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From: Rich Alderson <news@alderson.users.panix.com>
Newsgroups: comp.os.vms
Subject: OS emulation [was Re: Bootcamp]
Date: 03 Jul 2025 15:14:11 -0400
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cross@spitfire.i.gajendra.net (Dan Cross) writes:

Too many levels to be certain who actually wrote the following quoted by Dan:

>> 5) The idea of emulating one OS on another OS is questionable
>>     in itself.

> This really needs to be qualified, as it is common and has been
> done for decades.  Evaluation criteria must include a) the
> complexity of the emulation target, and b) its alignment with
> the existing system design.

Agreed.

> Consider PA1050 on TOPS-20, for example: this was a type-2 hypervisor that
> allowed the DECSYSTEM-20 to provide very faithful emulation of TOPS-10.  But
> TOPS-20 is argably closer to TOPS-10 than, say, VMS is to Linux.

Boggle.

Tops-10 and TENEX/TOPS-20 run on the same base hardware (the PDP-10), and
TOPS-20 shares part of its name with Tops-10, but other than that they are
entirely unrelated.

PA1050 (which was written by the BBN folks who created TENEX, the ancestor of
TOPS-20), emulates a subset of the system calls of Tops-10, in order to allow
utilities like the FORTRAN and COBOL compilers to run on an OS for which they
were not engineered.  It does this by mapping a set of routines into the user
program which then make TOPS-20 system calls invisibly.  These routines are
mapped in when the first Tops-10 system call (which is in essence an illegal
instruction that thereby triggers a trap to the monitor) is encountered in the
user program instruction stream.

PA1050 is, in this sense, not a hypervisor but something equivalent to the Wine
emulator on Linux ("Wine Is Not an Emulator", indeed).

> And since TOPS-20 used a different mechanism for trapping into the executive
> for system requests than TOPS-10, it was easy to distinguish between the two.

At the base level, no, it does not.  It simply uses a different "illegal"
instruction to trigger the context switch from user mode to monitor mode than
does Tops-10.

> On the other hand, things like gVisor, which emulates the Linux
> kernel interface, are very complex and difficult to get right.
> And of course the PDP-10 was a much simpler machine than x86_64.

Agreed.

-- 
Rich Alderson					  news@alderson.users.panix.com
      Audendum est, et veritas investiganda; quam etiamsi non assequamur,
	  omnino tamen proprius, quam nunc sumus, ad eam perveniemus.
									--Galen