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Tandem Computers

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Tandem Computers was an early manufacturer of fault tolerant computer systems, marketed to the growing number of transaction processing customers who used them for ATMs, banks, stock exchanges and other similar needs. Tandem systems used a number of redundant processors and storage devices to provide high-speed "failover" in the case of a hardware failure, an architecture that they called NonStop. Over the two decades from the 1970s into the mid-90s, Tandem systems evolved from custom hardware to commodity CPU designs. The company was eventually purchased by Compaq in 1997 in order to provide that company with more robust server offerings. Today it is still known as NonStop, as a separate product line offered by Hewlett-Packard, ironically the company that the original founders left to form Tandem in the first place.



Tandem Computers was founded in 1974 by a group of engineers from Hewlett-Packard: James Treybig, Mike Green, Jim Katzman, and Jack Loustaunou. Their business plan called for systems that were safe from "single-point failures" that were only marginally more expensive than competing non-fault tolerant systems. Tandem considered this to be very important to their business model. Limiting the additional expense was important since customers often developed procedural solutions to failures when the price of fault tolerant hardware was too high.

The first system was the Tandem/16 or T/16 (later called NonStop I after the introduction of its successor, the NonStop II). The system design was complete in 1975, and the first example was sold to Citibank in 1976. The machine consisted of between 2 and 16 processors, each capable of about 0.7 MIPS with their own memory, I/O buses, and dual connections to their custom inter-CPU computer bus, Dynabus. The modules were constructed with dual paths so that any single failure would always leave at least one bus (both I/O and Dynabus), free for use by the other modules. The CPU was influenced by the HP3000 CPU, a microprogrammed 16-bit stack-based machine with 16-bit user addressing. Like the HP3000, the NonStop CPU added a number of registers for fast access, such as base addresses for global and local variables.

The Tandem NonStop series ran a custom operating system, initially called T/TOS (Tandem Operating System) but soon named Guardian. It supported a "NonStop" programming paradigm that allowed a program to be completely fault tolerant. Several other companies introduced failover technologies but only Guardian supported completely fail-safe transaction processing. A properly constructed Guardian program could fail at any point and resume transaction processing without any loss of data.

While conventional systems of the era, including mainframes, had mean time between failure on the order of a few days, the NonStop system was designed to fail 100 times less, with "uptimes" measured in years. Nevertheless the NonStop was deliberately designed to be price-competitive with conventional systems, with a simple 2-CPU system priced at just over two times that of a competing single-processor mainframe, as opposed to four or more times of most competing solutions.

NonStop I was followed by the NonStop II in 1981, a slight improvement in speed to 0.8 MIPS, but a more measurable upgrade in memory from a maximum of 1 MB per CPU in the later versions of the NonStop I, to 2 MB in the II, and the addition of a revamped virtual memory system allowing for considerably larger address spaces. The NonStop I was limited to 4 virtual memory segments (System Data, System Code, User Data, User Code) each limited to 128 kB in size.

The NonStop II increased the number of memory maps from 4 to 16, 8 of which were used for I/O, and provided a 32 bit address mode with user-accessible "extended segments" virtually unlimited in size. The same basic system, including the physical packaging, was used in 1983's NonStop TXP system that more than doubled the speed to 2.0 MIPS, and increased the physical memory to 8 MB. In all of these machines the same Dynabus system was used, which had been overdesigned in the NonStop I so they could avoid changing it in the future.

Introduced along with the TXP was a new fibre optic bus system, FOX. FOX allowed a number of TXP and NonStop II systems to be connected together to form a larger system with up to 14 nodes. Like the CPU modules within the computers, Guardian could failover entire task sets to other machines in the network.

The company attempted to grab a piece of the rapidly-growing personal computer market in 1985 with its introduction of the MS-DOS based Dynamite PC/workstation. Sadly, numerous design compromises (including a unique 8086-based hardware platform incompatible with expansion cards of the day and extremely limited compatibility with IBM-based PC's) relegated the Dynamite to serving primarily as a smart terminal. It was quietly withdrawn from the market within a short period of time.

In 1986 a major upgrade to the system was introduced, the NonStop VLX. VLX used a new Dynabus, increasing speed from 13 Mbit/s to 40 Mbit/s (total, 20 Mbit/s per independent bus). They also introduced FOX II, increasing the size of the networks from 1 km to 4 km. Using the original FOX VLX systems could be used with the older NonStop II and TXP's, but these systems were not supported on FOX II.

