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From: xxremak@csduts1.lerc.nasa.gov (David A. Remaklus)
Newsgroups: comp.arch
Subject: Re: 370 channels and tape drives
Message-ID: <1991Feb13.172500.11664@eagle.lerc.nasa.gov>
Date: 13 Feb 91 17:25:00 GMT
References: <1991Feb8.153055.26279@yrloc.ipsa.reuter.COM>
Sender: news@eagle.lerc.nasa.gov
Reply-To: xxremak@csduts1.UUCP (David A. Remaklus)
Distribution: comp.arch
Organization: NASA/Lewis Research Center, Cleveland
Lines: 31

In article <1991Feb8.153055.26279@yrloc.ipsa.reuter.COM> rbe@yrloc.ipsa.reuter.COM (Robert Bernecky) writes:
>
(stuff deleted)
>
>At any rate, what you observed was the rewind operation being combined
>via chaining with some other operation, such as unload, etc. If the 
>rewind operation had been scheduled independently, the channel would have
>freed up during the rewind, and other tape operations could proceed 
>concurrently with the rewind.
>

Not necessarily.  Dynamic disconnect and reconnect for channel
protocol did not come until much later in the IBM 360/370 channel
protocol.  Under this protocol, certain devices (like disk and the
new 3480 cartridge tape) will return a status byte and disconnect
from the channel while the control unit and device service the
operation (usually some sort of positioning operation).  In the days
of the older tape drives, the control unit, when it had to take part
in the operation, was not able to disconnect from the channel so the
channel stayed busy.  The tape drive handles the rewind operation itself
freeing the control unit.  However, in the case of forward space
and backspace type operations, the control unit takes part in
identifying the tape marks and therefore does not return status until
the operation completes.  It is more likely that this is the type of
operation that the individual had observed being performed.

--
David A. Remaklus		   Currently at: NASA Lewis Research Center
Amdahl Corporation				 MS 142-4
(216) 642-1044					 Cleveland, Ohio  44135
(216) 433-5119					 xxremak@csduts1.lerc.nasa.gov