Sixth EditionUnix, 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 It was superseded by Version 7 Unix in 1978/1979, although V6 systems remained in regular operation until at least 1985. AT&T Corporation licensed Version 5 Unix to educational institutions only, but licensed Version 6 also to commercial users for $20,000, and it remained the most widely used version into the 1980s. 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 versions were based on V4 and V5. Whitesmiths produced and marketed a V6 clone under the name Idris.
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 languageC 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 was distributed with V7 and many later versions of Unix, and was used to produce the UNIX/32V port to the VAX.
A third Unix portability project was completed at Princeton, N.J. in 1976–1977, where the Unix kernel was adapted to run as a guest operating on IBM's VM/370 virtualization environment. This version became the nucleus of Amdahl's first internal UNIX offering.
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 a tape with the patchset was slipped to Lou Katz of USENIX, who distributed them. The University of Sydney released the Australian Unix Share Accounting Method 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 and for the Elektronika BK personal computer. V6 was used for teaching at MIT in 2002 through 2006, and subsequently replaced by a simpler clone called xv6.
