ref>{{Cite web |title=When was the PDP-11/50 released? |url=https://retrocomputing.stackexchange.com/a/28404/7208 |access-date=2024-02-05 |website=Retrocomputing Stack Exchange |language=en |archive-date=2024-07-01 |archive-url=https://web.archive.org/web/20240701172338/https://retrocomputing.stackexchange.com/questions/28397/when-was-the-pdp-11-50-released/28404#28404 |url-status=live }}</ref> and PDP–11/55 (1976)<ref name=pdp-11-faq-models/> – A much faster microprogrammed processor that can use up to 256&nbsp;[[kilobyte|KB]] of semiconductor memory instead of or in addition to [[Magnetic-core memory|core memory]] and support memory mapping and protection.{{r|Ritchie}} It was the first model to support an optional FP11 floating-point [[coprocessor]], which established the format used in later models.
* PDP–11/35 and PDP–11/40 – 1973.<ref name=pdp-11-faq-models/> [[Microprogram]]med successors to the PDP–11/20; the design team was led by Jim O'Loughlin.
* PDP–11/05 and PDP–11/10 – 1972.<ref name=pdp-11-faq-models/> A cost-reduced successor to the PDP–11/20. DEC Datasystem 350 models from 1975 include the PDP–11/10.<ref>{{cite journal |author=<!--Staff writer(s); no by-line.--> |date=July 30, 1975 |title=Time-Sharing Uses Emphasized for DEC Datasystem 350 Series |url=https://books.google.com/books?id=jT2fQqJplN8C&pg=PT29 |journal=Computerworld |volume=IX |issue=31 |pages=19 |access-date=November 4, 2022 |quote=All DEC Datasystem 350 models have PDP–11/10 CPUs |archive-date=March 6, 2023 |archive-url=https://web.archive.org/web/20230306042705/https://books.google.com/books?id=jT2fQqJplN8C&pg=PT29 |url-status=live }}</ref>
* PDP–11/70 – 1975.<ref name=pdp-11-faq-models/> The 11/45 architecture expanded to allow 4&nbsp;[[megabyte|MB]] of physical memory segregated onto a private memory bus, 2&nbsp;KB of cache memory, and much faster I/O devices connected via the Massbus.
* PDP–11/34 (1976<ref name=pdp-11-faq-models/>) and PDP–11/04 (1975<ref name=pdp-11-faq-models/>) – Cost-reduced follow-on products to the 11/35 and 11/05; the PDP–11/34 concept was created by Bob Armstrong. The 11/34 supports up to 256&nbsp;kB of Unibus memory. The PDP–11/34a (1978)<ref name=pdp-11-faq-models/> supports a fast floating-point option, and the 11/34c (same year) supported a [[cache memory]] option.
* PDP–11/60 – 1977.<ref name=pdp-11-faq-models/> A PDP–11 with user-writable microcontrol store; this was designed by another team led by Jim O'Loughlin.
* PDP–11/44 – 1979.<ref name=pdp-11-faq-models/> A replacement for the 11/45 and 11/70, introduced in 1980, that supports optional (though apparently always included) cache memory, optional FP-11 floating-point processor (one circuit board, using sixteen [[AMD]] [[Am2900|Am2901]] bit slice processors), and optional commercial instruction set (CIS, two boards). It includes a sophisticated serial console interface and support for 4&nbsp;MB of physical memory. The design team was managed by John Sofio. This was the last PDP–11 processor to be constructed using discrete [[logic gate]]s; later models were all microprocessor-based. It was also the last PDP–11 system architecture created by [[Digital Equipment Corporation]], later models were VLSI chip realizations of the existing system architectures.
* PDP–11/24 – 1979.<ref name=pdp-11-faq-models/> First VLSI PDP–11 for Unibus, using the "Fonz-11" (F11) chip set with a Unibus adapter.
* PDP–11/84 – 1985–1986.<ref name=pdp-11-faq-models/> Using the VLSI [[DEC J-11|"Jaws-11"]] (J11) chip set with a Unibus adapter.
* PDP–11/94 – 1990.<ref name=pdp-11-faq-models/> J11-based, faster than 11/84.

=== Q-bus models ===
[[File:DEC LSI11-23.jpg|thumb|A PDP–11/03 with cover removed to show the CPU board, with memory board beneath (Two of the CPU chipset's four 40-pin packages have been removed, and the optional [[Floating-point unit|FPU]] is also missing.)]]

