[js8]

Ah, sorry for the inaccuracies. I mean MVS as a predecessor of z/OS, of course.

What I mean by functional core/imperative shell is similar to what you mean by (the "kernel" is the "imperative shell" and the "userspace" is the "functional core"):

"for the most part operating systems design is an exercise in delegating as much as possible of those 'executive decisions' to userspace. 'mechanism, not policy' is the mantra for kernels and for system software in general, including things like device drivers and window servers"

And z/OS does that a lot, much more than Linux. On a typical z/OS, many of the functions that would be normally running inside Linux kernel are running in a separate address spaces, with limited authority.

But the intractable problem IMHO is, to decide the policy, you still need the authority to do so (you need to be able to invoke the commands to the kernel), so you can still wreak havoc in the system.

[kragen]

like what?

[js8]

For example, on z/OS, the whole disk storage subsystem (SMS, but there is more) is separate from the MVS (kernel). Security is also externalized in RACF server (in fact there are alternate products from non-IBM vendors). You can run multiple TCP/IP stacks, which are also running in their own address spaces. Sysplex serialization has its own address space.

All the address spaces involved in the operating system are coordinated through SVC or PC routines, which are like system calls, and scheduling of SRBs, which are kinda like kernel threads. I am not sure (although I am not aware of latest developments) if in Linux one can define a custom system call, like you can on z/OS. Or if you can schedule your own kernel thread from user space.

You seem to know about MVS, yet we probably disagree on whether it is to be called a microkernel or not. I am not an OS expert, and I never did kernel-level programming for Linux or z/OS. But I did read Lister's Fundamentals of Operating Systems long time ago, and that book is somewhat based on what MVS (the actual kernel of z/OS) does. It was written before the whole microkernel debate, which AFAICT might be just an artifact of enormous variety and complexity of x86 hardware.

So I would like to hear, in your opinion, what should have been different in MVS (or z/OS) for you to consider it a microkernel?

[kragen]

i don't know enough about it to have an opinion. thanks!