SDS Sigma series

The SDS Sigma series is a series of computers that were introduced by Scientific Data Systems in 1966. The first machines in the series are the 16-bit Sigma 2 and the 32-bit Sigma 7; the Sigma 7 was the first 32-bit computer released by SDS. At the time the only competition for the Sigma 7 was the IBM 360.

Memory size increments for all SDS/XDS/Xerox computers are stated in kWords, not kBytes. For example, the Sigma 5 base memory is 16K 32-Bit words (64K Bytes). Maximum memory is limited by the length of the instruction address field of 17 bits, or 128K Words (512K Bytes). Although this is a trivial amount of memory in today's technology, Sigma systems performed their tasks exceptionally well, and few were deployed with, or needed, the maximum 128K Word memory size.

The Xerox 500 series computers, introduced starting in 1973, are compatible upgrades to the Sigma systems using newer technology.

In 1975 Xerox sold its computer business to Honeywell, Inc. which continued support for the Sigma line for a time.

The Sigma 9 may hold the record for the longest lifetime of a machine selling near the original retail price. Sigmas 9s were still in service in 1993. In 2011 the Living Computer Museum in Seattle, Washington acquired a Sigma 9 from a service bureau (Applied Esoterics/George Plue Estate) and has made it operational. That Sigma 9 CPU was at the University of Southern Mississippi until Nov. 1985 when Andrews University purchased it and took it to Michigan. In Feb. 1990 Andrews University via Keith Calkins sold and delivered it to Applied Esoterics in Flagstaff. Keith Calkins made the Sigma 9 functional for the museum in 2012/13 and brought up the CP-V operating system in Dec. 2014. The various other system components came from various other user sites, such as Marquette, Samford, Xerox/Dallas.

The format for memory-reference instructions for the 32-bit Sigma systems is as follows:

For the Sigma 9, when real extended addressing is enabled, the reference address field is interpreted differently depending on whether the high-order bit is 0 or 1:

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