TheSDS 940 wasScientific Data Systems' (SDS) first machine designed to directly supporttime-sharing. The 940 was based on theSDS 930's24-bitCPU, with additional circuitry to provideprotected memory andvirtual memory.

It was announced in February 1966 and shipped in April, becoming a major part ofTymshare's expansion during the 1960s. The influentialStanford Research Institute"oN-Line System" (NLS) was demonstrated on the system. This machine was later used to runCommunity Memory, the firstbulletin board system.

After SDS was acquired byXerox in 1969 and became Xerox Data Systems, the SDS 940 was renamed as theXDS 940.

The design was originally created by theUniversity of California, Berkeley as part of theirProject Genie that ran between 1964 and 1969. Genie addedmemory management and controller logic to an existingSDS 930 computer to give itpage-mappedvirtual memory, which would be heavily copied by other designs. The 940 was simply a commercialized version of the Genie design and remained backwardly compatible with their earlier models, with the exception of the 12-bitSDS 92.

Like most systems of the era, the machine was built with a bank ofcore memory as the primary storage, allowing between 16 and 64 kilowords. Words were24 bits plus aparity bit.^[3] This was backed up by a variety of secondary storage devices, including a 1376 kword drum in Genie, orhard disks in the SDS models in the form of a drum-like 2097 kword "fixed-head" disk or a16384 kword traditional "floating-head" model. The SDS machines also included apaper tape punch and reader,line printer, and a real-time clock. Theybootstrapped from paper tape.

A file storage of 96 MB were also attached. Theline printer used was a Potter Model HSP-3502 chain printer with 96 printing characters and a speed of about 230 lines per minute.^[2]

Theoperating system developed at Project Genie was theBerkeley Timesharing System.^[3]By August 1968 a version 2.0 was announced that was just called the "SDS 940 Time-Sharing System".^[4]As of 1969, the XDS 940 software system consisted of the following:

• Time-Sharing Monitor (what is now usually called akernel)^[4]
• Time-Sharing Executive (what is now usually called acommand-line interface)^[4]
• CAL, the Conversational Algebraic Language^[5]
• QED, a text editor^[6]
• Fortran IV
• BASIC

The minimal configuration required to run the Software System included (partial list):

• Two 16-kword core-memory modules (with multiple access).^[7]
• Two rapid-access disc (RAD) storage units and couplers (just under 4M character capacity each); optionally two more could be connected
• Disc file and coupler, with 67M characters of storage
• Magnetic tape control unit and two magnetic-tape transports (controller supports up to 8)
• Asynchronous communication controller(s), supporting up to 64teletypewriter lines each

Additional software was available from the XDS Users' Group Library, such as astring-processing system, "SYSPOPs" (system programmed operators, which allow access to system services),CAL (Conversational Algebraic Language, a dialect ofJOSS),QED (a text editor), TAP (Time-sharing Assembly Program, anassembler), andDDT, a debugging tool.

Acathode-ray tube display with 26 lines that operated DDT loader-debugger that were originally designed to operate from ateletype terminal were also available.^[2]

Butler Lampson estimated that about 60 of the machines were sold.^[8]

• The major customer wasTymshare, who used the system to become the USA's best known commercial timesharing service in the late 1960s. By 1972 Tymshare alone had 23 systems in operation.^[9]
• Comshare, Inc, ofAnn Arbor, Michigan, was the second most important corporate customer. Tymshare, Comshare andUC Berkeley collaborated in much of the operating system development for the SDS 940.
• Douglas Engelbart'sAugmentation Research Center (ARC) used an SDS 940 for theiroN-Line System, more commonly known as NLS.^[10] It was with this computer that he gaveThe Mother of All Demos in December 1968, heralding many of the concepts associated with personal computing today.^[11]
• An SDS 940 host atStanford Research Institute was one of the four original nodes ofARPANET. It received the first message sent on the network in October 1969.^[12]
• ASan Francisco counterculture community action group called Resource One and located inProject One used a donated surplus XDS 940 as described inRolling Stone magazine in 1972.^[13]
• TheCommunity Memory project served as an early electronicbulletin board system.^[14]

• SDS Sigma series
• SDS 9 Series

• "SDS 940 Computer - console".Computer History Museum. RetrievedApril 17, 2011. (pictures)
• Software archive

• Mainframe computers
• Scientific Data Systems
• Time-sharing
• 24-bit computers

• Articles with short description
• Short description matches Wikidata