The following models use the [[Q-Bus]] as their principal bus:
* PDP–11/03 (also known as the LSI-11/03) – The first PDP–11 implemented with [[Integrated circuit#LSI|large-scale integration]] ICs, this system uses a four-package [[MCP-1600]] chipset from Western Digital and supports 60&nbsp;KB of memory.
* {{visible anchor|PDP-11/23}} – Second generation of LSI (F-11). Early units supported only 248&nbsp;KB of memory.
* PDP–11/23+/MicroPDP–11/23 – Improved 11/23 with more functions on the (larger) processor card. By mid-1982, the 11/23+ supported 4 MB of memory.<ref>{{cite magazine |magazine=Hardcopy |date=October 1982 |page=9 |title=TSX-Plus: Time Share RT-11}}</ref>
* [[PDP-11/73|MicroPDP–11/73]] – The third generation LSI-11, this system uses the faster "Jaws-11" ([[DEC J-11|J-11]]) chip set and supports up to 4&nbsp;MB of memory.
* MicroPDP–11/53 – Slower 11/73 with on-board memory.
* MicroPDP–11/83 – Faster 11/73 with PMI (private memory interconnect).
* MicroPDP–11/93 – Faster 11/83; final DEC Q-Bus PDP–11 model.
* KXJ11 – Q-Bus card (M7616) with PDP–11 based peripheral processor and DMA controller. Based on a J11 CPU equipped with 512&nbsp;KB of RAM, 64&nbsp;KB of ROM, and parallel and serial interfaces.
* [[Mentec#M100|Mentec M100]] – Mentec redesign of the 11/93, with J-11 chipset at 19.66&nbsp;MHz, four on-board serial ports, 1-4&nbsp;MB of on-board memory, and optional FPU.
* [[Mentec#M11|Mentec M11]] – Processor upgrade board; microcode implementation of PDP–11 instruction set by Mentec, using the TI 8832 ALU and TI 8818 microsequencer from [[Texas Instruments]].
* [[Mentec#M1|Mentec M1]] – Processor upgrade board; microcode implementation of PDP–11 instruction set by Mentec, using [[Atmel]] 0.35&nbsp;[[Micrometre|μm]] [[Application-specific integrated circuit|ASIC]].<ref>{{cite web |url=http://www.fuse-network.com/fuse/demonstration/30/24675/24675.pdf |title=Development Project Report |access-date=2014-04-14 |url-status=dead |archive-url=https://web.archive.org/web/20160412201307/http://www.fuse-network.com/fuse/demonstration/30/24675/24675.pdf |archive-date=2016-04-12 }}</ref>
* Quickware QED-993 – High performance PDP–11/93 processor upgrade board.
* DECserver 500 and 550 LAT terminal servers DSRVS-BA using the KDJ11-SB chipset
[[File:DEC-PDT-11-150.jpg|thumb|The PDT-11/150 smart terminal system had two 8-inch floppy disc drives.]]

=== Models without standard bus ===
* PDT-11/110
* PDT-11/130
* PDT-11/150

The PDT series were desktop systems marketed as "smart terminals". The /110 and /130 were housed in a [[VT100]] terminal enclosure. The /150 was housed in a table-top unit which included two 8-inch floppy drives, three asynchronous serial ports, one printer port, one modem port and one synchronous serial port and required an external terminal. All three employed the same chipset as used on the LSI-11/03 and LSI-11/2 in four "microm"s. There is an option which combines two of the microms into one dual carrier, freeing one socket for an EIS/FIS chip. The /150 in combination with a [[VT100#Variants|VT105]] terminal was also sold as [[MiniMINC]], a budget version of the [[MINC-11]].
[[File:DEC VT100 terminal.jpg|thumb|VT100 terminal]]
* PRO-325
* PRO-350
* PRO-380

The [[DEC Professional (computer)|DEC Professional]] series are desktop PCs intended to compete with IBM's earlier [[Intel 8088|8088]] and [[Intel 80286|80286]] based personal computers. The models are equipped with 5{{frac|1|4}} inch floppy disk drives and hard disks, except the 325 which has no hard disk. The original operating system was P/OS, which was essentially [[RSX-11]]M+ with a menu system on top. As the design was intended to avoid software exchange with existing PDP–11 models, the poor market response was unsurprising. The [[RT-11]] operating system was eventually ported to the PRO series. A port of the [[RSTS/E]] operating system to the PRO series was also done internal to DEC, but it was not released. The PRO-325 and -350 units are based on the DCF-11 ("Fonz") chipset, the same as found in the 11/23, 11/23+ and 11/24. The PRO-380 is based on the DCJ-11 ("Jaws") chipset, the same as found in the 11/53,73,83 and others, though running only at 10&nbsp;MHz because of limitations in the support chipset.

