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 Number	Option #	Option Name	Ship Date
PDP-7 #3			1100?
	7	PDP7 CPU unit?
	75D	Perforated paper tape punch and control
	173	Data interrupt multiplexer	07-68
	177B	Extended arithmetic element	1128?
	444B	Perforated tape reader and control
	550A	DECtape dual magnetic tape control	12-67
	649	Teleprinter and control
	CR01B	100Cpm card reader and control
	TU55	Single DECtape transport	12-67
	TU55	Single DECtape transport	12-67
	TU55	Single DECtape transport	03-69
PDP-7 #34			01-69
	75D	Perforated paper tape punch and control	07-65
	149B	Core memory module 8K, extends in 8K blocks	07-65
	177	Extended arithmetic element	07-65
	340	Precision incremental CRT display	07-65
	342	Symbol generator for 340 display, first 64 characters	07-65
	370	High speed light pen	07-65
	444B	Perforated tape reader and control	07-65
	649	Teleprinter and control	07-65
	CR01B	100Cpm card reader and control	12-66
	PDP7	CPU unit	07-65
	RC09	RB09 disk?	01-69
	76 05477	Custom Bell Labs Display?	01-69
PDP-7 #44			11-65
	75D	Perforated paper tape punch and control
	149B	Core memory module 8K, extends in 8K blocks	11-65
	177	Extended arithmetic element
	444B	Perforated tape reader and control
	649	Teleprinter and control
	PDP7	CPU Unit	11-65
PDP-7A #149			03-69
	149	Core memory module 4K, extends subsequent 4K blocks
	175	Information collector expansion
	175	Information collector expansion	03-69
	177B	Extended arithmetic element
	340	Precision incremental CRT display
	347C	CPU CRT subroutine interface
	370	High speed light pen
	550	DECtape dual magnetic tape control	03-69
	637	Bit synchronous data communication system
	CR01B	100Cpm card reader and control
	KA71A	I/O device package
	KA77A	Processor unit (PDP-7/A)
	KB03	Device selector expansion	03-69
	TU55	Single DECtape transport	03-69

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....