Path: utzoo!utgpu!attcan!uunet!husc6!rutgers!mailrus!ames!pacbell!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: <10243@well.UUCP>
Date: 6 Jan 89 00:54:34 GMT
References: <11605@dartvax.Dartmouth.EDU> <10205@well.UUCP> <27336@ucbvax.BERKELEY.EDU>
Reply-To: brecher@well.UUCP (Steve Brecher)
Organization: Software Supply, Sunnyvale, CA
Lines: 37


In article <27336@ucbvax.BERKELEY.EDU>, oster@dewey.soe.berkeley.edu
(David Phillip Oster), argues for the feasibility of timer interrupt
driven polling of SCSI disk head positioning:

> ... we are talking about 10s of instructions per millisecond.  All the
> rest of the cpu time goes to the application to get useful work done.  The
> i/o driver authors are just lazy.

Right except for the last sentence (the "lazy" accusation).  Macintosh
Technical Note #96, p. 5:

"The SCSI Manager (all machines) does not currently support interrupt-driven
(asynchronous) operations.  The Macintosh Plus can never support it since
there is no interrupt capability, although a polled scheme may be implemented
by the SCSI Manager.  The Macintosh SE [and II have] a maskable interrupt for
IRQ.  Apple is working on an implementation of the SCSI Manager that will
support asynchronous operations on the Macintosh II and probably on the SE as
well. Because the interrupt hardware will interact adversely with any
asynchronous schemes that are polled, it is strongly recommended that third
parties do not attempt asynchronous operations until the new SCSI Manager is
released.  Apple will not attempt to be compatible with with any products that
bypass some or all of the SCSI Manager.  In order to implement software-based
(polled) asynchronous operations it is necessary to bypass the SCSI Manager."

I am not sure that the tech note is correct in stating that the SE/II have
a maskable interrupt for IRQ.  (IRQ is the SCSI host adaptor signal for an
interrupt request.)  As far as I can tell from Macintosh Hardware Reference,
APDA Draft March 2, 1987, while IRQ is tied to a VIA it is not an available
source of VIA interrupts, i.e., there is no bit for the IRQ interrupt in
the VIA interrupt and flag registers, and no unused bits.

In sum, lack of asynchronous capability in disk drivers is not due to laziness
on the part of their authors.
-- 

brecher@well.UUCP (Steve Brecher)