Emulation’s a hobby in the secrets of SIMH

Emulation of legacy servers is a serious enterprise, except in those instances where it is not intended to be serious. Commercial is often the word that’s attached to a serious offering of emulation. There’s another brand of emulation, however, that is honest about its intentions and its targets for use. SIMH emulators have been created for several legacy hardware architectures. These software systems are not products, because they’re not supported by a company and are not sold.

The secrets of SIMH, though, stand in as a proof of two concepts. The first is the ability of software to mimic even the most advanced hardware. The history of emulators goes back as far as computing itself. Each time a new architecture was introduced, engineers felt certain that emulation would be too complex to succeed at replication.

In the OpenVMS ecosystem, SIMH delimited its scope of service. In a user guide for the software set called VAX MP the document said the software “targets primarily hobbyist use of OpenVMS. While the intent of VAX MP is to provide reasonably stable SMP VAX system simulation environment for hobbyist purposes, it cannot possibly have the same level of quality assurance or support as a commercial product. Therefore VAX MP is not recommended for production use.”

Sergey Obguev was behind the SIMH release for OpenVMS. The software emerged in 2012, the same year that another emulator for another legacy system went into production use. The HP 3000 on an emulator moved into commercial operations for the first time in the fall of 2012. Created for the MPE/iX market by Stromasys, that Charon product laid down guidelines for commercial success that even Obguev acknowledged in his VAX MP user guide. SIMH wouldn’t be powerful enough to drive production computers.

“If you are looking for a production solution, please examine other options. In addition to offering support and more thorough testing, the commercial solutions offer better per-processor performance as well, since they perform on-the-fly binary JIT compilation of VAX code into host processor code, which neither SIMH nor VAX MP as its derivative do not perform.”

SIMH serves its mission as a proof of emulation. It also proves the interest in legacy technology is everlasting. The efficiency tied to legacy computing needs a strong performance base for emulation, something stronger than a hobbyist-grade project.

“If you are looking for multiprocessor solution for production use,” the SIMH user guide states, “presumably your motive is increased performance, and that is where commercial JIT simulators significantly surpass interpreting simulators such as SIMH in general, and VAX MP specifically.”

An even more important feature of a commercial emulator is its ties with production operating systems. SIMH has only been tested with OpenVMS up to version 7.3. The emulation in Charon-VAX, for example, is tested through the most current version of OpenVMS.

Photo by timJ on Unsplash

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