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Comment by pjmlp | Hacker News Reader

OS/400 is very interesting OS for those interested in OS architectures.

The complete userspace is bytecode based, regardless of the language.

Applications are compiled on installation phase, or when the generated code is deemed not to be valid any longer.

When there was an architecture change for the PowerPC, many installations only required a regeneration of the installed software.

A concept that Microsoft tried with Longhorn and Windows Phone 7. Or we could even say, the model Android almost has as well.

Native Oberon and Inferno also tried a similar approach, to certain extent.

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> When there was an architecture change for the PowerPC, many installations only required a regeneration of the installed software.

Unfortunately it wasn't quite that clean in practice. Regen required TIMI to have access to the compilation templates for each program. These were intermediate compilation stages ( bytecode, I suppose ) that it could then translate to the new machine architecture.

However, these templates were often missing, deleted or out-of-sync. So we did an awful lot of recompiling from source, when we could find it...

Thanks for the input.

I only did a bit of AS/400 administration back in 1994's Summer, just logged into the system and started the backup procedure.

I was actually doing Clipper development and the company where I had a Summer job used AS/400 systems for accounting.

Most of I know about OS/400 bytecode system was found looking for compiler technologies a couple of years later.

The thing I find interesting about OS/400 (or whatever it's name is this month) is the single-level store. When your program needs access to the contents of a file, it's just a memory offset. The OS relies on the swapping mechanism to bring those pages into available memory. Which, when you have flat 64 (one could argue 65) bit addressing … why not?

That is why I always call it by the original name. :)

Yep that is also a nice feature.

Wait wait wait...so it effectively memory maps all the things?

Mainframe environments are weird.

And OS/400 files are libraries that you need to open, as another example of strangeness.

Or the OS/360 which uses virtualization for all OSs, like Hyper V does on Windows.

The first OS to boot from the hypervisor has master rights, but all OSs are virtualized.

> Or the OS/360 which uses virtualization for all OSs, like Hyper V does on Windows

No, the hypervisor was called VM. According to WP it most frequently ran CMS guests, which was a light weight single-app OS. But could also run OS/360 guests (which predated VM).

AS/400 isn't mainframe. It's midrange; smaller than a mainframe, and not as flexible.

"Everything's a file!"

"No, everything's memory!"

Android with ART sounds very much like that. They compile the bytecode to native code at installation or major arch change.

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