Operating system VSE: The great unknown in the mainframe

In 1964, IBM brought a family of compatible computers onto the market with the System/360, which has been further developed to this day as the zSeries. In addition to the “Operating System” OS/360, from which today’s z/OS emerged, there was the “Disk Operating System” DOS as an operating system for small and medium-sized models, which with the successor S/370 via DOS/VS, DOS/ VSE has been further developed into z/VSE.


Andreas Schmitter / Heinz Peter Maassen

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In the tenth episode of the mainframe interviews: Managing Director Hans-Dieter Lattwein (left) and Heinz Peter Maassen, Head of Software Development, take a look at the VSE operating system.

Today, z/VSE (Virtual Storage Extended) is still an operating system for IBM mainframes – but its further development was outsourced by IBM to the US company Century Software Technologies at the beginning of June 2021. The first version of the operating system developed by the new licensee has been marketed under the name VSEn V6.3 since May.

In this tenth edition of the mainframe interviews, we spoke about the history, present and future of VSE with two proven experts from the Düren-based software house Lattwein, which has been developing tools for application development for the mainframes under the name CPG since 1973: Managing Directors Hans-Dieter Lattwein and Heinz Peter Maassen, Head of Software Development.

Your company has been active in the VSE market practically from the start. Could you please briefly look back at the beginnings of your company with the VSE operating system?

Hans-Dieter Lattwein: The origins of our company lie with a medium-sized paper refiner in Düren. Around 1970, when a group of several companies decided to share and operate an IBM mainframe together, an S/360 model 20 was rented. This computer had the operating system “Disk Operating System” DOS and got by with 16 Kbytes of main memory; the next larger model, the 30, would have had 32 or 64 Kbytes of memory, but used the more expensive OS/360 operating system. At the time, that was more or less a cost decision – especially since the training for operating an OS environment would have taken much longer and would have been more extensive.

Heinz Peter Maassen: At the beginning of the 1970s we were still living in the era of punch cards, printers and the first hard drives were just coming onto the market. DOS only knew one partition, the background partition BG. It was therefore not yet possible to run any programs in parallel. OS/360 had advantages there because several regions could run in parallel there.

At the beginning of the 1970s, a spooling system was also added to DOS, which cached data for the slow I/O systems card readers and printers on fast hard disks and enabled parallel execution of up to four partitions.

In 1972, the first 3270 screens came onto the market in the USA, which could be used with the CICS transaction subsystem in both DOS and OS. With CICS, however, the computers needed more power and larger main memory. Thus, a System 370 model 135 was ordered for data center operation and replaced the 360-20 with 192 Kbytes of memory. The aim was to operate around 30 terminals locally. Virtual memory had not yet been invented at that time – only with the operating system DOS/VS Rel. 27 in 1972 was virtual memory also offered and used in DOS, which thus became DOS/VS. The online management system CICS also became CICS/VS and was able to benefit greatly from the virtual memory.

Later, new access methods for DOS/VS and OS/VS were developed called Virtual Storage Access Method (VSAM) and Virtual Telecommunications Access Method (VTAM). For the first time there was a compatible Application Programming Interface (API) for both systems, so that programs could be run under DOS/VSE as well as under the OS/370 successor MVS. Different programming techniques were only required for the so-called UR units, such as card readers and punches, as well as printers, and access methods such as Sequential Access Method (SAM) or native tape access.

At the end of the 1970s and beginning of the 1980s there was a switch to a new hardware technology: CPU chips in CMOS technology were cheaper than the previous bipolar technology and were used for the first time in the 4300 system. These systems came onto the market with 1 MB or 4 MB of memory and were able to quickly replace the mainframes used at the time. DOS/VSE appeared in the DOS area – and instead of OS/370, MVS became the popular operating system. Larger and more powerful computers were developed for both worlds and could be used by both z/VSE customers and z/OS customers.


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