> Domain/OS uses a single-level storage mechanism, whereby a program gains access to an object by mapping object pages directly into the process's address space.
It sounds similar in that respect to IBM i, and seems like an evolutionary branch that died off. What ever happened to this paradigm?
Even on IBM i it is in decline. Originally everything ran in the single-level store address space, but then they introduced additional non-single-level address spaces (teraspaces). And one of the major things that teraspaces are used for, is to run PASE, which is IBM i's AIX binary compatibility subsystem. And IBM appears to have a preference to ship new stuff in PASE. The single-level store environment is still used by "classic" apps (such as RPG and COBOL), but newer stuff – especially anything written in newer languages such as Java, Python, etc – runs outside of the single-level store in a PASE teraspace.
But one thing I think it demonstrates is a problem with non-mainstream operating system architectures. Even if a non-mainstream operating system architecture is technically superior, sooner or later you want to port software to it from a mainstream operating system, which means you need a compatibility layer implementing a more mainstream operating system architecture. And before you know it most of the code is running in the compatibility layer, because that's where all the new applications are coming from and there is no way you can keep up with that pace yourself. And then you have to ask what is the point of the innovative non-mainstream architecture if so much of the software you run doesn't actually use it. So eventually it leads you to moving off the non-mainstream architecture and on to a more mainstream one.
Is IBM i technically superior? It is a weird mixture of (a) advanced concepts like single-level store, an object-oriented operating system and bytecode virtual machine (b) legacy crud like EBCDIC, RPG, block mode terminals, 10 character limit on object names and a single-level filesystem (c) a severe lack of extensibility and openness in which a lot of OS concepts (e.g object types) are closed up so only IBM engineering can extend them (or possibly ISVs who pay big $$$$ for NDA manuals) (d) the completely different worlds of POSIX/AIX/Java grafted on the side, and increasingly taking over the rest. I grant that (a) could be said to be technically superior, but (b) and (c) clearly are not.
By the way, I'd argue about whether all of b) is technically inferior or not. Object name limits certainly are, but I got to really know data entry with block mode terminals long, long after its hey day (I've certainly come across it back then, but I was rarely a user). I feel that it can be enormously efficient for data entry and maintenance tasks. Many a person who had to move from intensive use of a block mode data entry terminal to performing the same tasks with a web app got quite annoyed at the clumsiness of it all.
The web was not created for "business apps" but for hypertext document retrieval, the other uses got bolted on and it still very much shows. It's sad, because proper terminal emulation well used to be a ubiquitous feature of the Internet, before browsers took over almost entirely.
I don't think block mode terminals are necessarily inferior. I see some big problems with 5250 though. The biggest is EBCDIC.
Another big problem is character-at-a-time interfaces let you build things like text editors (vim and emacs), spreadsheets (like Lotus 123), etc. Sure you can build a text editor for a block mode terminal (SEU on IBM i, XEDIT on z/VM, ISPF EDIT on z/OS) but there are just certain features and interaction styles that vim and emacs support that block mode terminals can't do as nicely (example: interactive search). Lotus 123 was actually ported to 3270 (to run under MVS and VM/CMS), I've never used it (I would love it if someone could find a copy so I could!) but from what I've heard it was pretty clunky compared to the MS-DOS / PC version.
Sometimes I think that block mode terminals could have exposed some kind of byte code to enable running some interactivity in the client. Actually real 3270s and 5250s generally had some kind of CPU in them (like an 8080) so I can't see why they couldn't have done that. And of course terminal emulators could do that. Then you could have these more flexible interaction styles that character mode terminals support even in a block mode terminal.
Oh yeah I agree, the actual implementation details in this case are icky.
> Another big problem is character-at-a-time interfaces let you build things like text editors (vim and emacs), spreadsheets (like Lotus 123)
That's true, but at the same time block mode allows for highly standardized and always latency free data entry and manipulation. I wonder if this is just a case for different technologies for different use cases.
> Sometimes I think that block mode terminals could have exposed some kind of byte code to enable running some interactivity in the client.
Hmm, it helps to preserve the zero latency aspect (if done correctly), but at the same time opens up the door for shoddy implementation and non-standard UX.
And then I'm sure people would come up with all sorts of "UI libraries" for terminals that they think are very clever, but just make everything fragmented and clumsy again, just like I often wish that a web site was just a plain old HTML page with maybe a standard web form, instead of whatever crazy js-backed UI the web framework du jour came up with...
Other vendors – such as DEC and HP – had dual-mode ASCII terminals that normally operated in character-at-a-time mode, but had an escape sequence you could use to switch them into block mode. Maybe that's the best of both worlds. However, in practice, few apps used the block mode, even "data entry" style apps which could use it often didn't. Part of that was that using block mode basically tied you to a single brand of terminal, whereas manually generating forms using character mode was more portable. A lot of clone terminals and emulators emulate DEC VT terminals but few of those clones and emulators included the block mode functions.