Path: utzoo!attcan!uunet!lll-winken!ncis.llnl.gov!ncis!helios.ee.lbl.gov!pasteur!ames!oliveb!tymix!antares!jms
From: jms@antares.UUCP (Joe Smith)
Newsgroups: comp.arch
Subject: Re: Shells everywhere
Summary: TOPS-10 has alternate contexts
Keywords: Release 7.03
Message-ID: <350@antares.UUCP>
Date: 18 Jan 89 05:40:57 GMT
References: <28200249@mcdurb> <451@babbage.acc.virginia.edu> <1951@scolex> <1121@raspail.UUCP> <2618@ficc.uu.net> <5657@cbmvax.UUCP> <10847@tekecs.TEK.COM>
Reply-To: jms@antares.UUCP (Joe Smith)
Organization: Tymnet QSATS, San Jose CA
Lines: 51

In article <10847@tekecs.TEK.COM> andrew@frip.gwd.tek.com (Andrew Klossner) writes:
>A shell requires that the system supply fork() and exec(), or perhaps
>start_task() (fork and exec combined) system services, or something
>analogous and reasonably efficient.  Lots of "usable computers" don't
>have these.  Two examples that come to mind are the original PDP-10
>running TOPS-10, and IBM CMS under VM/370.
> [deleted]
>write a shell for it by slipping a few tentacles into the (unprotected)
>kernel.  This wouldn't work under TOPS-10, because there's no way for a
>shell coexisting with a user task to get control when the task exits.
>  -=- Andrew Klossner   (uunet!tektronix!hammer!frip!andrew)    [UUCP]

Your example is referring to how TOPS-10 used to work.  Release 7.03
introduced alternate contexts.  From the command level, you can type
Control-C to stop execution, type PUSH to get an alternate context, do
a bunch of stuff, type POP to get back to the original context, and finally
type CONTINUE.  Some commands (such as HELP) are defined as auto-push, so
they do not wipe out your "core image".

From the system call level, a program can execute a CTX. uuo which will
run a program in another context and report back the ending status.

A similar feature exists in the OS that Tymshare created for their PDP-10's.
The frame-run option to the FRMOP uuo creates a child frame, runs a program,
and reports ending status.
	______________________________________________________________

There is one feature of TOPS-10 that I have yet to see duplicated.  That is
the ability to scan up the directory heirarchy automatically when a file is
openned for reading and does not exist in the current directory. 

For example:
I have the sources to TOPS-10 in [10,11,TOPS10,703]*.MAC and my default
directory is [10,11,TOPS10,703,TEST] with the /SCAN bit set. If I type
".COPY COMCON.MAC=COMCON.MAC" and [10,11,TOPS10,703,TEST]COMCON.MAC does not
exist, the OS will find the file in [10,11,TOPS10,703]COMCON.MAC on the LOOKUP
and PIP will duplicate it in the default directory.  Editing this file will
modify only the copy in the TEST directory, leaving the original alone.
When it comes time to assemble and load the monitor, the linker will use
the modified modules from the TEST directory and automatically find the
unmodified modules in the directory above it.

With TOPS-10 it is easy to tell when to stop scanning up the directory tree;
it stops when it comes UFD (User File Directory).  I could see where an OS
like UNIX might have a problem in determining where to stop the scan (so as
to not go all the way up to the root).
-- 
Joe Smith (408)922-6220 | JMS@antares.Tymnet.COM[131.146.3.1] or jms@opus
McDonnell Douglas FSCO  | UUCP: {ames|pyramid}oliveb!tymix!antares!jms
PO Box 49019, MS-D21    | PDP-10: JMS@F74.Tymnet   CA license plate: "POPJ P,"
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