Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!ncar!gatech!rutgers!apple!well!brecher
From: brecher@well.UUCP (Steve Brecher)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Apple HD SC 80 does not support Asynch i/o.
Keywords: Hall of Shame entry?
Message-ID: <10205@well.UUCP>
Date: 3 Jan 89 21:11:49 GMT
References: <11605@dartvax.Dartmouth.EDU>
Reply-To: brecher@well.UUCP (Steve Brecher)
Organization: Software Supply, Sunnyvale, CA
Lines: 28


In article <11605@dartvax.Dartmouth.EDU>, earleh@eleazar.dartmouth.edu
(Earle R. Horton) writes:

> The following is an excerpt from David Phillip Oster's Asynch IO test,
> posted to info-mac:

>>    I want to get Macintosh programmers up in arms about the shameful state
>> of 3rd party Macintosh hard disk driver software. The Mac operating system
>> supports a wonderful feature that could let our programs run much faster,
>> but the developers of disk driver software have cheated us out of it.
>>
>>    I am referring to asynchronous disk i/o...

> The Apple hard drive and driver do not support asynchronous reads.  For
> shame! ... Does anyone know of drives that do?

It is not the driver software that is to be "blamed," it is the Macintosh.
Asynchronous I/O is possible only if interrupts and/or DMA are supported
by the hardware, and the Macintosh supports neither for SCSI I/O.  It might
be theoretically possible for a SCSI disk driver to perform seeks
asynchronous to program execution, but it would have to poll for seek
completion by using a VBL task or Time Manager task; that would probably add
enough overhead to make it infeasible.  As for SCSI data transfer, it all goes
through the CPU and hence cannot be performed asynchronously.
-- 

brecher@well.UUCP (Steve Brecher)