Version 6 Unix
|Unixes by Bell|
Sixth Edition Unix, also called Version 6 Unix or just V6, was the first version of the Unix operating system to see wide release outside Bell Labs. It was released in May 1975 and, like its direct predecessor, targeted the DEC PDP-11 family of minicomputers. It was superseded by Version 7 Unix in 1978/1979, although V6 systems remained in regular operation until at least 1985.
An enhanced V6 was the basis of the first ever commercially sold Unix version, INTERACTIVE's IS/1. Bell's own PWB/UNIX 1.0 was also based on V6, where earlier (unreleased) versions were based on V4 and V5. Whitesmiths produced and marketed a (binary-compatible) V6 clone under the name Idris.
The Lions book 
Since source code was available and the license was not explicit enough to forbid it, V6 was taken up as a teaching tool, notably by the University of California, Berkeley, Johns Hopkins University and the University of New South Wales (UNSW). Berkeley distributed a set of add-on program called the First Berkeley Software Distribution or 1BSD, which later became a full-fledged operating system. UNSW professor John Lions' famous Commentary on UNIX 6th Edition was an edited selection of the main parts of the kernel as implemented for a Digital PDP-11/40, and was the main source of kernel documentation for many early Unix developers. Due to license restrictions on later Unix versions, the book was mainly distributed by samizdat photo-copying.
In 1977, Richard Miller and Ross Nealon, working under the supervision of professor Juris Reinfelds at Wollongong University, completed a port of V6 Unix to the Interdata 7/32, thus proving the portability of Unix and its new systems programming language C in practice. Their Wollongong Interdata UNIX, Level 6 also included utilities developed at Wollongong, and later releases had features of V7, notably its C compiler. Wollongong Unix was the first ever port to a platform other than the PDP series of computers, proving that portable operating systems were indeed feasible, and that C was the language in which to write them. In 1980, this version was licensed to The Wollongong Group in Palo Alto that published it as Edition 7.
Around the same time, a Bell Labs port to the Interdata 8/32 was completed, but not externally released. The goal of this port was to improve the portability of Unix more generally, as well to produce a portable version of the C compiler. The resulting Portable C Compiler (PCC) was distributed with V7 and many later versions of Unix.
Variants and extensions 
Bell Labs developed several variants of V6, including the stripped-down MINI-UNIX for low-end PDP-11 models, LSI-UNIX or LSX for the LSI-11, and the real-time operating system UNIX/RT, which merged V6 Unix and the earlier MERT hypervisor.
After AT&T decided the distribution by Bell Labs of a number of pre-V7 bug fixes would constitute support (disallowed by an antitrust settlement) a tape with the patchset was slipped to Lou Katz of USENIX, which distributed them.
The University of Sydney released the Australian Unix Share Accounting Method (AUSAM) in November 1979, a V6 variant with improved security and process accounting.
In the Eastern Bloc, clones of V6 Unix appeared for local-built PDP-11 clones (MNOS, later augmented for partial compatibility with BSD Unix) and for the Elektronika BK personal computer (BKUNIX, based on LSX).
- V6 source code
- Wollongong Interdata UNIX source code
- Unix V6 Manuals - Web interface to the V6 manual pages.
- Unix V6 documents, e.g. C Reference, and man pages
- The First Unix Port; Richard Miller's account of porting Unix to the Interdata 7/32
- Unix V5, V6 and V7 for PDP-11 installed in SIMH emulator on MS Windows
- Unix v6 for PDP-11 online emulator
- Quarterman, John S.; Abraham Silberschatz and James L. Peterson (December 1985). "4.2BSD and 4.3BSD as examples of the Unix system". Computing Surveys 17 (4): 379–418. doi:10.1.1.117.9743. "There are even some Version 6 systems still in regular operation."
- Johnson, S. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX System". Bell System Tech. J. 57 (6): 2021–2048. Retrieved 16 December 2012.
- Bayer, D. L.; Lycklama, H. (1975). "MERT - a multi-environment real-time operating system". Fifth ACM Symposium on Operating Systems Principles. Austin, TX. doi:10.1145/800213.806519.
- Toomey, Warren (December 2011). "The Strange Birth and Long Life of Unix". IEEE Spectrum (IEEE). Retrieved December 15, 2012.