Path: utzoo!utgpu!water!watmath!clyde!att!rutgers!gatech!linus!mbunix!eachus
From: eachus@mitre-bedford.ARPA (Robert Eachus)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Need info on exceptions
Summary: Virtual memory does not require virtual address spaces.
Keywords: Virtual memory, Address space
Message-ID: <39432@linus.UUCP>
Date: 31 Aug 88 17:36:47 GMT
References: <8808272322.AA03740@cory.Berkeley.EDU> <125@fishpond.UUCP>
Sender: news@linus.UUCP
Reply-To: eachus@mitre-bedford.arpa (Robert I. Eachus)
Organization: The MITRE Corporation, Bedford, Mass.
Lines: 64

In article <125@fishpond.UUCP> fnf@fishpond.UUCP (Fred Fish) writes:
>In article <8808272322.AA03740@cory.Berkeley.EDU> (Matt Dillon) writes:
>>: [somebody besides Matt writes]
>>:				An idea central to the Amiga
>>:is that an address in any tasks address space is not a virtual 
>>:address but a physical memory address. That means that you can hand 
>>:a pointer off to another task and it will be valid in THAT tasks's address
>>:space.
>
>>	Yes, this *IS* a nice idea.
>
>Personally I think this is the single biggest problem with the Amiga.  For
>a process running in a multitasking environment, it is akin to living in
>a society where it is perfectly acceptable for strangers to walk up to you
>at any time and stick their fingers in your mouth to examine your teeth.
>There is absolutely no concept of enforced privacy between processes.

     Fred  (and   many other people,   but not Matt  :-) are confusing
virtual  address spaces (not   virtual memory) with memory management.
AmigaDOS 2.0 (or  some such) will  probably  provide memory management
(assuming that you have a 68020 + 688851 or a 68030 or  ...) such that
all memory not declared MEMF_PUBLIC cannot be read or written by other
tasks, etc.  This is a not  a difficult job.  AmigaDOS  2.0 might also
provide  virtual memory, such that not  all valid memory addresses are
mapped  to  hardware memory.    (Again  not  too   difficult.   Memory
requested as CHIP memory, of course  cannot be made  virtual, and some
DMA disk controllers would  need to  be made smarter, but overall  not
too bad.)

     HOWEVER, to add    virtual address  spaces to  KickStart/AmigaDOS
would  be a messier project than  replacing  MS-DOS with OS/2! (And it
would  sell about as  well  because it  would be   at least as fat and
slow.)

     This is  not  a blanket  condemnation of  virtual   address space
systems.  Many such  OS's (but not  all) on  mainframes work well  and
don't  impose major burdens on  users.  (For example, many versions of
Unix on  VAXes.)  Some, such   as  Multics,  even  provide  low   cost
interprocess   memory sharing.  But   such  systems don't  belong   on
desktops. In single-user multitasking systems, unlimited address space
(as opposed to a finite but large address space) is not worth the pain
and agony.

     To sum up: 

     Adding   memory management  to   AmigaDOS  will   be a  win  (and
MEMF_PUBLIC is just one feature present  NOw to make the eventual move
less painful), but it will require a hardware MMU.

     Adding virtual memory also requires an  MMU, and could be done at
the same time as the above, but I'm not sure how important it will be.
(I'm typing this on an Amiga 2000 with 5 Meg of  RAM and a 40 Meg Hard
Disk.  If I had  VM, I'd probably use a  (880K) floppy as  the backing
store.   With   5  Meg or  20 Meg removables,   I'd find it  much more
interesting.)

     Adding virtual address spaces to AmigaDOS would be a big mistake
under any circumstances.

					Robert I. Eachus

with STANDARD_DISCLAIMER;
use  STANDARD_DISCLAIMER;
function MESSAGE (TEXT: in CLEVER_IDEAS) return BETTER_IDEAS is...