In a discovery bridging five decades of computing history, a magnetic tape containing the complete source code for Fourth Edition Research Unix—previously thought lost to time—was recovered from the University of Utah and successfully restored. This 1973 release from AT&T Bell Laboratories represents a watershed moment in operating system development, marking the transition of kernel components from PDP-11 assembly to the early C programming language.

"This tape contained a full system dump: kernel, utilities, compiler, and even binaries—a digital time capsule from the dawn of modern operating systems," noted restoration lead Diomidis Spinellis, who integrated the code into the Unix History Repository. "Only the Fourth Edition's manual had survived until now."

Spinellis employed forensic techniques to validate the tape's provenance amid debates that its contents resembled Fifth Edition Unix. By comparing file structures and leveraging git blame lineage analysis across editions, key findings emerged:

# File comparison between Fourth (V4) and Fifth (V5) Editions
comm -3 <(cd v4; find . -type f | sort) <(cd v5; find . -type f | sort)
  • 52,000 lines of Fourth Edition code persisted into Fifth Edition
  • Fifth Edition introduced 11,000 new lines—significant for 1970s development pace
  • File timestamps confirm an 8-month gap between editions

Crucially, the Fourth Edition's composition analysis revealed:

Code Origin Lines Percentage
New in V4 42K 89.6%
From V3 4K 8.5%
From V1/V2 1K 1.9%

Original Bell Labs developers, including Ken Thompson (SNOBOL III) and Robert H. Morris (math library), contributed attribution details. The restored code—now publicly accessible—offers unprecedented insight into C's early evolution and the architectural decisions that shaped modern computing, silencing doubts about this edition's historical footprint through data-driven archaeology.

Source: Diomidis Spinellis, Unix History Repository analysis (CC BY-NC 4.0)