PDP-7
PDP-7
The PDP-7 should be a footnote — a modest DEC midline machine: 18-bit, introduced 1964, $72,000, only 120 sold, essentially the same instruction set as the PDP-4 and PDP-9 it sat between. It was DEC's first wire-wrapped machine and its first to use Flip-Chip modules, and it ran DECsys — DEC's first small-computer OS. Unremarkable. Except for one spare unit sitting in a corner at Bell Labs.[1]
In 1969, Ken Thompson — exiled from the Multics project when Bell Labs pulled out — wanted to keep playing Space Travel, a planetary-motion game he'd written, and wanted a machine to play it on. The spare PDP-7 was available. So he wrote a file system for it. And a kernel. And a shell, an assembler, an editor — a complete operating environment in assembly, initially without even a name: Brian Kernighan dubbed it Unics, a pun on Multics (one user, not many). Within a year they'd moved to a PDP-11 — and dropped the joke spelling for Unix.[2]
The most consequential spare machine ever
The PDP-7 version of Unix is the ur-Unix: everything since — the hierarchical file system, the shell, the idea that an OS is a program you can carry — descends from what one man typed into a machine DEC sold 120 of. Thompson wrote it in PDP-7 assembly on a machine so minor DEC barely marketed it; the seed says it best — an unremarkable machine made immortal by what was skunkworks'd onto it. It's the same pattern as MUMPS, which was itself born on a PDP-7: the cheapest, most neglected machines kept becoming the cradles of the most durable software.[3]
Where it sits
Between PDP-1 (where the games and the hackers began) and PDP-8 (where the minicomputer became a product category), the PDP-7 is the quiet middle child whose only job was to be spare — and whose spareness is exactly why Thompson could build Unix on it without asking anyone's permission.
Check your understanding
- Who wrote the first Unix, on what machine, and why?
- What was Unix originally called — and what was the pun?
- How many PDP-7s were built, and what did one cost?
- What was DECsys?
Show answers
1 — Ken Thompson, 1969, on a spare PDP-7 at Bell Labs — initially to run his Space Travel game after Multics collapsed. 2 — Unics — Kernighan's pun on Multics ("one user, not many"). 3 — 120, at $72,000 — a deliberately cheap midline machine. 4 — DEC's first operating system for its 18-bit family — shipped 1965 for Fortran/assembly work.