VLX was partnered with the NonStop CLX, a minicomputer sized machine for smaller installations. The CLX had roughly the same performance as the earlier TXP, but was much smaller and less expensive. By the end of its lifetime the CLX had increased in speed considerably, and competed with the VLX, 1991's CLX 800 was only about 20% slower than the VLX, with the main difference being more limited expansion abilities.

HP NonStop ServernetIn 1986 Tandem also introduced the first fault-tolerant SQL database, NonStop SQL. Developed totally in-house, NonStop SQL included a number of features based on Guardian to ensure data validity across nodes. NonStop SQL was famous for scaling linearly in performance with the number of nodes added to the system, whereas most databases of the era had performance that plateaued quite quickly, often after two CPUs. A later version released in 1989 added transactions that could be spread over nodes, a feature that remained unique for some time. Later, the SQL database group was first co-opted then absorbed into Microsoft's SQL development effort. One outcome of this collaboration was Microsoft's clustered system technology.

The NonStop Cyclone was introduced in 1989, introducing a new superscalar CPU design. It was otherwise similar to earlier systems, although much faster. In general terms the Cyclone was about four times as fast as the CLX 800, which Tandem used as their benchmark. On the downside the new CPU was complex and expensive, requiring four circuit boards to implement a single CPU.

In 1991, Tandem released the Cyclone/R and CLX/R, based on the MIPS R3000, taking advantage of the improved cost and performance opportunities presented by the RISC CPU. The initial customer release ran all software emulated, including the operating system, allowing the machine to reach the market years before native compilers were available. A static object code translator accepted programs written for the earlier stack-based CPU design and produced optimized MIPS code, which worked in concert with an interpreter. These tools delivered a seamless migration path for all software, including programs that depend on unique features of the legacy Tandem architecture. The fidelity and performance of this emulation technique meant that applications needed no reprogramming and programmers needed little additional training, even for debugging.

In 1993 the NonStop Himalaya K-Series using the MIPS R4400 was shipped. The next year a POSIX compatible environment, Open System Services (OSS), was added to the operating system. The Guardian personality remained available, and the operating system supporting both of these was named the NonStop Kernel (NSK).

In 1997 Tandem introduced the NonStop Himalaya S-Series. The S-Series machines were the first systems that changed the underlying architecture of the NonStop family, basing both the I/O and inter-CPU communication on their new ServerNet interconnect. Whereas Dynabus and FOX linked the CPU's together into a ring network, ServerNet was a true point-to-point network replacing both, and ran at much higher speeds. ServerNet later was used as the basis of the InfiniBand industry standard. The S-Series machines continued the use of MIPS processors, including the R4400 and R10000.

HP Integrity NonStopAll the more recent systems are based on commodity microprocessors, the internal circuits of which are not fully checked. To assure correct computation, each logical processor has two microprocessors operating in lockstep. If the results coming out ever disagree, the processor is considered to be faulting and instantly stopped. At that point Guardian moves that task to another processor as in earlier systems, guaranteeing that bad data is never written out due to hardware failures.

A different approach was used in a separate family of computers, the Integrity line. These computers used additional redundant CPUs running the same instruction stream. When a fault was detected (e.g. by lockstep mismatch), the failing module was disabled but the redundant module continued processing the instruction stream without interruption. Since this was handled primarily in hardware, it could be used with a slightly modified conventional operating system; Integrity used a Unix variant ("NonStop UX") rather than Guardian. The line was introduced in 1989, apparently as a response to the machines of Stratus Technologies (which were remarketed by IBM as IBM System/88). Although distinct from the NonStop line, the Integrity designs were also based on the MIPS processors. With the introduction of the Integrity S4000 in 1995, the line was the first to use ServerNet and moved towards sharing hardware designs with the NonStop line.

Tandem was acquired by Compaq in 1997. Compaq was in turn acquired by HP in 2002, bringing Tandem back to its original roots.

After being acquired by HP, the NonStop line has moved to Itanium based processors, called Integrity NonStop Servers. The original Integrity line is no longer produced but the name 'Integrity' has been adopted by HP for all Itanium based servers. The NSK operating system, now termed NonStop OS, continues as the base software environment for the NonStop Servers, and has been extended to include support for Java and integration with popular development tools like Visual Studio and Eclipse.