=== Models that were planned but never introduced ===
* PDP–11/74 – A PDP–11/70 that was extended to contain multiprocessing features. Up to four processors could be interconnected, although the physical cable management became unwieldy. Another variation on the 11/74 contained both the multiprocessing features and the Commercial Instruction Set. A substantial number of prototype 11/74s (of various types) were built and at least two multiprocessor systems were sent to customers for beta testing, but no systems were ever officially sold. A four processor system was maintained by the RSX-11 operating system development team for testing and a [[uniprocessor]] system served PDP–11 engineering for general purpose timesharing. The 11/74 was due to be introduced around the same time as the announcement of the new 32-bit product line and the first model: the VAX 11/780. The 11/74 was cancelled because of concern for its field maintainability,<ref>{{cite web|title=Multiprocessor FAQ|year=2005|publisher=Machine Intelligence|author1=Bruce Mitchell|author2=Brian S. McCarthy|url=http://www.miim.com/faq/hardware/multipro.shtml|access-date=August 20, 2019}}{{Dead link|date=March 2022 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> though employees believed the real reason was that it outperformed the 11/780<ref>{{cite mailing list|url=http://www.classiccmp.org/pipermail/cctech/2006-February/057197.html|title=Original 11/74 front panel|author=Don North|date=February 7, 2006|mailing-list=cctech|archive-url=https://web.archive.org/web/20110718232453/http://www.classiccmp.org/pipermail/cctech/2006-February/057197.html|archive-date=July 18, 2011|access-date=March 15, 2007|url-status=dead}}</ref> and would inhibit its sales. In any case, DEC never entirely migrated its PDP–11 customer base to the VAX. The primary reason was not performance, but the PDP–11's superior real-time responsiveness.{{citation needed|date=May 2011}}
* PDP–11/27 – A Jaws-11 implementation that would have used the [[VAXBI Bus]] as its principal I/O bus.
* PDP–11/68 – A follow-on to the PDP–11/60 that would have supported 4&nbsp;MB of physical memory.

[[File:GT40 Lunar Lander.jpg|thumb|DEC GT40 running ''[[Lunar Lander (video game genre)|Moonlander]]''|229x229px]]

