SDS Sigma series


The SDS Sigma series is a series of third generation 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. Maximum memory is limited by the length of the instruction address field of 17 bits, or 128K Words. 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.
, Seattle, Washington, 2014
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 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, Arizona. 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 other user sites, such as Marquette, Samford and Xerox/Dallas.

Models

32-bit systems

ModelDateFloating pointDecimalByte stringMemory mapMax memory
Sigma 71966optionaloptionalstandardoptional128
Sigma 51967optionalN/AN/AN/A128
Sigma 61970optionalstandardstandardstandard128
Sigma 91971standardstandardstandardstandard512
Sigma 81972standardN/AN/AN/A128
Sigma 9 model 2?standardstandardstandardstandard256
Sigma 9 model 31973standardN/AN/Astandard512

16-bit systems

Instruction format

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

 +-+--------------+--------+------+---------------------------+
 |*| Op Code | R | X | Reference address |
 +-+--------------+--------+------+---------------------------+
bit 0 1 7 8 1 1 1 1 3
 1 2 4 5 1
Bit 0 indicates indirect address.
Bits 1-7 contain the operation code 
Bits 8-11 encode a register operand 
Bits 12-14 encode an index register. 0 indicates no indexing.
Bits 16-31 encode the address of a memory word.

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:

 +-+--------------+--------+------+-+-------------------------+
 | | | | |0| Address in 1st 64K words|
 |*| Op Code | R | X +-+-------------------------+
 | | | | |1| Low 16 bits of address |
 +-+--------------+--------+------+-+-------------------------+
bit 0 1 7 8 1 1 1 1 1 3
 1 2 4 5 6 1

If the high-order bit is 0, the lower 16 bits of the address refer to a location in the first 64K words of main memory; if the high-order bit is 1, the lower 16 bits of the address refer to a location in a 64K-word block of memory specified by the Extension Address in bits 42-47 of the Program Status Doubleword, with the Extension Address being concatenated with the lower 16 bits of the reference address to form the physical address.

Features

CPU

Sigma systems provided a range of performance, roughly doubling from Sigma 5, the slowest, to Sigma 9 Model 3, the fastest. For example, 32-bit fixed point multiply times ranged from 7.2 to 3.8 μs; 64-bit floating point divide ranged from 30.5 to 17.4 μs.
Most Sigma systems included two or more blocks of 16 general-purpose registers. Switching blocks is accomplished by a single instruction, providing fast context switching, since registers do not have to be saved and restored.

Memory

Memory in the Sigma systems can be addressed as individual bytes, halfwords, words, or doublewords.
All 32-bit Sigma systems except the Sigma 5 and Sigma 8 used a memory map to implement virtual memory. The following description applies to the Sigma 9, other models have minor differences.
The effective virtual address of a word is 17 bits wide. Virtual addresses 0 thru 15 are reserved to reference the corresponding general purpose register, and are not mapped. Otherwise, in virtual memory mode the high-order eight bits of an address, called virtual page number, are used as an index to an array of 256 13-bit memory map registers. The thirteen bits from the map register plus the remaining nine bits of the virtual address form the address used to access real memory.
Access protection is implemented using a separate array of 256 two-bit access control codes, one per virtual page, indicating a combination of read/write/execute or no access to that page.
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. The system allows pages to be marked "unlocked", or the key to be a "master key". Otherwise the key in the PSD had to match the lock in the access register in order to reference the memory page.

Peripherals

Input/output is accomplished using a control unit called an IOP. An IOP provides an 8-bit data path to and from memory. Systems support up to 8 IOPs, each of which can attach up to 32 device controllers.
An IOP can be either a selector I/O processor or a multiplexer I/O processor. The SIOP provides a data rate up to 1.5 megabytes per second, but allows only one device to be active at a time. 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.

