20190709

The PDP-7 Where Unix Began

Serial Number of First Unix System

In preparation for a talk on Seventh Edition Unix this fall, I stumbled upon a service list from DEC for all known PDP-7 machines. From that list, and other sources, I believe that PDP-7 serial number 34 was the original Unix machine.
PDP-7 System from DEC sales literature

Building The Case

We know from simh sources, the restored PDP-7 Unix version 0 sources, and recollections from the time that the original machine used by Ken Thompson and Dennis Ritchie had, or likely had, the following hardware:
  1. 8k word of memory (option 149B)
  2. A tape reader (option 444B)
  3. A tape punch (option 75D)
  4. A 1MB disk drive (RB09 same as an RD10)
  5. A tty controller for a teletype (option 649)
  6. A standard video display (option 340)
  7. A custom video display (Unknown option number)
  8. A keyboard for input (also option 340)


We know from the service list that Bell Labs had three PDP-7s and one PDP-7A. Several of these machines had the standard options (the tape reader and teletype) and extra memory. Only one system, serial number 34, also had a disk drive, a custom unknown board that could be a Bell Custom display, and the standard display. In addition, that system shipped to Bell Labs in 1965 and appears to have been refurbished in 1969. This timeline matches the oral histories describing a discarded PDP-7 used to bring up the system in late 1969.

Here's the full table of all the systems shipped to Bell Labs with each system's options, taken from the 18 bit service list provided by Bob Supnik. You can check the list for other contenders.

Serial NumberOption #Option NameShip Date
PDP-7 #31100?
7PDP7 CPU unit?
75DPerforated paper tape punch and control
173Data interrupt multiplexer07-68
177BExtended arithmetic element1128?
444BPerforated tape reader and control
550ADECtape dual magnetic tape control12-67
649Teleprinter and control
CR01B100Cpm card reader and control
TU55Single DECtape transport12-67
TU55Single DECtape transport12-67
TU55Single DECtape transport03-69
PDP-7 #3401-69
75DPerforated paper tape punch and control07-65
149BCore memory module 8K, extends in 8K blocks07-65
177Extended arithmetic element07-65
340Precision incremental CRT display07-65
342Symbol generator for 340 display, first 64 characters07-65
370High speed light pen07-65
444BPerforated tape reader and control07-65
649Teleprinter and control07-65
CR01B100Cpm card reader and control12-66
PDP7CPU unit07-65
RC09RB09 disk?01-69
76 05477Custom Bell Labs Display?01-69
PDP-7 #4411-65
75DPerforated paper tape punch and control
149BCore memory module 8K, extends in 8K blocks11-65
177Extended arithmetic element
444BPerforated tape reader and control
649Teleprinter and control
PDP7CPU Unit11-65
PDP-7A #14903-69
149Core memory module 4K, extends subsequent 4K blocks
175Information collector expansion
175Information collector expansion03-69
177BExtended arithmetic element
340Precision incremental CRT display
347CCPU CRT subroutine interface
370High speed light pen
550DECtape dual magnetic tape control03-69
637Bit synchronous data communication system
CR01B100Cpm card reader and control
KA71AI/O device package
KA77AProcessor unit (PDP-7/A)
KB03Device selector expansion03-69
TU55Single DECtape transport03-69

Another surprise

V0 Unix could run on only one of the PDP-7s. Of the 99 PDP-7s produced, only two had disks. Serial number 14 had an RA01 listed, presumably a disk, though of a different type. In addition to the PDP-7 being obsolete in 1970, no other PDP-7 could run Unix, limiting its appeal outside of Bell Labs. By porting Unix to the PDP-11 in 1970, the group ensured Unix would live on into the future. The PDP-9 and PDP-15 were both upgrades of the PDP-7, so to be fair, PDP-7 Unix did have a natural upgrade path (the PDP-11 out sold the 18 bit systems though ~600,000 to ~1000). Ken Thompson reports in a private email that there were 2 PDP-9s and 1 PDP-15 at Bell Labs that could run a version of the PDP-7 Unix, though those machines were viewed as born obsolete.


Please see this followup post where I make the case that footage of the PDP-7 Ken would later use has been found on youtube....

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