=== Special-purpose versions ===
* [[DEC GT40|GT40]] – VT11 [[vector graphics]] terminal using a PDP–11/10.<ref name="gt40/42">{{Cite web |date=February 1975 |title=GT40/GT42 user's guide |url=http://www.bitsavers.org/pdf/dec/graphics/VT11/EK-GT40-OP-002_GT40_GT42_Users_Guide_Feb75.pdf |page=29 |access-date=2022-12-22 |archive-date=2022-12-22 |archive-url=https://web.archive.org/web/20221222163442/http://www.bitsavers.org/pdf/dec/graphics/VT11/EK-GT40-OP-002_GT40_GT42_Users_Guide_Feb75.pdf |url-status=live }}</ref>
* GT42 – VT11 vector graphics terminal using a PDP–11/10.<ref name="gt40/42" />
* GT44 – VT11 vector graphics terminal using a PDP–11/40.
* GT62 – VS60 vector graphics workstation using a PDP–11/34a and VT48 graphics processor.
* [[Heathkit H11|H11]] – [[Heathkit]] OEM version of the LSI-11/03.
* VT20 – Terminal with PDP–11/05 with direct mapped character display for text editing and typesetting (predecessor of the VT71).
* [[File:PDP-11-34 front panel.jpg|thumb|229x229px|PDP–11/34 front panel which was a replacement for toggle switches in earlier PDP–11 computers]]VT71 – Terminal with LSI-11/03 and Q-Bus backplane with direct mapped character display for text editing and typesetting.
* [[VT103]] – VT100 with backplane to host an LSI-11.
* VT173 – A high-end editing terminal containing an 11/03, which loaded its editing software over a serial connection to a host minicomputer. Used in various publishing environments, it was also offered with DECset, Digital's VAX/VMS 3.x native mode OEM version of the [[Datalogics]] Pager automated batch composition engine. When VT173 inventory was exhausted in 1985, Digital discontinued DECset and transferred its customer agreements to Datalogics. (HP now uses the name HP [[DECset]] for a software development toolset product.)[[File:DEC-MINC-23.jpg|thumb|MINC-23 laboratory computer|229x229px]]
{{anchor|MINC-11}}
* [[MINC-11]] – Laboratory system based on 11/03 or 11/23;<ref>{{cite web|url=http://www.binarydinosaurs.co.uk/Museum/Digital/minc/index.php |title=Digital MINC-11 |publisher=Binary Dinosaurs |access-date=2014-04-14}}</ref> when based on the 11/23, it was sold as a 'MINC-23', but many MINC-11 machines were field-upgraded with the 11/23 processor. Early versions of the MINC-specific software package would not run on the 11/23 processor because of subtle changes in the instruction set; MINC 1.2 is documented as compatible with the later processor.
* [[C.mmp]] – Multiprocessor system from [[Carnegie Mellon University]].
[[File:Unimation controller internals.jpg|thumb|This [[Unimation]] robot arm controller used DEC LSI-11 series hardware.|229x229px]]
* The [[Unimation]] robot arm controllers used Q-Bus LSI-11/73 systems with a DEC M8192 / KDJ11-A processor board and two DEC DLV11-J (M8043) async serial interface boards.
* SBC 11/21 (boardname KXT11) Falcon and Falcon Plus – single board computer on a Q-Bus card implementing the basic PDP–11 instruction set, based on T11 chipset containing 32&nbsp;KB static RAM, two ROM sockets, three serial lines, 20&nbsp;bit parallel I/O, three interval timers and a two-channel DMA controller. Up to 14&nbsp;Falcons could be placed into one Q-Bus system.
* KXJ11 Q-Bus card (M7616) with PDP–11 based peripheral processor and DMA controller. Based on a J11 CPU equipped with 512&nbsp;KB RAM, 64&nbsp;KB ROM and parallel and serial interfaces.
* HSC high end CI disk controllers used backplane mounted J11 and F11 processor cards to run the CHRONIC operating system.<ref name="ReferenceA">{{cite book|url=http://vaxhaven.com/images/b/ba/EK-HSCMN-IN-002.pdf|id=EK-HSCMN-IN-002|title=HSC Controller Installation Manual|at=p. 4-28|date=July 1991|publisher=Digital Equipment Corporation|access-date=2017-05-29|archive-date=2019-09-03|archive-url=https://web.archive.org/web/20190903150133/http://vaxhaven.com/images/b/ba/EK-HSCMN-IN-002.pdf|url-status=live}}</ref>
* VAX Console – The [[DEC Professional (computer)|DEC Professional Series]] PC-38N with a real-time interface (RTI) was used as the console for the [[VAX 8000|VAX 8500 and 8550]]. The RTI has two serial line units: one connects to the VAX environmental monitoring module (EMM) and the other is a spare that could be used for data transfer. The RTI also has a programmable peripheral interface (PPI) consisting of three 8-bit ports for transferring data, address, and control signals between console and the VAX console interface.<ref>{{cite book |author=<!--Staff writer(s); no by-line.--> |title=VAX 8500/8550 System Hardware User's Guide |publisher=Digital Equipment Corporation |pages=1–8 |date=1986 }}</ref>
* [[DEC T-11|T-11]] is a microprocessor that implements the PDP-11 instruction set architecture. It was developed for embedded systems and was the first single-chip microprocessor developed by DEC. It was sold on the open market.<ref>{{cite web |date=March 24, 1982 |title=T-11 Engineering Specification |url=http://www.bitsavers.org/pdf/dec/pdp11/t11/T11_Engineering_Specification_Rev_E_Mar82.pdf |access-date=May 15, 2023 |archive-date=March 8, 2023 |archive-url=https://web.archive.org/web/20230308003715/http://www.bitsavers.org/pdf/dec/pdp11/t11/T11_Engineering_Specification_Rev_E_Mar82.pdf |url-status=live }}</ref>

