Editing Digital Equipment Corporation (section)

=== PDP-11 (1970) ===
{{Main|PDP-11}}

[[File:Locale_RS6_Digital PDP11-IMG 1498 cropped.jpg|thumb|PDP-11/20, the first model of PDP-11 on display at [[École Polytechnique Fédérale de Lausanne|EPFL]] ]]

DEC's successful entry into the computer market took place during a fundamental shift in the underlying organization of the machines from [[Word (computer architecture)|word]] lengths based on 6-bit characters to those based on 8-bit words needed to support [[ASCII]].{{efn|Although ASCII is a 7-bit standard, units-of-8-bits are typically used for machines that support it.}} DEC began studies of such a machine, the PDP-X, but [[Ken Olsen]] did not support it as he could not see how it offered anything their existing 12-bit or 18-bit machines didn't.<ref>{{cite web |url=http://simh.trailing-edge.com/docs/pdpx.pdf |archive-url=https://web.archive.org/web/20041210092501/http://simh.trailing-edge.com/docs/pdpx.pdf |archive-date=2004-12-10 |url-status=live |title=What Was The PDP-X? |first=Bob |last=Supnik |date=January 10, 2004}}</ref> This led the leaders of the PDP-X project to leave DEC and start [[Data General]], whose 16-bit [[Data General Nova]] was released in 1969 and was a huge success.<ref>{{cite interview |interviewer=Gardner Hendrie |first=Edson |last=de Castro |author-link=Edson de Castro |title=Oral History of Edson (Ed) D. de Castro |date=November 22, 2002 |url=http://archive.computerhistory.org/resources/access/text/2012/07/102702207-05-01-acc.pdf |archive-url=https://web.archive.org/web/20141227091345/http://archive.computerhistory.org/resources/access/text/2012/07/102702207-05-01-acc.pdf |archive-date=2014-12-27 |url-status=live}}</ref>

The success of the Nova finally prompted DEC to take the switch seriously, and they began a crash program to introduce a 16-bit machine of their own. The new system was designed primarily by Harold McFarland, [[Gordon Bell]], Roger Cady, and others.<ref name="birth">{{cite web|first=Larry|last=McGowan|url=http://hampage.hu/pdp-11/birth.html|title=How the PDP-11 Was Born, according to Larry McGowan|date=August 19, 1998}}</ref> The project was able to leap forward in design with the arrival of Harold McFarland, who had been researching 16-bit designs at [[Carnegie Mellon University]]. One of his simpler designs became the basis for the new design, although when they first viewed the proposal, management was not impressed and almost cancelled it.<ref name=birth/>

The result was the [[PDP-11]], released in 1970. It differed from earlier designs considerably. In particular, the new design did not include many of the [[addressing mode]]s that were intended to make programs smaller in memory, a technique that was widely used on other DEC machines and [[Complex instruction set computing|CISC]] designs in general. This would mean the machine would spend more time accessing memory, which would slow it down. However, the machine also extended the idea of multiple "General Purpose Registers" (GPRs), which gave the programmer flexibility to use these high-speed memory caches as they needed, potentially addressing the performance issues.

[[File:Locale_RS6_PDP-11-34 unibus slots.jpg|thumb|PDP-11/34 top view, showing the [[:en:Unibus|Unibus]] slots with the CPU, [[:en:RK05|DK drive controller]] and other options]]
A major advance in the PDP-11 design was DEC's [[Unibus]], which supported all peripherals through [[Memory-mapped I/O|memory mapping]]. This allowed a new device to be added easily, generally only requiring plugging a hardware interface board into the backplane and possibly adding a jumper to the [[wire wrap]]ped backplane, and then installing software that read and wrote to the mapped memory to control it. The relative ease of interfacing spawned a huge market of third party add-ons for the PDP-11, which made the machine even more useful.

The combination of architectural innovations proved superior to competitors and the "11" architecture was soon the industry leader, propelling DEC back to a strong market position. The design was later expanded to allow [[Paging|paged physical memory]] and [[memory protection]] features, useful for [[Computer multitasking|multitasking]] and [[time-sharing]]. Some models supported separate instruction and data spaces for an effective virtual address size of 128&nbsp;KB within a physical address size of up to 4&nbsp;MB. Smaller PDP-11s, implemented as single-chip CPUs, continued to be produced until 1996, by which time over 600,000 had been sold.<ref name=miller452/>

[[File:Locale_RS6_RT-11 help.jpg|thumb|The [[RT-11]] interactive help screen displayed on a [[VT100]] display terminal]]

The PDP-11 supported several operating systems, including [[Bell Labs]]' new [[Unix]] operating system as well as DEC's [[DOS-11]], [[RSX-11]], IAS, [[RT-11]], DSM-11, and [[RSTS/E]]. Many early PDP-11 applications were developed using standalone paper-tape utilities. DOS-11 was the PDP-11's first disk operating system, but was soon supplanted by more capable systems. RSX provided a general-purpose [[computer multitasking|multitasking]] environment and supported a wide variety of [[programming language]]s. IAS was a [[time-sharing]] version of RSX-11D. Both RSTS and Unix were time-sharing systems available to educational institutions at little or no cost, and these PDP-11 systems were destined to be the "sandbox" for a rising generation of engineers and computer scientists. Large numbers of PDP-11/70s were deployed in telecommunications and industrial control applications. [[AT&T Corporation]] became DEC's largest customer.

RT-11 provided a practical real-time operating system in minimal memory, allowing the PDP-11 to continue DEC's critical role as a computer supplier for [[embedded system]]s. Historically, RT-11 also served as the inspiration for many microcomputer OS's, as these were generally being written by programmers who cut their teeth on one of the many PDP-11 models. For example, [[CP/M]] used a command syntax similar to RT-11's, and even retained the awkward [[Peripheral Interchange Program|PIP]] program used to copy data from one computer device to another. As another historical footnote, DEC's use of "/" for "switches" (command-line options) would lead to the adoption of "\" for pathnames in [[MS-DOS]] and [[Microsoft Windows]] as opposed to "/" in [[Unix]].<ref name="afu-faq">{{cite web|url=http://www.netwhatever.com/faq/inicio.html#queIII15|title=III.15 - Why does MS-DOS use '\' as the path separator, while Unix uses '/'?|work=alt.folklore.computers List of Frequently Asked Questions}}</ref>

The evolution of the PDP-11 followed earlier systems, eventually including a single-user deskside personal computer form, the MicroPDP-11. In total, around 600,000 PDP-11s of all models were sold, and a wide variety of third-party peripheral vendors had also entered the computer product ecosystem. It was even sold in kit form as the [[Heathkit H11]], although it proved too expensive for [[Heathkit]]'s traditional hobbyist market.