Tandem treated its employees with a great deal of respect, especially in the years leading to the company's first billion-dollar yearly sales figure. Innovative programs included:

  • TOPS ("Tandem Outstanding PerformerS") - every employee in the company could be nominated for this award, which was awarded to about the top 5% of employees annually. Winners (and a guest of their choosing) were treated to an all-expense paid trip to locations such as Hawaii, Vail, and similar resort areas for several days of fun and teambuilding. Management actually worked the event as hosts. TOPS was known, among other things, for its 24-hour open bar, where one could encounter senior VPs and even the company CEO dishing out drinks and stories of the company's early years.
  • Annual stock option - every employee of the company received a 100-share stock option each fall. As the company's stock rose (or split), employees could share in the company's financial success.
  • Sabbaticals - all US employees earned a six-week paid sabbatical (contiguous vacation) every four years, which could be augmented with personal vacation. Employees who chose to perform public service during their sabbatical could apply for an additional three weeks.
  • "First Friday" - the award-winning in-house Tandem TV staff produced a monthly program, broadcast live to all Tandem locations world-wide. While generally educational about some aspect of the company, the programs usually featured some member of the senior management team in a humorous way.
  • "Beer Bust" - Tandem sponsored a weekly get-together for its employees world-wide. It was called "beer bust" due to the availability of beer and wine, paid for by the company, in addition to other beverages and prepared food. This gave employees a way to cross barriers. It was not uncommon to see employees from various functions huddled in a corner, beer in hand, working to solve a problem.
  • "Third Class Mail" - Tandem was one of the first companies in which every employee had access to e-mail, which was divided into first, second, and third classes. Third Class mail allowed employees to buy and sell goods, ask questions, and share information that was not company-related. A wide variety of "SIGs" (Special Interest Groups) allowed employees to share a variety of interests with each other. 


Tandem Computers / HP NonStop Summary


(from 1974)
(from 1997)
(from 2002)

1974 Tandem Computers Incorporated is born.
1976 The first NonStop servers are sold to Citibank.
1979 The U.S. Treasury Department purchases NonStop servers to handle electronic transfer of US$10 billion weekly.
1980 The 1,000th NonStop processor ships.
1981 The NonStop II server is introduced. Nasdaq begins using NonStop severs for its securities transaction processing. Tandem Computers Incorporated joins the Fortune 500.
1983 NonStop TXP system that over doubled the speed to 2.0 MIPS, and the physical memory to 8MB.
1985 The V8 and XL8 are introduced, the first disk storage facilities that can be serviced while online.
1986 a major upgrade to the system was introduced, the NonStop VLX. VLX used a new Dynabus, increasing speed from 13MBps to 40MBps (total, 20MBps per independant bus). They also introduced FOX II, increasing the size of the networks from 1Km to 4Km. The world’s 30 largest telecommunications companies use NonStop technology, and NonStop solutions are used in 21 of the top 25 U.S. banks and 46 of the top banks outside the United States.
1986 Tandem introduced the first fault-tolerant SQL database, NonStop SQL. Developed on the famous Ingres code base.
1987 The NonStop CLX server is introduced, providing low-cost OLTP for distributed processing. NonStop CLX was a minicomputer sized machine for smaller installations. The CLX had roughly the same performance as the earlier TXP, but was much smaller and less expensive. By the end of it's lifetime the CLX had increased in speed considerably, and competed with the VLX, 1991's CLX 800 was only about 20% slower than the VLX, with the main difference being more limited expansion abilities.
1989 NonStop Cyclone was introduced. A later version of NonStop SQL released that added transactions that could be spread over nodes, a feature that remained unique for some time.
1990 The S2 server is introduced, the first fault-tolerant UNIX server based on RISC technology.
1991 The NonStop Cyclone/R and NonStop CLX/R servers are introduced, dramatically improving the economics of fault-tolerant OLTP. The NonStop K-series server is launched.
1994 ServerNet technology is introduced—the first system area network to enable high-speed connections between processors and I/O devices without having to use the processor as a concentrator.
1997 Tandem Computers Incorporated is acquired by Compaq Computer Corporation.
1999 Zero Latency Enterprise (ZLE) solutions are introduced, providing a way to achieve real-time, enterprise-wide access to up-to-the-minute information.
2001 Compaq announces that the entire NonStop product line will migrate to the powerful new Intel Itanium processor.
2002 HP merged with Compaq Computer Corporation.

Tandem Computers Incorporated was founded in Cupertino in November 1974. Decades later Compaq Computer Corporation acquired Tandem Computers Incorporated in 1997. After the merger of Hewlett-Packard Company and Compaq Computer Corporation in 2002 the HP NonStop Enterprise Division takes care about – now called – HP NonStop.
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