=== Unlicensed clones ===
The PDP–11 was sufficiently popular that many unlicensed PDP–11-compatible minicomputers and microcomputers were produced in [[Eastern Bloc]] countries. Some were pin-compatible with the PDP–11 and could use its peripherals and system software. These include:
* [[SM-4]], [[SM-1420]], [[SM-1600]], [[Electronika 100-25]], [[Electronika BK]] series, [[Electronika 60]], [[Electronika 85]], [[DVK]], [[UKNC]], and some models of the [[SM EVM]] series (in the [[Soviet Union]]).
* [[SM-4]], [[SM-1420]], [[IZOT-1016]] and peripherals (in [[Bulgaria]]).
* MERA-60 in [[Poland]].
* SM-1620, SM-1630 (in [[East Germany]]).
* [[SM-4]], TPA-1140,<ref>{{Cite web |title=Múzeum - KFKI TPA 1140 |url=http://hampage.hu/tpa/e_tpa1140.html |access-date=2023-04-30 |website=hampage.hu |archive-date=2023-04-06 |archive-url=https://web.archive.org/web/20230406010424/http://hampage.hu/tpa/e_tpa1140.html |url-status=live }}</ref> TPA-1148,<ref>{{cite web |author=Ákos Varga |url=http://hampage.hu/tpa/e_tpa1148.html |title=TPA-1148 |publisher=Hampage.hu |access-date=2014-04-14 |archive-date=2015-07-12 |archive-url=https://web.archive.org/web/20150712212348/http://hampage.hu/tpa/e_tpa1148.html |url-status=live }}</ref> TPA-11/440<ref>{{cite web |author=Ákos Varga |url=http://hampage.hu/tpa/e_tpa11440.html |title=TPA-11/440 |publisher=Hampage.hu |access-date=2014-04-14 |archive-date=2016-03-03 |archive-url=https://web.archive.org/web/20160303235921/http://hampage.hu/tpa/e_tpa11440.html |url-status=live }}</ref> (in [[Hungary]]).
* SM-4/20, SM {{not a typo|52-11}}, JPR-12R (in Czechoslovakia).
* CalData – Made in US, ran all DEC OSes.<ref>{{cite web |url=http://www.bitsavers.org/pdf/calData/CalData_Brochures_1974.pdf |title=CalData_brochure |access-date=2014-04-14 |archive-date=2012-09-12 |archive-url=https://web.archive.org/web/20120912170014/http://www.bitsavers.org/pdf/calData/CalData_Brochures_1974.pdf |url-status=live }}</ref> The CalData hardware was sufficiently DEC-compatible that CalData memory boards could be used in DEC PDP–11 systems.
* CORAL series (made at [[ICE Felix]] in [[Bucharest]]) and the INDEPENDENT series (made at ITC [[Timișoara]])<ref name="Dragomirescu2003">{{cite book|author=Ion Glodeanu (coord.), Oscar Hoffman, Doina Dragomirescu|title=Actorii sociali ai promovării tehnologiilor, informaţiei şi comunicaţiilor|url=https://books.google.com/books?id=Z2aoVu7XheMC&pg=PA122|year=2003|publisher=Editura Mica Valahie|isbn=978-973-85884-4-8|page=122|language=ro|access-date=2014-04-14}}</ref> running the [[RSX-11M]] operating system (in [[Romania]]). The CORAL series had several models: the CORAL 4001 was roughly equivalent to the PDP–11/04, the CORAL 4011 was a PDP 11/34 clone, while the CORAL 4030 was a PDP–11/44 clone.<ref>{{Cite web |url=http://uknc.narod.ru/Doc/rt11book.txt |title=Archived copy |access-date=2014-02-13 |archive-url=https://web.archive.org/web/20140223131154/http://uknc.narod.ru/Doc/rt11book.txt |archive-date=2014-02-23 |url-status=dead }}</ref> These were used in state-owned companies and in public universities, originally operated with [[punched card]]s, later through video terminals like the Romanian [[DAF-2020]], to teach FORTRAN and Pascal, until replaced by IBM PC compatibles, starting in 1991.
* [[Systime Computers]] models 1000, 3000, 5000 – OEM agreement for sales in the UK and Western Europe, but disputes originated over both intellectual property infringement and indirect sales to the [[Eastern Bloc]].<ref>{{cite news | url=<!--BD https://www.cbronline.com/news/systime_sets_80386_s_series_box_100_user_unix_system/ -->| title=Systime sets 80386 S-series box, 100-user Unix System | work=[[Computergram International]] | publisher=Computer Business Review | date=1 February 1987}}</ref><ref>{{cite news | url=https://books.google.com/books?id=Vzni1LqxEEsC&pg=PA29 | title=Who will tear the Silicon Curtain? | author-first=Mary | author-last=Fagan | magazine=New Scientist | date=24 September 1987 | pages=28–29 }}{{Dead link|date=November 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

== Operating systems ==
Several [[operating system]]s were available for the PDP–11.

