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PDP-11

Introduction

The PDP-11 was a series of 16-bit minicomputers sold by Digital Equipment Corporation from 1970 into the 1990s, one of a set of products in the Programmed Data Processor (PDP) series. 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 alone. In total, around 600,000 PDP-11s of all models were sold, making it one of DEC's most successful products. The PDP-11 is considered by some experts to be the one of the most popular minicomputers.

Its features established the PDP-11 as DEC’s flagship computer for science, engineering and industry. The system’s time-sharing capabilities made it an important system for the spread of ARPANET: by March 1977, PDP-11’s accounted for over 30% of the nodes on the network. DEC PDP-11 sales ended in 1990.

The PDP-11 included a number of innovative features in its instruction set and additional general-purpose registers that made it much easier to program than earlier models in the PDP series. Additionally, the Unibus system allowed external devices to be easily interfaced to the system using direct memory access, opening the system to a wide variety of peripherals. The PDP-11 replaced the PDP-8 in many real-time applications, although both products lived in parallel for well over 10 years. The ease of programming of the PDP-11 made it very popular for general-purpose computing uses as well.

The design of the PDP-11 inspired the design of late-1970s microprocessors including the Intel x86 and the Motorola 68000. The design features of PDP-11 operating systems, and other operating systems from Digital Equipment, influenced the design of CP/M and MS-DOS operating systems. The first version of Unix ran on the PDP-11/20 in 1970. It is commonly stated that the C programming language took advantage of several low-level PDP-11–dependent programming features, albeit not originally by design. An effort to expand the PDP-11 from 16 to 32-bit addressing led to the VAX-11 (Virtual Address eXtension) design, which took part of its name from the PDP-11.

Epilogue

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 bottleneck, and the 16-bit logical address limitation hampered the development of larger software applications. The article on 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 overcame the 16-bit limitation, but was initially a super-minicomputer 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; this capability was dropped with the first MicroVAX.

For a decade, the PDP-11 was the smallest system that could run Unix, but in the 1980s, the IBM PC and its clones largely took over the small computer market. BYTE in 1984 reported that the Intel 8088 microprocessor could outperform the PDP-11/23 when running Unix. Newer microprocessors such as the Motorola 68000 (1979) and Intel 80386 (1985) also included 32-bit logical addressing.

Personal computers constituted less of a threat to DEC's business, although technically these systems could also run Unix derivatives. Microsoft Xenix first ran on the PDP-11; the first port was for the Zilog Z8001 16-bit processor. Altos shipped a version for their Intel 8086 based computers early in 1982, Tandy Corporation shipped TRS-XENIX for their 68000-based systems in January 1983, and SCO released their port to the IBM PC in September 1983. A port to the 68000-based Apple Lisa also existed. At the time, Xenix was based on AT&T's UNIX System III. Version 2.0 of Xenix was released in 1985 and was based on UNIX System V. An update numbered 2.1.1 added support for the Intel 80286 processor. Subsequent releases improved System V compatibility. In 1986, SCO ported Xenix to the 386 processor, a 32-bit chip. Xenix 2.3.1 introduced support for i386, SCSI and TCP/IP.

The mass-production of processor chips for the personal computer market by Intel, Zilog and Motorola eliminated any cost advantage for the 16-bit PDP-11. Meanwhile, the DEC Professional series of personal computers based on the PDP-11, failed commercially, along with other DEC personal computer offerings.

In 1994, DEC sold the PDP-11 system-software rights to 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. In 2006 Mentec Inc. declared bankruptcy while Mentec Ltd. was acquired by Irish firm Calyx in December 2006. The PDP-11 software, which was owned by Mentec Inc. was then bought by XX2247 LLC, which is the owner of the software today. It is unclear if new commercial licenses are possible to buy at this time. Hobbyists can run RSX-11M (version 4.3 or earlier) and RSX-11M Plus (version 3.0 or earlier) on the SIMH emulator thanks to a free license granted in May 1998 by Mentec Inc.

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