Editing GE-600 series (section)

==Operating systems==
Originally the operating system for the 600-series computers was [[GECOS]], developed by GE beginning in 1962. GECOS was initially a [[batch processing]] system, but later added many features seen on more modern systems, including [[Computer multitasking|multitasking]] and multi-user support.

Between 1963 and 1964, GE worked with [[Dartmouth College]] on their [[Dartmouth BASIC]] project, which also led to the development of a new [[timesharing]] system to support it on the [[GE-200_series|GE-235]]. This was a great success and led to a late 1967 proposal for an improved version of the system running on the 635. The first version, known to Dartmouth as "Phase I" and GE as "Mark II", the original on the GE-235 becoming "Mark I", was a similar success. "Phase II" at Dartmouth was released as the [[Dartmouth Time-Sharing System]] (DTSS), while GE further developed Mark II into the improved Mark III.

The [[Computer History Museum]]'s Corporate Histories Collection describes GE's Mark I history this way:<ref>[http://corphist.computerhistory.org/corphist/view.php?s=select&cid=5&PHPSESSID=8921e36474c6ff040ecb6c3b587a6be0 Computer History Museum's Corporate Histories Collection: Company Details - General Electric Information Services (GEIS)]</ref>
:The precursor of General Electric Information Services began as a business unit within General Electric formed to sell excess computer time on the computers used to give customer demos. In 1965, Warner Sinback recommended that they begin to sell time-sharing services using the time-sharing system (Mark 1) developed at Dartmouth on a General Electric 265 computer. The service was an instant success and by 1968, GEIS had 40% of the $ 70 million time-sharing market. The service continued to grow, and over time migrated to the GE developed Mark II and Mark III operating systems running on large mainframe computers.

The GE Mark II operating system (later Mark III) was used by GE Information Services as the basis for its timesharing and networked computing business. Although Mark II / Mark III was originally based on the Dartmouth system, the systems quickly diverged. Mark II/III incorporated many features normally associated with on-line transaction-processing systems, such as journalization and granular [[file locking]]. In the early-to-mid-1970s, Mark III adopted a high-reliability cluster technology, in which up to eight processing systems (each with its own copy of the operating system) had access to multiple file systems.

The [[Multics]] operating system was begun in 1964 as an advanced new operating system for the 600 series, though it was not production-ready until 1969. GE supplied the hardware to the project and was one of the development partners (the others were [[Massachusetts Institute of Technology]] and [[Bell Labs]]). GE saw this project as an opportunity to clearly separate themselves from other vendors by offering this advanced OS which would run best only on their machines. Multics required a number of additional features in the CPU to be truly effective, and John Couleur was joined by Edward Glaser at MIT to make the required modifications. The result was the [[GE 645]], which included support for [[virtual memory]]. Addressing was modified to use an 18-bit ''segment'' in addition to the 18-bit address, dramatically increasing the theoretical memory size and making virtual memory much easier to support.