Path: utzoo!attcan!uunet!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: Mac ][ bashing / blitters are not everything
Message-ID: <4516@cbmvax.UUCP>
Date: 18 Aug 88 21:26:59 GMT
References: <8808100909.AA00265@cory.Berkeley.EDU>
Organization: Commodore Technology, West Chester, PA
Lines: 36

in article <8808100909.AA00265@cory.Berkeley.EDU>, dillon@CORY.BERKELEY.EDU (Matt Dillon) says:
> 
>>> 	-It doesn't multitask ...

>>Funny, we run A/UX (Almost Unix) on our Mac-II and it multitasks ...

> 	John, surely you have been following the various arguments on *THAT*
> issue.  The jist is essentially: You might be able to run an operating system
> on some machine and get multitasking, for instance OS 9 on an Atari, but that
> doesn't let you run all the old software under multitasking, does it.  I.E.
> the software under to original/standard OS.

> 					-Matt

And that's only part of it.  If most users are single-tasking on a machine,
there's a really good chance that not many hardware vendors are considering
the hardware ramifications of multi-tasking.  Which may mean that when you do
start multitasking on such a machine, you may hit hard performance walls that
never surface when you're single tasking.

For example, consider hard disks.  At least on some Macs (don't personally know
about the Mac II), they handle disk I/O using a clever little hack that
basically turns your data ready signal from the hard disk control chip into
DTACK* for the CPU, so you read a full block just about as fast as possible
with the CPU.  Not all that bad for a single-tasking setup, as the program is
sleeping during this transfer anyway.  But come a multitasking OS, and all
of a sudden this becomes a long atomic operation that takes a bite out of 
your performance.  The standard Amiga hard drive controller is a DMA/FIFO
combo that works better than interrupt or Mac-style I/O when you have the
CPU sliced up amoung a number of tasks.


-- 
Dave Haynie  "The 32 Bit Guy"     Commodore-Amiga  "The Crew That Never Rests"
   {ihnp4|uunet|rutgers}!cbmvax!daveh      PLINK: D-DAVE H     BIX: hazy
		"I can't relax, 'cause I'm a Boinger!"