Path: utzoo!yunexus!geac!syntron!jtsv16!uunet!ncrlnk!ncrcae!hubcap!mark
From: mark@hubcap.UUCP (Mark Smotherman)
Newsgroups: comp.os.misc
Subject: Re: a very naive Question???
Summary: old 360 TSO
Message-ID: <3228@hubcap.UUCP>
Date: 12 Oct 88 21:54:14 GMT
Article-I.D.: hubcap.3228
References: <835@amethyst.ma.arizona.edu> <5085@medusa.cs.purdue.edu>
Organization: Clemson University, Clemson, SC
Lines: 33

In article <5085@medusa.cs.purdue.edu>, spaf@cs.purdue.edu (Gene Spafford) writes:
> Swapping is not per se virtual memory.  If you swap images in and out
> of the exact same addresses, and if there is no translation of the
> addresses after any swap, then that is not virtual memory.

This is how IBM OS/360 TSO worked.  Since the base registers could be
accessed (and changed!) by the user, they could not be used for memory
relocation.  Essentially, they were used to offload relocation from the
loader (poor choice, since loader does relocation once and for all, while
base registers incur an execution time penalty each time the instruction
is executed) and to provide address abbreviation (fairly good choice, since
16 bits were used as the base:offset field in the instruction instead of
24; however, catenation of high-order bits as in the old PDP-8 is
sometimes better than addition since it avoids the addition time - the
drawback is the corresponding alignment requirement.)

To distinguish between swapping with execution-time relocation and
the IBM 360 technique of transfer to and from the same addresses, the
latter was called roll-out/roll-in.  The OS effect was that there was
no load balancing between multiple TSO partitions.  Once a command
interpreter was initiated, it was bound to its initial partition for
the duration of the terminal session.  Thus a pathological case could
be that half the users are allocated to partition A, half to partition
B - then if it so happened that all the partition A users logged off
or were in extended terminal I/O wait, then all the partition B users
still contended for that one partition.

Fred Brooks has indicated that early 360 design hopes were that the base
registers could be used for execution-time relocation.

-- 
Mark Smotherman, Comp. Sci. Dept., Clemson University, Clemson, SC 29634
INTERNET: mark@hubcap.clemson.edu    UUCP: gatech!hubcap!mark