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From: rjc@geech.ai.mit.edu (Ray Cromwell)
Newsgroups: comp.sys.amiga.misc,comp.sys.amiga.advocacy
Subject: Multitasking.
Message-ID: <12916@life.ai.mit.edu>
Date: 23 Jan 91 04:42:55 GMT
References: <1991Jan21.032207.6127@cunixf.cc.columbia.edu> <4827@skye.cs.ed.ac.uk> <12913@life.ai.mit.edu>
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  It seems a few people misread my original message about preemptive vs
cooperative multitasking and my chart. Re-read my message. I said
that my chart assumes each task uses ALL of its timeslice, this means
no waiting is occuring.  Mac multitasking will become jerky and bogged
down before Amiga tasks will in a heavily loaded system. Consider the
following psuedo-code.

Mac:

GetNextEvent();
HandleTheEvent();
while(condition) {
  do compute intensive loop
  such as uncrunch, fractal, trace, search a buffer, fibonacci,etc
}

Amiga:
CheckMsgPort();
HandleTheMsg();
while(condition) {
  compute loop
}

Now run 3 or 4 copies of this program on each machine. On the Mac,
the loop will grab 100% of CPU time and the other 3 copies will be DEAD.
On the Amiga, all 4 will run but at a slower speed (assuming equal priority).

  The Mac programmer must decide CAREFULLY where to place I/O calls to 
give up the CPU. If the compute loop is sufficiently complex( nested),
he will have to put them in many places.

  One can readily test this on the Amiga by starting up 2 or 3 copies of
MandelVroom or BioMorph. I did it, and on my 7mhz Amiga, the fractal
slows down linearly, but it does NOT become jerky. On the Mac, the other
fractal geneators will become jerky(possibly dead).
  Mac users must consider what Amiga users typically do. An artist may
place a long ray-trace task at -1 priority in the background and let it
run for a week! A programmer will put a compile in the background while
editing or reviewing source.
  I suspect the Mac's lack of priorities and preemptive tasking would cause
foreground tasks to become jerky if compute intensive tasks are placed in
the background (such as a long ray-trace).

  Also, I made the comment 'I don't consider the Mac to be multitasking
since ithasn't been multitasking since the start.' Let me clarify this.
On the Amiga, programming rules about multitasking have been around
since day one. However, on the Mac, I suspect the vast quantities of
software published before Multifinder was invented will be CPU hogs.
The Amiga faces the same problem with device independent graphics.
Most Amiga programs assume bitplaned graphics and alot of them write
directly to display memory. It will be hard for the Amiga to become device
independent and still work with older programs. The MS-DOS world has
an even bigger problem. No matter how many kludges IBM companies come up
with, they will always have to contend with MS-DOS and 8088 compatibility.
If they don't, they have to break millions of dollars of software out
there. 

  Now look at the Mac. Apple is attempting to layer a multitasking
interface ontop of a non-multitasking core. This is almost the same
as layering MS Windows ontop of MS-DOS. As far as I know, the Mac
has none of the facilities the Amiga has for sharing interupts,
exceptions, handlers devices, console windows, keyboard strokes.
Consider the Amiga's Task Control Block, or struct Process, can anybody
tell me what the Mac equivelent is?

--
Destroy Iraq, send them the Mac!