=== From Digital ===
{{div col|colwidth=48em}}
* [[Commercial Operating System]]
* [[DEC BATCH-11/DOS-11|BATCH-11/DOS-11]]
* [[CAPS-11]] ('''Ca'''ssette '''P'''rogramming '''S'''ystem)<ref name="CAPS-11 USER'S GUIDE">{{cite web |url=http://bitsavers.org/pdf/dec/pdp11/caps-11/DEC-11-OTUGA-A-D_CAPS-11_Users_Guide_Oct73.pdf |title=CAPS-11 User's Guide |publisher=Digital Equipment Corporation |date=1973 |access-date=2021-01-26 |archive-date=2021-08-24 |archive-url=https://web.archive.org/web/20210824045117/http://bitsavers.org/pdf/dec/pdp11/caps-11/DEC-11-OTUGA-A-D_CAPS-11_Users_Guide_Oct73.pdf |url-status=live }}</ref>
* CHRONIC Hierarchical Storage Controller executive<ref name="ReferenceA" />
* [[GAMMA-11]]<ref name="village.org">{{cite web |url=http://www.village.org/pdp11/faq.pages/pdpOSes.html |title=The PDP-11 FAQ |publisher=Village.org |date=2000-04-18 |access-date=2014-04-14 |url-status=dead |archive-url=https://web.archive.org/web/20150321163350/http://www.village.org/pdp11/faq.pages/pdpOSes.html |archive-date=2015-03-21 }}</ref>
* [[MUMPS|DSM-11]]
* [[RSX-11|IAS]]
* [[RSX-11|P/OS]]
* [[RSTS/E]]
* [[RSX-11]]
* [[RT-11]]
* [[TRAX (Transaction Processing system)|TRAX]] (Transaction Processing system)<ref name="village.org" /><ref>{{cite book|url=http://www.bitsavers.org/pdf/dec/pdp11/trax/TRAX_Brochure_1978.pdf|title=TRAX - The Complete On-Line Transaction Processing System|publisher=Digital Equipment Corporation|access-date=2019-10-21|archive-date=2019-05-28|archive-url=https://web.archive.org/web/20190528214709/http://bitsavers.org/pdf/dec/pdp11/trax/TRAX_Brochure_1978.pdf|url-status=live}}</ref>
* [[Ultrix]]-11
* ''OS/45'' was a proposed operating system for the PDP-11/45 capable of batch processing, real time and timesharing.<ref>{{cite web|url=http://www.bitsavers.org/pdf/dec/pdp11/memos/711020_OS45_Proposal_Meeting.pdf|title=OS/45 Proposal Meeting|publisher=Digital|author=Peter van Roekens|date=1971-10-20|access-date=2023-09-22|archive-date=2024-07-01|archive-url=https://web.archive.org/web/20240701172352/http://www.bitsavers.org/pdf/dec/pdp11/memos/711020_OS45_Proposal_Meeting.pdf|url-status=live}}</ref> It was cancelled during development as its requirements led to a system which was too large for the intended hardware.<ref>{{cite interview |last=Cutler|first=Dave|subject-link=Dave Cutler|interviewer=Grant Saviers|title=Dave Cutler Oral History|url=https://www.youtube.com/watch?v=29RkHH-psrY |archive-url=https://ghostarchive.org/varchive/youtube/20211211/29RkHH-psrY| archive-date=December 11, 2021 |url-status=live|publisher=Computer History Museum|date=February 25, 2016|website=YouTube |access-date=2023-09-22}}{{cbignore}}</ref>
{{div col end}}

=== From third parties ===
{{div col|colwidth=48em}}
* [[ANDOS]]
* [[CSI-DOS]]
* [[DEIMOS]] (University of Edinburgh)
* [[DEMOS]] (Soviet Union)
* [[Duress (operating system)|Duress]] ([[University of Illinois at Urbana–Champaign]]/[[Datalogics]])<ref name="village.org" />
* LOS/C, a small unitasking system written by BRL for the BRL routers and the I/O controller for the Denelcor HEP
* [[Multi-Environment Real-Time|MERT]]<ref name="village.org" />
* [[Micropower Pascal]]<ref name="village.org" />
* [[MK-DOS]]
* [[MONECS]]
* [[Multi-Tasking System|MTS]] (Multi-Tasking System written in [[RTL/2]] by SPL)<ref name="village.org" />
* [[MUMPS]]
* [[MUSS-11]]
* PC11 (Decus 11-501/[[Pilkington]])<ref name="village.org" />
* polyForth, Forth Inc.'s [[Forth (programming language)|Forth]] for the PDP-11
* ROSTTP (Realtime Operating System for Terminal Teletype Processing/Simpact){{citation needed|date=January 2011}}
* SHAREeleven, SHAREplus
* Solo by [[Per Brinch Hansen]]<ref name="Solo">{{Citation|last=Brinch Hansen|first=Per|title=The Solo Operating System: A Concurrent Pascal Program|year=1976|url=http://brinch-hansen.net/papers/1976b.pdf|access-date=22 June 2011|archive-date=25 July 2011|archive-url=https://web.archive.org/web/20110725215859/http://brinch-hansen.net/papers/1976b.pdf|url-status=live}}</ref>
* [[Sphere (operating system)|Sphere]] (Infosphere – Portland Oregon 1981–87)<ref name="village.org" />
* [[Softech Microsystems]] [[UCSD System]] with [[UCSD Pascal]]<ref name="village.org" />
* [[TRIPOS]]
* [[TSX-Plus]]
* [[Unix]]<ref name="byte198308">{{cite news |url=https://archive.org/stream/byte-magazine-1983-08/1983_08_BYTE_08-08_The_C_Language#page/n189/mode/2up |title=The History of Unix |work=BYTE |date=August 1983 |access-date=31 January 2015 |pages=188}}</ref>{{r|fiedler198310}} (many versions, including [[Version 6 Unix]], [[Version 7 Unix]], [[UNIX System III]], and [[Berkeley Software Distribution#PDP-11 beginnings|2BSD]])
* [[Xinu]] OS for instructional purposes
* [[Venix]] (implementation/port of Unix developed by [[VenturCom]])<ref name="village.org" /><ref name="hinnant198408">{{cite news |url=https://archive.org/stream/byte-magazine-1984-08/1984_08_BYTE_09-08_Modula-2#page/n137/mode/2up |title=Benchmarking UNIX Systems |work=BYTE |date=Aug 1984 |access-date=23 February 2016 |author=Hinnant, David F. |pages=132–135, 400–409}}</ref>
{{div col end}}

