Jump to content

PDP-6

From Wikipedia, the free encyclopedia
PDP-6
Gordon BellandAlan Kotokusing a PDP-6 in 1964
DeveloperDigital Equipment Corporation
Product familyProgrammed Data Processor
TypeMainframe computer
Release date1964;60 years ago(1964)
Operating systemearly version of what later becameTOPS-10,custom versions of the system,ITS,WAITS
PlatformDEC 36-bit
Mass1,300 pounds (590 kg), 1,700 pounds (770 kg) with "Fast Memory"
SuccessorPDP-10

ThePDP-6,short forProgrammed Data Processormodel 6, is acomputerdeveloped byDigital Equipment Corporation(DEC) during 1963 and first delivered in the summer of 1964.[1][a]It was an expansion of DEC's existing18-bit systemsto use a 36-bit data word, which was at that time a commonword sizefor large machines likeIBM mainframes.The system was constructed using the samegermanium transistor-basedSystem Modulelayout as DEC's earlier machines, like thePDP-1andPDP-4.[2]

The system was designed withreal-time computinguse in mind, not justbatch processingas was typical for most mainframes. This made it popular in university settings and its support for theLisplanguage made it particularly useful inartificial intelligencelabs likeProject MACatMIT.It was also complex, expensive, and unreliable as a result of its use of so many early-model transistors. Only 23 were sold, at prices ranging from $120,000 to $300,000.

The lasting influence of the PDP-6 was its re-implementation using modernsilicon transistorsand the newerFlip-Chip modulepackaging to produce thePDP-10.Theinstruction setsof the two machines are almost identical. The PDP-10 was less expensive and more reliable, and about 1500 were sold during its lifetime.

History

[edit]

DEC's first products were not computers but a series of plug-in circuits known as Digital Laboratory Modules that performed digital logic. Users could wire the modules together to perform specific tasks. DEC soon introduced thePDP-1which was built out of large numbers of these modules, now known as System Building Blocks orSystem Modules.[3]

The PDP-1 used an 18-bit word. Word lengths in the early 1960s were generally some multiple of six bits, as thecharacter codesof the era were6 bits longand it was also a useful size for storingbinary-coded decimaldigits with an optional sign, as commonly used onIBMmachines of the era.[4]Large machines generally used a36-bit word length,but there were many variations. The PDP-1's 18-bit length allowed the machine to be simpler and less expensive than these, atUS$120,000(equivalent to $1,254,247 in 2023). Proposals for a PDP-2 and PDP-3 were turned down, and DEC's next machine, thePDP-4,was essentially a smaller and less-expensive PDP-1 that sold for roughly half the cost. ThePDP-5was built from the same components, but used a 12-bit word for even greater savings and sold for aboutUS$27,000(equivalent to $268,709 in 2023).[5]

The PDP-6 was DEC's first "big" machine. It used36-bitwords,in common with other large computers at the time from companies likeIBM,HoneywellandGeneral Electric.Unlike those machines, the PDP-6 was supplied with atimesharingsystem "out of the box". Timesharing had been available for other machines, most famously the PDP-1 at Project MAC, but the PDP-6 was the first such system to be supported by the manufacturer.[6]

Worldwide, only 23 PDP-6's were sold,[6][7]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". Nevertheless, the company later considered the PDP-6 to be a success:

Because the PDP-6 was the first computer to offer elegant, powerful capabilities at a low price, a great many of the PDP-6s built found their way into university and scientific environments, giving DEC a strong foothold in that market and providing both educated customer input for future models and a source of bright young future employees to assist in the hardware and software development for those future models.[8]

The sales were so slow that DEC eventually decided to abandon the system and announced that they would not build any more 36-bit machines.[9]Fortunately, this decision was reversed and a new 36-bit system was designed using the newFlip-Chipmodules and much smaller system boards to improve density and reliability. The resulting design was released in 1966 as thePDP-10.DEC describes this machine as the successor to the PDP-6. Although it was program-compatible with the PDP-6, it ran roughly twice as fast, had both timesharing and batch processing features, and offered a wide variety of expansions andinput/outputoptions.[10]It was far more successful and eventually sold about 1,500 machines.[11]

Description

[edit]

Architecture

[edit]

Addressing remained18-bit,as in earlier DEC machines, allowing for a 256 kwordmain memory,about 1 MB in modern terms. Memory was implemented usingmagnetic cores;a typical system included 32,768 words (equivalent to 144 kB on modern machines).[12]The use of a 36-bit word allowed two 18-bit addresses to be stored in a single memory location; this made it suited to the storage of acons,a widely used structure in theLisplanguage, meaning the PDP-6 could store a cons in a single word and read and write one in a single operation.[13]

