The GE-600 series was a family of 36-bit mainframe computers originating in the 1960s, built by General Electric (GE). When GE left the mainframe business the line was sold to Honeywell, who built similar systems into the 1990s as the division moved to Groupe Bull and then NEC.
The 600 series used 36-bit words and 18-bit addresses. They had two 36-bit accumulators, eight 18-bit index registers, and one 8-bit exponent register. It supported floating point in both 36-bit single-precision and 2 x 36-bit double precision, the exponent being stored separately, allowing up to 71 bits of precision (one bit being used for the sign). It had an elaborate set of addressing modes, many of which used indirect words, some of which were auto-incrementing or auto-decrementing. It supported 6-bit and 9-bit bytes through addressing modes; these supported extracting specific bytes, and incrementing the byte pointer, but not random access to bytes.
It also included a number of channel controllers for handling I/O. The CPU could hand off short programs written in the channel controller's own machine language, which would then process the data, move it to or from the memory, and raise an interrupt when they completed. This allowed the main CPU to move on to other tasks while waiting for the slow I/O to complete, a primary feature of time sharing systems.
Originally the operating system for the 600-series computers was GECOS, developed by GE beginning in 1962.
The GE-635 was used for the Dartmouth Time Sharing System starting in 1965.
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 was hardware supplier to the project and one of 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.
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