==Communications==

The DECSA communications server was a communications platform developed by DEC based on a PDP–11/24, with the provision for user installable I/O cards including asynchronous and synchronous modules.<ref>{{cite web|url=http://www.bitsavers.org/pdf/dec/comm/EK-CMIV5-RM-005_Communications_Options_Minireference_Manual_Vol5_Aug88.pdf|title=Communications Options Minireference Manual, Volume 5, Ethernet Devices (Part 1)|id=EK-CMIV5-RM-005|publisher=Digital Equipment Corporation|date=August 1988|at=p. DECSA-1|access-date=2017-09-19|archive-date=2017-09-19|archive-url=https://web.archive.org/web/20170919234702/http://www.bitsavers.org/pdf/dec/comm/EK-CMIV5-RM-005_Communications_Options_Minireference_Manual_Vol5_Aug88.pdf|url-status=live}}</ref> This product was used as one of the earliest commercial platforms upon which networking products could be built, including X.25 gateways, [[Systems Network Architecture|SNA]] gateways, [[router (computing)|routers]], and [[terminal server]]s.

Ethernet adaptors, such as the DEQNA [[Q-Bus]] card, were also available.

Many of the earliest systems on the [[ARPANET]] were PDP–11's

== Peripherals ==
[[File:DEC TU10 tape drive.jpg|thumb|The DEC TU10 [[9-track tape]] drive was also offered on other DEC computer series.]]
A wide range of peripherals were available; some of them were also used in other DEC systems like the [[PDP-8|PDP–8]] or [[PDP-10|PDP–10]].
The following are some of the more common PDP–11 peripherals.
* CR11 – [[punched card]] reader
* DL11 – single [[serial line]] for either [[RS-232]] or [[current loop]]
* LA30/LA36 – [[DECwriter]] [[Dot matrix printer|dot-matrix]] printing keyboard terminal
* LP11 – high speed [[line printer]]
* PC11 – high speed [[papertape]] reader/punch
* RA, RD series – fixed platter [[hard disk]]
* [[RK05|RK series]] – hard disk with exchangeable platter
* [[RL02|RL01/RL02]] – hard disk with exchangeable platter
* RM, RP series – exchangeable multi-platter hard disk
* RX01/RX02 – 8-inch [[floppy disk]]
* RX50/RX33 – 5.25-inch floppy disk
* TU10 – [[9-track tape]] drive
* [[TU56]] – [[DECtape]] [[Block (data storage)|block-addressed]] tape system
* [[VT05]]/VT50/[[VT52]]/[[VT100]]/[[VT220]] – video display terminal

== Use ==
The PDP–11 family of computers was used for many purposes. It was used as a standard minicomputer for general-purpose computing, such as [[timesharing]], scientific, educational, medical, government or business computing. Another common application was [[Real-time computing|real-time]] [[process control]] and [[factory automation]].

Some [[OEM]] models were also frequently used as [[embedded system]]s to control complex systems like traffic-light systems, medical systems, [[numerical control]]led [[machining]], or for network management. An example of such use of PDP–11s was the management of the packet switched network [[Datanet]] 1. In the 1980s, the UK's [[air traffic control]] radar processing was conducted on a PDP 11/34 system known as PRDS – Processed Radar Display System at RAF West Drayton.{{Citation needed|date=April 2010}} The software for the [[Therac-25]] medical [[linear particle accelerator]] also ran on a 32K PDP 11/23.<ref>{{cite journal |last1=Leveson |first1=Nancy G. |first2=Clark S. |last2=Turner |title=An Investigation of the Therac-25 Accidents |journal=[[IEEE Computer]] |date=July 1993 |volume=26 |issue=7 |pages=18-41 |doi=10.1109/MC.1993.274940 |url=https://www.cs.columbia.edu/~junfeng/08fa-e6998/sched/readings/therac25.pdf}}</ref>

