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Mighty Mainframe OS

By Destination Z posted Mon December 23, 2019 03:33 PM


For decades—essentially since birth of the IBM System/370 in the early 1970s—mainframers have known and used the three primary members of the mainframe OS family: z/OS, z/VM and z/VSE.

Ask a hundred mainframers to name z Systems OSes and at least 99 of them might only mention those three. And seek the best solution for sustained mission-critical high-volume transaction processing against massive databases—the strategic foundation for leaders in many consumer and business serving industries—the answer will likely be z/OS running CICS or IMS, combined with middleware.

But simultaneously and quietly, as these systems evolved through multiple generations to current "z/" versions, another powerful environment was little known in the mainstream community, even though it was critical to multiple key industries and was used on, helped market and influenced evolution of the biggest of IBM's big iron.

That's the mainframe system software sibling: IBM z/Transaction Processing Facility (TPF).

Transaction-Oriented System

z/TPF's origin was the Semi-Automated Business Research Environment (Sabre), a joint development of American Airlines and IBM in the early 1960s. It progressed through Programmed Airline Reservation System (PARS), Airline Control Program (ACP) and ACP/TPF.

Today, z/TPF is unique in that it’s specifically a transaction-oriented OS highly optimized for the z Systems platform. I/O rates and memory usage are ultra-efficient; minimal instruction path lengths for system services yield more efficiency and capacity for customers.

According to Wikipedia, TPF delivers fast, high-volume, high-throughput transaction processing, handling large, continuous loads of essentially simple transactions across large, geographically dispersed networks. The world's largest TPF-based systems are easily capable of processing tens of thousands of transactions per second, three billion per day. TPF is also designed for highly reliable, 24-7 operation. It’s not uncommon for TPF customers to have continuous online availability of a decade or more, even with system and software upgrades.

As a narrowly focused transaction-processing environment, z/TPF lacks general user functions such as GUI, compiler/assembler, text editor and concept of a desktop.

Active customers are growing installed MIPS year-over-year. z/TPF has implemented API management, REST, JSON and SOA support to externalize applications on z/TPF and all industries are seeing increased transaction volumes.


A recent z/TPF user group presentation showed four key attributes for which the system was built: speed, efficiency, reliability and availability. Other attributes include:

  1. A short existence time for messages and transactions
  2. High throughput
  3. Quick response times
  4. Short instruction path lengths
  5. High availability
  6. Lightweight process model
  7. Fast restart and recovery
  8. Process protection
  9. Open-ended growth capacity
  10. Ability to make updates while taking traffic
Most other transaction processing models run on top of another OS, reducing efficiency. As a native transaction processing system, z/TPF simply needs fewer architectural layers.

The 3.5 MB memory-resident assembler language control program runs a dispatch loop. It includes efficient and hardware-specific device drivers and memory/storage management, and exploits hardware and software security features. z/TPF environments include uni (single engine), tightly coupled (multi-engine) and loosely coupled (multi-server).

These aspects make z/TPF a standout cost-effective engine for business, providing low cost processing for high-volume real-time transaction processing.

Source Code

Mainframe veterans who remember IBM removing system source code under the Object Code Only policy in the 1980s will be intrigued to learn that z/TPF is unique in providing full source code to licensed customers (with the exception of specific IBM proprietary code included in other systems).

In the past, some customers customized source to provide unique functionality. But over time they found that asking IBM for features/functions is a better option. IBM also creates many user exits where sites can easily implement customized actions without supporting IBM-specific code. Jeff van Minde, IBM Client Solutions professional, notes that providing source code is a win/win for both customer and IBM. "Customers can better understand internals and help troubleshoot problems more quickly than only IBM having the source code," he says.

z/TPF is delivered electronically, including control program; system services; continual APAR updates; yearly PUT releases; and drivers, automation, etc.

Industries Served

A colorful page on the z/TPF Users Group website shows dozens of marquee-name companies using z/TPF including Sabre, Delta Airlines, IRS, Bank of America, Visa, United, Marriott, Travelport, Amadeus, American Express and more.

As an example of processing power dedicated to z/TPF, in 2015 Marriott installed three IBM z13s and a BC12 to support z/TPF and z/OS applications and the disaster recovery site. They also upgraded DASD to EMC VMAX 40K and IBM virtual tape subsystems. Marriott International Inc. is a global leading lodging company based in Bethesda, Maryland, with more than 4,300 properties in 85 countries and territories. It regularly books over a million room nights a day with a value of $150 million dollars ($4,000/second in peak hours) on the z/TPF reservation system (MARSHA).


z/TPF development works closely with z/VM and z/OS teams since customers typically use multiple z Systems products together. They share interest in expanding the z Systems platform.

They also work closely with z Systems hardware and support most new features and functions so z/TPF tracks technology evolution. For example, hardware cryptography Central Processor Assist for Cryptographic Functions is integrated as are high performance FICON and other new features. And there have been z/TPF system-specific hardware enhancements for processors such as 3081, 3083 and 3090.

IBM uses Linux on z Systems (with the GNU Compiler Collection compiler and High Level Assembler) to build applications and kernel code; z/VM provides test environments. z/TPF can interact with Linux on the same mainframe for services and apps. Transaction time is minimal—milliseconds—letting customers explore ways to exploit this unique environment.

Most z/TPF developers and client support staff stay loyal to the environment. Since it's a very close-knit community and people tend to be involved in all aspects of z Systems, learning opportunities and career development are plentiful.


Though small in comparison to other mainframe systems' communities, the z/TPF "village" is active, closely knit, productive and well connected to IBM. The TPF User Group website links to resources and activities, including a yearly conference.

Other readings and resources include:
  1. IBM z/TPF page
  2. YouTube z/TPF channel
  3. z/TPF YouTube introduction presentation
  4. "Understanding zTPF" presentation slides

Gabe Goldberg has developed, worked with, and written about technology for decades. Email him at