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From: jb10320@uxa.cso.uiuc.edu (Desdinova)
Newsgroups: comp.sys.apple2
Subject: Re: Multitasking on a II
Message-ID: <1990Dec15.093137.17304@ux1.cso.uiuc.edu>
Date: 15 Dec 90 09:31:37 GMT
References: <11297.apple.net@pro-angmar>
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Organization: University of Illinois at Urbana
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m.tiernan@pro-angmar.UUCP (Michael Tiernan) writes:

>In-Reply-To: message from floyd@pawl.rpi.edu

>One thought, the 8088 and subsequent processors of that bastardized family
>have all had one little thing that none of the others (6502, Z80, etc) have
>had and that's some kind of instruction that CANNOT under ANY circumstances be
>preempted. 

   The 65816 has the TSB and TRB instructions.  Only one of the two is
necessary for preemtpive multitasking. Trust me on this, I just took a final
exam on this stuff.  The 65c02 also has this instruction. The gist of this
is that you must (in an uninterruptible step) test the value of a location
and then set it to some value.  This is used to implement simple mutex,
and from that semaphores, monitors, ad naseum.  In short, and FOR THE LAST TIME,
there is nothing in the architecture that prevents preemptive multitasking.
Arguments have been presented that a MMU would be nice, but it's definitely
NOT necessary (witness the Amiga).

> Now, I am not dead sure of the 8088 but the children of it have
>had this instruction.  Now you can't still say that this legitimizes the idea
>of multiprocessing (or any other word prefixed by "multi") but it did allow
>you to do more than you can without it.

   Well, actually- without an uninterruptible Test-And-Set (the aforementioned
instructions) you can't have preemptive multitasking without resorting to
some pretty clumsy and inefficient software techniques.

>But as has been shown many times you ain't close without hardware level memory
>management.

   Oh, I suppose the Amiga "ain't close" then?  What IS close- a Sequent?
a Pyramid? Cray perhaps?  What do you people want, a personal computer or
a bloody UNIX mainframe? (I would like to have Unix on my GS, as a sort of
intellectual exercise- I'd much prefer a truly multitasking GS/OS)

><< MCT >>
--
Jawaid Bazyar               | Being is Mathematics 
Senior/Computer Engineering | Love is Chemistry
jb10320@uxa.cso.uiuc.edu    | Sex is Physics
   Apple II Forever!        | Babies are engineering