PDP-6
Encyclopedia
The PDP-6 was a computer
model developed by Digital Equipment Corporation
(DEC) in 1963. It was influential primarily as the prototype (effectively) for the later PDP-10
; the instruction set
s of the two machines are almost identical.
The PDP-6 was DEC's first "big" machine. It used 36-bit words
, in common with other large computers at the time from companies like IBM
, Honeywell
and General Electric
. Addressing remained 18-bit
, as in earlier DEC machines, allowing for a 256 kword main memory. Memory was implemented using magnetic cores
; a typical system included 32,768 words (equivalent to 160kB on modern machines).
The instruction set architecture could be categorized as "one-and-a-half address"; instructions contained one full 18-bit memory address, and a second four-bit address that could specify one of the first sixteen memory locations as an "accumulator" or "AC". Another four-bit field in the instruction allowed for any AC other than AC0 to be used as an index register.
Most if not all PDP-6 systems were equipped with the optional Type 162 "Fast Memory", which provided 16 memory locations constructed from discrete-transistor flip-flops. The Fast Memory (also known as "fast accumulators" or "fast ACs") substituted for the first 16 words of core memory and operated four times faster.
The PDP-6 supported time sharing through the use of a status bit selecting between two operating modes ("Executive" and "User", with access to I/O
, etc., being restricted in the latter), and a single relocation/protection register which allowed a user's address space
to be limited to a set section of main memory (a second relocation/protection register for shareable "high segments" was added on the PDP-10). The main operating system
used on the machine was an early version of what later became TOPS-10
, and several sites made custom versions of the system, which was available in source code
form. MIT's ITS
operating system also began on the PDP-6.
Worldwide, only 23 PDP-6's were sold http://research.microsoft.com/~gbell/Digital/timeline/1964-1.htm, the smallest number of any DEC machine. It was complex and expensive to build, as well as difficult to install and get operational at the customer's site. Additionally the sales force found the PDP-6 to be a "hard sell". After a short period in the market, DEC let it be known that they were exiting the 36-bit market to concentrate on their smaller machines again. Not long after this rumors started to spread that they were, in fact, working on a new 36-bit design, which was eventually released as the PDP-10
.
The PDP-6 was infamous because of the 6205 board, a large (11 x 9 inches) board which contained 1 bit of AR, MB, and MQ (thus there were 36 of these). It had 88 transistors, a 2-sided PC etch, two 18-pin and two 22-pin connectors (two on each side of the module). Because of all these connectors, swapping this module was a major undertaking, and the mechanical coupling made it highly likely that fixing one fault would cause another. There was also a great fear of powering off a PDP-6, since it would generally result in at least one 6205 board failing.
The experience with the 6205 led the designers of the first models of PDP-10
, the KA10 and KI10, to use only small boards. It was not until the KL10 that large boards were used again.
DEC management still considered the system useful because those sales were to technical leaders such as universities. That gave DEC a number of advantages, including a foothold in that market, access to advice on future technical direction from a group of advanced and technically knowledgeable users, and finally a source of intelligent young employees as the business grew.
Stanford's
PDP-6 was shown at DECUS in 1984. The machine was transferred to a DEC warehouse after that event. There are no records of this machine being given to the Computer Museum, which was not part
of DEC in 1984. In the late 90's Compaq donated the contents of the DEC internal archives to The Computer Museum History Center. The Fast Memory cabinet from the Stanford PDP-6 was part of that donation. There is no evidence that
the modules sold at the Boston computer museum gift shop were from the Stanford PDP-6, nor is there any evidence that the museum had ever had this machine in its possession.
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
model developed by Digital Equipment Corporation
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
(DEC) in 1963. It was influential primarily as the prototype (effectively) for the later PDP-10
PDP-10
The PDP-10 was a mainframe computer family manufactured by Digital Equipment Corporation from the late 1960s on; the name stands for "Programmed Data Processor model 10". The first model was delivered in 1966...
; the instruction set
Instruction set
An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...
s of the two machines are almost identical.
The PDP-6 was DEC's first "big" machine. It used 36-bit words
36-bit word length
Many early computers aimed at the scientific market had a 36-bit word length. This word length was just long enough to represent positive and negative integers to an accuracy of ten decimal digits . It also allowed the storage of six alphanumeric characters encoded in a six-bit character encoding...
, in common with other large computers at the time from companies like IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
, Honeywell
Honeywell
Honeywell International, Inc. is a major conglomerate company that produces a variety of consumer products, engineering services, and aerospace systems for a wide variety of customers, from private consumers to major corporations and governments....
and General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
. Addressing remained 18-bit
18-bit
18 binary digits have unique combinations.-Example 18-bit computer architectures:* Possibly the most well-known 18-bit computer architectures are the PDP-1, PDP-4, PDP-7, PDP-9 and PDP-15 minicomputers produced by Digital Equipment Corporation from 1960 to 1975.* UNIVAC produced a number of...
