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From: wagner@utcs.UUCP
Newsgroups: comp.sys.amiga
Subject: Re: Amiga Multi-tasking Overhead
Message-ID: <1986Dec21.165050.4430@utcs.uucp>
Date: Sun, 21-Dec-86 16:50:50 EST
Article-I.D.: utcs.1986Dec21.165050.4430
Posted: Sun Dec 21 16:50:50 1986
Date-Received: Sun, 21-Dec-86 17:35:20 EST
References: <1986Dec2.102542.4658@utcs.uucp> <1085@cbmvax.cbmvax.cbm.UUCP> <1986Dec11.185731.13459@utcs.uucp> <926@midas.UUCP>
Reply-To: wagner@utcs.UUCP (Michael Wagner)
Organization: University of Toronto Computing Services, general purpose UNIX
Lines: 29
Checksum: 41098

In article <926@midas.UUCP> jacc@midas.TEK.COM (Jac Colby) writes:
>When I was trying to figure out how to add my own external disks,
>I noticed that the select lines went active once every second if a
>disk was not present in a drive.  When a disk is placed in a
>drive, its presence is always detected within one second, and the
>validator is run.  However, the select lines *continue* to be
>pulsed, and at an even higher rate.  I presume that this is done
>to check for disk removal.  The disk-removed signal is active low,
>so detection of disk removal or insertion must be done by polling.
>An obvious question, though, is why doesn't the trackdisk.device
>just check for disk-removed prior to each disk access?

A very interesting observation, that sent me scurrying for my manuals.

In the RKM (part2), the discussion of the trackdisk.device includes the
command TD_REMOVE.  This nominates your routine to get control when a 
disk is removed.  I guess you could use this support to put up a requestor
and say PUT THAT BACK! or something.  Sounds marginally useful.  The
trackdisk driver promises to check and tell you, in any case, when you go
to do I/O.

So, you ask, how does it do this?  Well, the hardware manual claims there 
is a signal.  However, it doesn't interrupt at the point of removal.  Rather,
'the drives that support this signal latch it until the next time the heads
are stepped'.  Is it perhaps stepping the heads on a regular basis to see if
the disk got removed?  If so, could it be induced to be a little less 
eager?

Michael