Mass storage

The primary mass storage device, known as a RAD, contains 512 fixed heads and a large vertically mounted disk spinning at relatively low speeds. Because of the fixed head arrangement, access is quite fast. Capacities range from 1.6 to 6.0 megabytes and are used for temporary storage. Large-capacity multi-platter disks are employed for permanent storage.
DeviceDevice typeCapacity Avg seek time Avg rotational delay Avg transfer rate
3214RAD2.75N/A8.5647
7202RAD.7N/A17166
7203RAD1.4N/A17166
7204RAD2.8N/A17166
7232RAD6.0N/A17355
3231Cartridge disk2.4 removable3812.5246
3232Cartridge disk4.9 removable3812.5246
3233Cartridge disk4.9 fixed
4.9 removable		3812.5246
3242Cartridge disk5.7 removable3812.5286
3243Cartridge disk5.7 fixed
5.7 removable		3812.5286
7251Cartridge disk2.3 removable3812.5225
7252Cartridge disk2.3 fixed
2.3 removable		3812.5225
3277Removable disk95308.3787
7271Removable disk46.83512.5245

Communications

The Sigma 7611 Character Oriented Communications subsystem supports one to seven Line Interface Units. Each LIU can have one to eight line interfaces capable of operating in simplex, half-duplex, or full-duplex mode. The COC was "intended for low to medium speed character oriented data transmissions."

System control unit

The System Control Unit 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 SCU executes horizontal microinstructions with a 32-bit word length. A cross-assembler running on a Sigma system can be used to create microprograms for the SCU.

Carnegie Mellon Sigma 5

The Sigma 5 computer owned by Carnegie Mellon University was donated to the Computer History Museum in 2002. The system consists of five full-size cabinets with a monitor, control panel and a printer. It is possibly the last surviving Sigma 5 that is still operational.
The Sigma 5 sold for US$300,000 with 16 kilowords of random-access magnetic-core memory, with an optional memory upgrade to 32 kW for an additional $50,000. The hard disk drive had a capacity of 3 megabytes.

32-bit software

Operating systems

Sigma 5 and 8 systems lack the memory map feature,
The Sigma 5 is supported by the Basic Control Monitor and the Batch Processing Monitor. The Sigma 8 can run the Real-time Batch Monitor as well as BPM/BTM.
The remaining models initially ran the Batch Processing Monitor, later augmented with a timesharing option ; the combined system was usually referred to as BPM/BTM. The Universal Time-Sharing System became available in 1971, supporting much enhanced time-sharing facilities. A compatible upgrade of UTS, Control Program V became available starting in 1973 and added real-time, remote batch, and transaction processing. A dedicated real-time OS, Control Program for Real-Time was also available for Sigma 9 systems. The Xerox Operating System, intended as an IBM DOS/360 replacement, also runs on Sigma 6/7/9 systems, but never gained real popularity.

Third party operating systems

Some third party operating systems were available for Sigma Machines. One was named GEM , and was said to be "rather UNIX-like". A second was named JANUS, from Michigan State University.

Applications software

The Xerox software, called processors, available for CP-V in 1978 included:
†Program product, chargeable

16-bit software

Operating systems

The Basic Control Monitor for the Sigma 2 and 3 provided "Full real-time capability with some provision for batch processing in the background." The Sigma 3 could also run RBM.

Clones

After Honeywell discontinued production of Sigma hardware — Xerox had sold most of the rights to Honeywell in July, 1975 — several companies produced or announced clone systems. The Telefile T-85, introduced in 1979, was an upward compatible drop-in replacement for 32-bit Sigmas. Ilene Industries Data Systems announced the MOD 9000, a Sigma 9 clone with an incompatible I/O architecture. Realtime Computer Equipment, Inc. designed the RCE-9, an upward compatible drop-in replacement that could also use IBM peripherals. The Modutest Mod 9 was redesigned and built by Gene Zeitler, Lothar Mueller and Ed Drapell, is 100% hardware and software compatibility with the Sigma 9. It was manufactured and sold to Telefile, Utah Power and Light, Minnesota Power, Taiwan Power and Ohio College Library Center.