Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!rutgers!sri-spam!mordor!lll-lcc!ames!ucbcad!ucbvax!decvax!decwrl!delni.dec.com!goldstein
From: goldstein@delni.dec.com.UUCP
Newsgroups: comp.sys.atari.st
Subject: multitask discussion
Message-ID: <8019@decwrl.DEC.COM>
Date: Wed, 11-Feb-87 11:59:17 EST
Article-I.D.: decwrl.8019
Posted: Wed Feb 11 11:59:17 1987
Date-Received: Thu, 12-Feb-87 19:52:34 EST
Sender: daemon@decwrl.DEC.COM
Organization: Digital Equipment Corporation
Lines: 48


    Okay, just how tough a nut to crack is multi-tasking on the ST?
    Hackers are hereby solicited.  I'm a user, not a developer.
    
    I don't want to learn 68000 machine language in order to figure
    out what's going on in the ST.  I've heard various rants on the
    net about how you can't do decent multi-tasking on a 68k without
    a hardware memory manager.  Others claim it's possible, if a bit
    tricky.
    
    Here's what I've gleaned so far.  I hereby solicit mavens to correct
    me, add details, etc.  The 68000 has numerous addressing modes.
    Absolute addressing permits the entire 24-bit address bus to be
    used, sans segmentation, but such programs are not relocatable.
    (My RT-11 background reminds me of "background" .SAV files.)  Other
    address modes permit relocatable code, but the PC relative pointers
    are only 16 bits wide, so these programs must be segmented a la
    80-series programs.  Macintosh progams are required to do this,
    but ST programs aren't.  (Developer's guidelines and all that.)
    
    If this is true, then for multitasking to work, any given program
    must be able to go anywhere in memory.  This means that either the
    program is coded/linked as relocatable (a pain if this means segmenting,
    or just a useful guideline if it doesn't mean segmenting), or there
    must be something in the O/S (image activator?) that actually mungs
    all the absolute addresses in the image to fit the available memory
    location.  Could that be done reasonably, or would it be too complex?
    
    One proposal (in the discussion of porting MINIX) said that you could 
    outswap each task (into memory, not physical disk) every time the 
    context swapped. That sounds a bit slow -- shuffling a good-sized image 
    into high memory might slow things down quite a bit, though a blitter 
    might make it reasonable!  Plus you'd need enough memory for the
    largest image to run in plus for all the images to outswap to.
    On the other hand a 1040ST has an awful lot of memory.
    
    I'd be very willing to pay a bit for, say, a cartridge that added
    multi-tasking to the ST.  If some hacker out there wrote a
    p.d. program that added it, with GEM intact (not like MT-C Shell,
    which provides its own environment), then he'd be a bit of a hero,
    don't you think?
    
    We'll save the discussion about stealing the whole screen, vs. using
    a window for a program to run in, for later.  (Desk accessories
    seem to be the only ones that usually don't grab it all.  I suppose
    it's the efficiency of bypassing GEM, which doesn't do text too 
    well.)
          fred