SDS Sigma series

Maximum memory is limited by the length of the instruction address field of 17 bits, or 128K words (512K bytes).

The Xerox 500 series computers, introduced starting in 1973, were also 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[citation needed].

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.

In February 1990, Andrews University via Keith Calkins sold and delivered it to Applied Esoterics in Flagstaff, Arizona.

Independently, an array of 256 2-bit access control registers for the first 128k words of real memory function as a "lock-and-key" system in conjunction with two bits in the program status doubleword.

Otherwise the key in the PSD had to match the lock in the access register in order to reference the memory page.

The MIOP, intended to support slow speed peripherals allows up to 32 devices to be active at any time, but provides only a .3 MBPS aggregate data rate.

The Sigma 7611 Character Oriented Communications subsystem (COC) supports one to seven Line Interface Units (LIUs).

[10] The System Control Unit (SCU) was a "microprogrammable data processor" which can interface to a Sigma CPU, and "to peripheral and analog devices, and to many kinds of line protocol.

The remaining models initially ran the Batch Processing Monitor (BPM), later augmented with a timesharing option (BTM); the combined system was usually referred to as BPM/BTM.

A compatible upgrade (or renaming) of UTS, Control Program V (CP-V) became available starting in 1973 and added real-time, remote batch, and transaction processing.

[15][16] The Xerox software, called processors, available for CP-V in 1978 included:[17] †Program product, chargeable The Basic Control Monitor (BCM) for the Sigma 2 and 3 provided "Full real-time capability with some provision for batch processing in the background.

[4] The Modutest Mod 9 was redesigned and built by Gene Zeitler (President), Lothar Mueller (Senior VP) and Ed Drapell, is 100% hardware and software compatibility with the Sigma 9.