, as in earlier DEC machines, allowing for a 256 kword main memory. Memory was implemented using magnetic cores
Magnetic core memory
Magnetic-core memory was the predominant form of random-access computer memory for 20 years . It uses tiny magnetic toroids , the cores, through which wires are threaded to write and read information. Each core represents one bit of information...
; a typical system included 32,768 words (equivalent to 160kB on modern machines).
The instruction set architecture could be categorized as "one-and-a-half address"; instructions contained one full 18-bit memory address, and a second four-bit address that could specify one of the first sixteen memory locations as an "accumulator" or "AC". Another four-bit field in the instruction allowed for any AC other than AC0 to be used as an index register.
Most if not all PDP-6 systems were equipped with the optional Type 162 "Fast Memory", which provided 16 memory locations constructed from discrete-transistor flip-flops. The Fast Memory (also known as "fast accumulators" or "fast ACs") substituted for the first 16 words of core memory and operated four times faster.
The PDP-6 supported time sharing through the use of a status bit selecting between two operating modes ("Executive" and "User", with access to I/O
I/O
I/O may refer to:* Input/output, a system of communication for information processing systems* Input-output model, an economic model of flow prediction between sectors...
, etc., being restricted in the latter), and a single relocation/protection register which allowed a user's address space
Address space
In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.- Overview :...
to be limited to a set section of main memory (a second relocation/protection register for shareable "high segments" was added on the PDP-10). The main operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
used on the machine was an early version of what later became TOPS-10
TOPS-10
The TOPS-10 System was a computer operating system from Digital Equipment Corporation for the PDP-10 mainframe computer launched in 1967...
, and several sites made custom versions of the system, which was available in source code
Source code
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...
form. MIT's ITS
Incompatible Timesharing System
ITS, the Incompatible Timesharing System , was an early, revolutionary, and influential time-sharing operating system from MIT; it was developed principally by the Artificial Intelligence Laboratory at MIT, with some help from Project MAC.In addition to being technically influential ITS, the...
operating system also began on the PDP-6.
Worldwide, only 23 PDP-6's were sold http://research.microsoft.com/~gbell/Digital/timeline/1964-1.htm, the smallest number of any DEC machine. It was complex and expensive to build, as well as difficult to install and get operational at the customer's site. Additionally the sales force found the PDP-6 to be a "hard sell". After a short period in the market, DEC let it be known that they were exiting the 36-bit market to concentrate on their smaller machines again. Not long after this rumors started to spread that they were, in fact, working on a new 36-bit design, which was eventually released as the PDP-10
PDP-10
The PDP-10 was a mainframe computer family manufactured by Digital Equipment Corporation from the late 1960s on; the name stands for "Programmed Data Processor model 10". The first model was delivered in 1966...
.
The PDP-6 was infamous because of the 6205 board, a large (11 x 9 inches) board which contained 1 bit of AR, MB, and MQ (thus there were 36 of these). It had 88 transistors, a 2-sided PC etch, two 18-pin and two 22-pin connectors (two on each side of the module). Because of all these connectors, swapping this module was a major undertaking, and the mechanical coupling made it highly likely that fixing one fault would cause another. There was also a great fear of powering off a PDP-6, since it would generally result in at least one 6205 board failing.
The experience with the 6205 led the designers of the first models of PDP-10
PDP-10
The PDP-10 was a mainframe computer family manufactured by Digital Equipment Corporation from the late 1960s on; the name stands for "Programmed Data Processor model 10". The first model was delivered in 1966...
, the KA10 and KI10, to use only small boards. It was not until the KL10 that large boards were used again.
DEC management still considered the system useful because those sales were to technical leaders such as universities. That gave DEC a number of advantages, including a foothold in that market, access to advice on future technical direction from a group of advanced and technically knowledgeable users, and finally a source of intelligent young employees as the business grew.
Stanford's
Stanford University
The Leland Stanford Junior University, commonly referred to as Stanford University or Stanford, is a private research university on an campus located near Palo Alto, California. It is situated in the northwestern Santa Clara Valley on the San Francisco Peninsula, approximately northwest of San...
PDP-6 was shown at DECUS in 1984. The machine was transferred to a DEC warehouse after that event. There are no records of this machine being given to the Computer Museum, which was not part
of DEC in 1984. In the late 90's Compaq donated the contents of the DEC internal archives to The Computer Museum History Center. The Fast Memory cabinet from the Stanford PDP-6 was part of that donation. There is no evidence that
the modules sold at the Boston computer museum gift shop were from the Stanford PDP-6, nor is there any evidence that the museum had ever had this machine in its possession.
External links
- DEC PDP-6 Serial numbers
- DEC's PDP-6 was the worlds first commercial time-sharing system Gordon Bell interview at the Smithsonian
- http://www.inwap.com/pdp10/usenet/pdp6
- http://research.microsoft.com/~gbell/Digital/timeline/1964-1.htm
- http://research.microsoft.com/~gbell/Digital/timeline/1964-2.htm
- The PDP-6 at Stanford
- Tom Knight's PDP-6 pages