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With our chart addictions this sub is a VMS.
Upgrading anything comes with a certain amount of risk. Ripping the OS out from underneath also increases that exponentially. So often it’s a safety/risk thing. When people’s lives and money are on the line risk becomes a big deal. That said, last time I worked in defence (which was 15 years ago) they were starting to move parts of command systems away from VMS. Some parts were even using extremely stripped down versions of windows. After defence I moved to (investment) banking. The place I worked at had a large legacy back office system. In terms of that the difficulty is that the system is so complex and unmigratable that the effort/cost to write a whole new system was prohibitive and when it provides minimal new functionality to clients the will/money to do it isn’t there. Having said that, again, that same place did start working on a replacement while I was there. They thought it would take 1yr with a big team. I wasn’t on that team but I left like 4yrs later and it was still nowhere near production ready 😂
Probably VMS. You’d be surprised how widespread it still is in banking and defence
Go back to engineering class. The core is most certainly not. Windows 3.11 ran on dos so did Windows 95 but after 98 dos was emulated. Have you ever built a backwards compatible API yet rewrote the code base? > The winapi is a layer on top of the native operating system. It was designed to resemble the api of Windows version 3, intended to make porting programs easy. It was one of three api layers, OS/2 and Posix where the other two. But have been removed because nobody used them, the winapi won by a land-slide. The original name was "win32", distinguishing it from the 16-bit version, but that caused too much confusion when 64-bit Windows came around so everybody calls it "winapi" today. The native operating system looks very different, it resembles VMS a lot. Which is no coincidence, they had the same designer. David Cutler, he used to work for DEC before it imploded. Money changed hands when DEC complained about it. The native api is undocumented. Intentionally to allow Microsoft to innovate on the OS while still allowing old programs to run. It is not otherwise a well-kept secret, it has been heavily reverse-engineered. And some of it did get documented, like the functions whose name start with "Nt" and "Rtl". Like NtCreateFile and RtlFillMemory. The Rtl functions are part of the glue that marry the winapi to the native api. You'll also get much more exposure to the native api when you write a device driver.