The instruction set architecture could be categorized as "one-and-a-half address". Theopcodewas stored in themost significant bitsof the 36-bit word, using 9 bits. The next four bits indicate which of sixteen registers to apply the instruction to. The last 18 bits indicated an address. Thus, a typical instruction might be "add the value in memory location 1234 to the value in register 4". Thus the format contains one-and-a-half addresses, the half being the register.[12]

This left another five bits in the instruction word, bits 13 through 17. Bit 13 indicated the address was indirect; instead of the value stored in address 1234 being added to the selected register, the value in 1234 was interpreted as another address, the value in that location used. For instance, if the value in 1234 is 2345, the resulting instruction would add the value in 2345 to register 4.[12]This sort of access pattern was common as it allowed tables to be scanned over using a single instruction and then changing the value in memory to point to another location. The remaining four bits offered a similar functionality by selecting a second register as anindex register,allowing memory to be stepped through by changing the value in the register rather than inmain memory,which is generally faster.[12]

The registers in the PDP-6 were simply the first 16 memory locations of main memory. Most, if not all, PDP-6 systems were equipped with the optional Type 162 "Fast Memory", which constructed these 16 memory locations from discrete-transistor flip-flops. These operated four times as fast as thecore memory.[14]

Hardware

[edit]

The PDP-6 weighed about 1,300 pounds (590 kg), 1,700 pounds (770 kg) with "Fast Memory".[15]

The PDP-6 was infamous because of the 6205 board, a large (11 × 9 inches) board which contained 1 bit of arithmetic register (AR), memory buffer (MB), and multiplier-quotient register (MQ). The CPU was built from 36 such cards. It had 88 transistors, a two-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 fi xing 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.[16]

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.

Operating system

[edit]

The PDP-6 supportedtime-sharingthrough the use of a status bit selecting between two operating modes ( "Executive" and "User", with access toinput/output(I/O), etc., being restricted in the latter), and a single relocation/protection register which allowed a user'saddress spaceto be limited to a set section ofmain memory(a second relocation/protection register for shareable "high segments" was added on the PDP-10). The mainoperating systemused on the machine was an early version of what later becameTOPS-10,and several sites made custom versions of the system, which was available insource codeform. MIT'sIncompatible Timesharing System(ITS) operating system also began on the PDP-6.

Although it was possible to time-share a PDP-6 without a disk drive,[17]configuring it with four dualDECtapedrives "could effectively support about 4-6 simultaneous users." The same[b]system, with a single[c]disk drive, resulted in "real time-sharing (and) could easily handle 20-30 users."

Museum

[edit]

Stanford'sPDP-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 1990s 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.

Notes

[edit]
  1. ^Ed Thelen puts the delivery date in June 1964.
  2. ^64K words
  3. ^RP02

References

[edit]

Citations

[edit]
  1. ^Bell et al. 1978,pp. 487, 489: "The project from which the PDP-6, DECsystem-10, and DECSYSTEM-20 series of scientific, timeshared computers evolved began in the spring of 1963 and continued with the delivery of a PDP-6 in the summer of 1964.".
  2. ^Knight, Tom."PDP-6 Home Page".MIT CSAIL.Archived fromthe originalon 2 February 2004.
  3. ^"A Proposal to American Research and Development Corporation 27 May 1957"(PDF).Retrieved30 March2023.
  4. ^IBM Corporation (1954).704 electronic data-processing machine: manual of operation(PDF).p. 35.
  5. ^History 1975,pp. 3–5.
  6. ^abHistory 1975,p. 6.
  7. ^"PDP-6".DIGITAL Computing Timeline.Digital Equipment Corporation.
  8. ^Bell et al. 1978,p. 487-488.
  9. ^Bell et al. 1978,p. 488.
  10. ^History 1975,p. 8.
  11. ^Larry Lettieri (November 1980). "Foonly challenges DEC patents with emulator".Mini-Micro Systems.pp. 15, 17.
  12. ^abcdLin & Griffee,p. 2.
  13. ^"History".Common Lisp HuperSpec.The PDP-10 computer and its predecessor the PDP-6 computer were, by design, especially well-suited to Lisp because they had 36-bit words and 18-bit addresses
  14. ^Fast Memory Type 162 and Core Memory Type 161C(Technical report). DEC. 1965.
  15. ^"PDP-6 documents".bitsavers.org.F-68_PDP-6_Installation_Manual_Sep64.pdf, p. 2.
  16. ^Lin & Griffee,p. 3.
  17. ^DEC Timesharing (1965), by Peter Clark, The DEC Professional (magazine), volume 1, number 1

Bibliography

[edit]
[edit]
Retrieved from "https://en.wikipedia.org/w/index.php?title=PDP-6&oldid=1226232202"