Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!rutgers!mcnc!rti!bcw
From: bcw@rti.UUCP (Bruce Wright)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Why unix doesn't catch on
Summary: Virtual Machine operating systems
Keywords: multitasking
Message-ID: <2901@rti.UUCP>
Date: 21 Apr 89 03:14:22 GMT
References: <1922@dataio.Data-IO.COM> <101000047@hpcvlx.HP.COM> <199@isctsse.UUCP> <1308@ns.network.com>
Distribution: usa
Organization: Research Triangle Institute, RTP, NC
Lines: 25

In article <1308@ns.network.com>, ddb@ns.network.com (David Dyer-Bennet) writes:
> The "test for character available" and "read
> character" (from tty) hardware instructions were trapped and emulated by
> TSS-8, as were most of the other character-at-a-time device instructions.
> In fact TSS-8 was smart enough to recognize the busy wait loop and stop
> running it (providing its own non-busy wait).  There were TSS-8 native
> sys calls that avoided this, but these emulations were provided for
> compatibility with previous standalone programs.

Interesting.  I have used some PDP-8 systems as a user, but never did
much on the assembler level with it.  That capability would make it
the only small machine OS I'm aware of that had anything like that
kind of capability.  I've used quite a few small multitasking OSs on
various machines and although I've been pretty familiar with the innards
of some of them I have never seen that sort of thing.

It is, of course, not at all unusual on large machines (able to do a
nearly complete job of hardware emulation) - look at VM/370.  But this
isn't there to allow you to run ill-behaved application programs - it's
there to be able to run (and debug) different operating systems.

It does sound sort of limited though if it only caught single-character
hardware calls.

						Bruce C. Wright