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From: wwtaroli@rodan.acs.syr.edu (Bill Taroli)
Newsgroups: comp.sys.mac.system
Subject: Re: Preemptive scheduling
Message-ID: <1991Jan17.083351.3764@rodan.acs.syr.edu>
Date: 17 Jan 91 08:33:51 GMT
References: <48172@apple.Apple.COM> <1991Jan16.214809.25818@maverick.ksu.ksu.edu> <48184@apple.Apple.COM>
Organization: Syracuse University, Syracuse, NY
Lines: 66

In article <48184@apple.Apple.COM> heksterb@Apple.COM (Ben Hekster) writes:
>jxf@orion.cis.ksu.edu (Jerry Frain) responds to my latest posting:
>
>> How about "mouse response on the Mac goes away during volume recognition."
>
>Yes, but I mean, how does that relate to (or improve under) different types
>of multitasking?

Exactly. Disk access in the Mac would not be improved if preemtive multi-
tasking were used. Since all disk operations generate interrupts, you're 
going to get jerky mouse movement anyway.

>> [...] Haven't
>> you ever tried to open the folder that the file is in while the Mac is
>> trying to figure out what the name of your disk is?
>>
>> The jerkiness of the mouse caused by volume recognition is just damn
>> _unnecessary_, and irritating.
>
>I'm not familiar with details of the Disk Driver, but I always assumed that
>since the CPU (on machines other than the IIfx) is doing the transfer, it
>would not be possible to enable mouse interrupts as well without losing some
>of the sector.

Correct. However, again the assumption is made that preemptive multitasking
will solve this problem... WRONG! What will solve this problem is if Apple's
development team gets DMA transfers going. I understand the only reason they
weren't available in the IIfx is that the Virtual Memory and DMA teams did
something incompatible with one another, so they had to drop one in System 7.
So, DMA hardware sits on the IIfx motherboard pretty much useless.

Thus, if it's disk access interrupts that this guy is griping about, have
him get a job with Apple with both hardware and software teams so that he
can help speed the development of DMA...

[More misconceptions on multitasking and DMA hardware.]

>> You cannot tell me that during the time period that volume mounting is
>> taking place, that the CPU is _really_ busy.  Probably less than 1/1000
>> of the delay experienced is actually due to the fact that the CPU is
>> too busy to move your mouse.

Sure we can! The CPU in the Mac deals with every aspect of disk transfers. So,
if a disk is accessed, the CPU is involved all the way. True, you could 
probably get situations where the CPU sat idle for a relatively long amount of
time waiting for a disk transfer to complete (esp. for floppies), but this is
what DMA is designed to solve -- completely free the CPU from doing the 
intensive parts of disk access.  Again, this is a hardware and OS problem.

>> I think that *both* mouse responsiveness and volume mounting should be
>> provided at the same time.  It can be done.  Why do you not desire this?
>
>But I do.  That's why I like my IIfx so much.  It can be done, but it requires
>hardware support.  Because the IIfx's IOPs relieve the CPU from doing the disk
>accessing it is free to handle mouse movement stuff.

Yes and no. DMA isn't even fully supported in System 7. It is DMA that would
really give you the capability that you describe. You're just working with one
extremely high-power system.


-- 
______  Bill Taroli  --  Syracuse University  |  "The only thing necessary for
\ PT /                                        |   the triumph of evil is for
 \  /   Internet: wwtaroli@rodan.acs.syr.edu  |   good men to do nothing."
  \/    BITNET: wwtaroli@sunrise.acs.syr.edu  |                 -- Edmund Burke