Another use was for storage of test programs for [[Teradyne]] [[Automatic test equipment|ATE]] equipment, in a system known as the TSD (Test System Director). As such, they were in use until their software was rendered inoperable by the [[Year 2000 problem]]. The US Navy used a PDP–11/34 to control its Multi-station Spatial Disorientation Device, a simulator used in pilot training, until 2007, when it was replaced by a PC-based emulator that could run the original PDP–11 software and interface with custom Unibus controller cards.<ref>{{cite web |url=http://www.migrationspecialties.com/pdf/MSDD.pdf |title=PDP-11 Replacement Keeps the Navy's MSDD Spinning |first=Bruce |last=Claremont |date=February 2008 |access-date=October 15, 2017 |archive-date=April 17, 2016 |archive-url=https://web.archive.org/web/20160417131718/http://www.migrationspecialties.com/pdf/MSDD.pdf |url-status=dead }}</ref>

A PDP–11/45 was used for the experiment that discovered the [[J/ψ meson]] at the [[Brookhaven National Laboratory]].<ref>{{cite journal |title=Experimental Observation of a Heavy Particle J |first=J.J. |last=Aubert |journal=[[Physical Review Letters]] |date=November 1974 |volume=33 |issue=23 |pages=1404–1406 |doi=10.1103/PhysRevLett.33.1404 |bibcode=1974PhRvL..33.1404A |display-authors=etal |doi-access=free }}</ref> In 1976, [[Samuel C. C. Ting]] received the [[Nobel Prize]] for this discovery. Another PDP–11/45 was used to create the Death Star plans during the briefing sequence in ''[[Star Wars]]''.{{citation needed|date=March 2023}}

== Emulators ==
[[File:Modern PDP-11 70 replica and keyboard.jpg|thumb|upright=1.3|PiDP-11, a 6:10-scale PDP-11/70 console replica with a [[Raspberry Pi]] running SIMH]]
===Ersatz-11===
Ersatz-11, a product of D Bit,<ref>{{cite web| url = http://www.dbit.com/| title = D Bit Ersatz-11 PDP-11 emulator| access-date = 2014-05-06| archive-date = 2014-05-06| archive-url = https://web.archive.org/web/20140506044153/http://www.dbit.com/| url-status = live}}</ref> emulates the PDP–11 instruction set running under DOS, OS/2, Windows, Linux or [[bare metal]] (no OS). It can be used to run RSTS or other PDP–11 operating systems.

===SIMH===
[[SIMH]] is an emulator that compiles and runs on a number of platforms (including [[Linux]]) and supports hardware emulation for the DEC PDP–1, PDP–8, PDP–10, PDP–11, VAX, AltairZ80, several machines from IBM, and other minicomputers.

== See also ==
* [[Heathkit H11]], a 1977 Heathkit personal computer based on the PDP–11
* [[MACRO-11]], the PDP–11's native assembly language
* [[PL-11]], a high-level assembler for the PDP–11 written at [[CERN]]
* [[H8 Family]], a family of microcontrollers with an instruction set inspired by the PDP-11

== Notes ==
===Explanatory citations===
{{notelist}}

===Citations===
{{Reflist|30em}}

== References ==
* {{citation|title=PDP11 processor handbook – PDP11/05/10/35/40|publisher=Digital Equipment Corporation|year=1973}}
* {{citation|title=PDP11 processor handbook – PDP11/04/34a/44/60/70|publisher=Digital Equipment Corporation|year=1979}}

== Further reading ==
* {{Citation|first1=Richard H. Jr.|last1=Eckhouse|first2=L. Robert|last2=Morris|title=Minicomputer Systems: Organization, Programming, and Applications (PDP-11)|publisher=[[Prentice-Hall]]|location=[[Englewood Cliffs, New Jersey]]|year=1979|isbn=0-13-583914-9}}
* {{citation |first=Michael |last=Singer |title=PDP-11. Assembler Language Programming and Machine Organization |publisher=Wiley |year=1980 |isbn=0-471-04905-0 |hdl=2027/mdp.39076005031633}}

== External links ==
{{Commons category}}
* {{cite journal |last1=Bell |first1=Gordon |last2=Strecker |first2=William D. |title=Computer structures: What have we learned from the PDP-11? |journal=ACM SIGARCH Computer Architecture News |volume=4 |issue=4 |date=1976-01-17 |issn=0163-5964 |doi=10.1145/633617.803541 |pages=1–14}}
* [http://research.microsoft.com/users/GBell/Digital/DECMuseum.htm Gordon Bell's CyberMuseum for Digital Equipment Corp (DEC)]
* {{citation |first=Günter |last=Dotzel |url=http://www.modulaware.com/history/Vrsmot.pdf |title=On LSI-11, RT-11, Megabytes of Memory and Modula-2/VRS |journal=DEC Professional: The Magazine for DEC Users |publisher=Professional Press |location=Spring House, Pennsylvania, US |date=January 1986}}

{{DEC hardware}}
{{Authority control}}

[[Category:PDP-11| ]]
[[Category:Computer-related introductions in 1970]]
[[Category:16-bit computers]]