The PDP-11 was a series of 16-bit minicomputers sold by Digital Equipment Corporation (DEC) from 1970 into the 1990s, one of a succession of products in the PDP series.[1][2] The PDP-11 replaced the PDP-8 in many real-time applications, although both product lines lived in parallel for more than 10 years. The PDP-11 had several uniquely innovative features, and was easier to program than its predecessors with its use of general registers. Its successor in the mid-range minicomputer niche was the 32-bit VAX-11.
Design features of the PDP-11 influenced the design of microprocessors such as the Motorola 68000; design features of its operating systems, as well as other operating systems from Digital Equipment, influenced the design of other operating systems such as CP/M[3] and hence also MS-DOS.[4] The first officially named 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,[5] albeit not originally by design.[6]
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DEC developed the 16-bit PDP 11 as a response to the introduction of the Data General NOVA, which had a 16-bit word length; DEC's previous PDP-8 had only 12 bit words. 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.[7] Initially manufactured of small-scale Transistor-transistor logic, a single-board large scale integration version of the processor was developed in 1975. A single-chip processor, the J-11 was developed in 1979. The last models of the PDP-11 line were the PDP-11/94 and -11/93 introduced in 1990. [2]
The PDP-11 processor architecture had a mostly orthogonal instruction set. For example, instead of instructions such as load and store, the PDP-11 had a move instruction for which either operand (source and destination) could be memory or register. There were no specific input or output instructions; the PDP-11 used memory-mapped I/O and so the same move instruction was used; orthogonality even enabled moving data directly from an input device to an output device. More complex instructions such as add likewise could have memory, register, input, or output as source or destination.
Most operands could apply any of eight addressing modes to eight registers. The addressing modes provided register, immediate, absolute, relative, deferred (indirect), and indexed addressing, and could specify autoincrementation and autodecrementation of a register by one (byte instructions) or two (word instructions). Use of relative addressing let a machine-language program be position-independent.
Early models of the PDP-11 had no dedicated bus for input/output, but only a memory 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. 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.
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, starting with the PDP-11/45 Unibus and 11/83 Q-bus systems, departed from the single-bus approach. Instead, memory was interfaced by dedicated circuitry and space in the CPU cabinet, while the Unibus continued to be used for I/O only. 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.
The PDP-11 supported hardware interrupts at four priority levels. Interrupts were serviced by software service routines, which could specify whether they themselves could be interrupted (achieving interrupt nesting). The event that caused the interrupt was indicated by the device itself, as it informed the processor of the address of its own interrupt vector.
Interrupt vectors were 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 contained 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.
The article on PDP-11 architecture provides more details on interrupts.
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-wrapped backplane. That is, the printed circuit boards plugged into a backplane connector. The backplane connectors had square pins that could be connected to by wrapping wires around them. The corners of the pins would bite into the wire to form a gas-tight (i.e. corrosion-proof, therefore reliable) connection.
The LSI-11 (PDP-11/03), introduced in February, 1975[2] was the first PDP-11 model produced using large-scale integration; the entire CPU was contained on four LSI chips made by Western Digital (the MCP-1600 chip set; a fifth chip could be added to extend the instruction set, as pictured on the right). It used a bus which was a close variant of the Unibus called the LSI Bus or Q-Bus; it differed from the Unibus primarily in that addresses and data were multiplexed onto a shared set of wires, as opposed to having separate sets of wires, as in the Unibus. It also differed slightly in how it addressed I/O devices and it eventually allowed a 22-bit physical address (whereas the Unibus only allowed an 18-bit physical address) and block-mode operations for significantly improved bandwidth (which the Unibus did not support).
The CPU's microcode includes a debugger: firmware with a direct serial interface (RS-232 or current loop) to a terminal. This let 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 could 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 disabled the microcode itself). The operator could also specify which disk to boot from.
Both innovations increased the reliability and decreased the cost of the LSI-11.
Later Q-Bus based systems such as the LSI-11/23, /73, and /83 were 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 peripherals, 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).
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 (for "Virtual Address eXtension") overcame the 16-bit limitation, but was initially a superminicomputer aimed at the high-end time-sharing market. The early VAXes provided a PDP-11 compatibility mode under which much existing software could be immediately used, in parallel with newer 32-bit software.
In the 1980s, the IBM PC and its clones largely took over the small computer market, and DEC was unable to counter this competition.
Newer microprocessor chips such as the Motorola 68000 (1979) and Intel 80386 (1985) also included 32-bit logical addressing. 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 series, failed commercially, along with other non-PDP-11 PC offerings from DEC.
DEC discontinued PDP-11 production in 1997, and in 1994 [8] 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. 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.
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.
The PDP-11 processors tended to fall into several natural groups depending on the original design upon which they are based and which I/O bus they used. Within each group, most models were offered in two versions, one intended for OEMs and one intended for end-users. Although all models shared 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.
The following models used the Unibus as their principal bus:
The following models used the Q-Bus as their principal bus:
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 was an option which combined two of the microms into one dual carrier, freeing one socket for an EIS/FIS chip. The /150 in combination with a VT-105 terminal was also sold as MiniMINC, a budget version of the MINC-11.
The DEC Professional series were desktop PCs intended to compete with IBM's earlier 8088 and 80286 based personal computers. The models were equipped with 5¼ inch floppy disk drives and hard disks, except the 325 which had no hard disk. The original operating system was P/OS, which was essentially RSX-11M+ with a menu system on top. As the design was intended to avoid software exchange with existing PDP-11 models, their ill fate in the market was no surprise for anyone except DEC. The RT-11 was eventually ported to the PRO series. A port to the PRO for RSTS/E was also done internal to DEC, but was not released. The PRO-325 and -350 units were based on the DCF-11 ("Fonz") chipset, the same as found in the 11/23, 11/23+ and 11/24. The PRO-380 was based on the DCJ-11 ("Jaws") chipset, the same as found in the 11/53,73,83 and others, though running only at 10 MHz because of limitations in the support chipset.
However conspiracy or not, DEC was never able to successfully migrate its entire PDP-11 customer base to the VAX. The primary reason was not performance, but the PDP-11's superior real-time responsiveness.
The PDP-11 was sufficiently popular that many unauthorized PDP-11-compatible minicomputers and microcomputers were produced in Eastern Bloc countries. At least some of these were pin-compatible with DEC's PDP-11s and could share peripherals and system software. These include:
Several operating systems were available for the PDP-11
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A wide range of peripherals were available; some of them were also used in other DEC systems like the PDP-8 or PDP-10. The following are some of the more common PDP-11 peripherals.
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, or business computing. Another common application was real-time process control and factory automation.
Some OEM models also were frequently used as embedded systems to control complex systems like traffic-light systems, medical systems, numerical controlled 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. The software for the Therac-25 medical linear accelerator also ran on a 32K PDP 11/23.[21]
Another use was for storage of test programs for Teradyne 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.
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