Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!chiba!khb
From: khb%chiba@Sun.COM (Keith Bierman - Sun Tactical Engineering)
Newsgroups: comp.society.futures
Subject: Re: Memories...
Message-ID: <83550@sun.uucp>
Date: 4 Jan 89 00:45:58 GMT
References: <8812301210.aa25361@note.nsf.gov>
Sender: news@sun.uucp
Reply-To: khb@sun.UUCP (Keith Bierman - Sun Tactical Engineering)
Organization: Sun Microsystems, Mountain View
Lines: 42

In article <8812301210.aa25361@note.nsf.gov> fbaube@NOTE.NSF.GOV ("F.Baube") writes:
>
>With the advent of cheap non-volatile RAMs (such as ferro-
>electric), isn't there an operating system design issue relating
>to whether the dichotomy of core memory versus device-based
>storage isn't a bit artificial ? Is there an OS that can run a
>program without first loading it, by applying (virtual) program
>addresses directly to the device that has been determined to be
>byte addressable ?  To rephrase, why should an executable stored
>in a RAM disk first have to be copied to main memory in order to
>be executed ?
>
>Pardon me if this is an old question.

Consider the IBM mainframe extended storage options, or the Cray solid
state disk (common on XMP systems).

Both these system's lack the hardware support for enough memory, so
the RAMdisk is very different that real memory. Cray does not support
any sort of VM, so the RAMDISK is really just that (albeit very, very fast).

IBM does support VM (the OS and the memory system :>), there are
several levels of memory (2 levels of cache on some) followed by real
memory (RAM), followed by extended storage (which can only be spoken
to in special ways) followed by actual disks (and often these are
tiered w.r.t. performance). There is a large overhead in dealing with
overhead memory, so fetching (or writing) byte by byte would be a very
bad move.

With today's machines the different memory systems have very different
characteristics. Loading the program into system memory and then
paging out what turns out to not be useful works out fairly well.

Demand paged systems do pretty much what you suggest, in that
"loading" is very short, execution begins, followed (in nearly all
cases) a page fault, which results in some IO, followed by some
exeuction, etc. 

There was a very cheap 16032 kit for the PC and z80 machines which
used this form of demand paged memory, and it worked out fairly well.
Keith H. Bierman
It's Not My Fault ---- I Voted for Bill & Opus