Path: utzoo!mnetor!tmsoft!itcyyz!yrloc!rbe
From: rbe@yrloc.ipsa.reuter.COM (Robert Bernecky)
Newsgroups: comp.arch
Subject: 370 channels and tape drives
Message-ID: <1991Feb8.153055.26279@yrloc.ipsa.reuter.COM>
Date: 8 Feb 91 15:30:55 GMT
Reply-To: rbe@yrloc.ipsa.reuter.COM (Robert Bernecky)
Distribution: comp.arch
Organization: I P Sharp Associates, Toronto
Lines: 35


I think it was this group, or alt.folklore.computers, or whatever it's called,
that someone asked why IBM 360 tape drives tie up the channel during rewind.
This is not a 360 problem as much as an operating system access method coder
problem. 

The 360 channel allows chaining of commands, so that you can, for example,
write a disk command chain which will seek to a cylinder, search for a specific
record on a track, and read/write one or more records from that point. If the
commands were not chained, another process or cpu could move the arm between
the commands, and you'd read gibberish.

The device signals (channel end) when the data transfer associated with the
operation has completed. Concurrently, or later, (device end) will be signalled
indicating the device has completed any mechanical motion required, etc.
For example, disk seek, tape rewind, and forms eject on a printer all signal
channel end immediately, and device end much later.

If command chaining occurs, it does not take place until device end time,
and on old selector channels, ties up the channel during this time. 
The newer block multiplexor channels do not have this problem, as they
can disconnect, under some circumstances.

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.

Bob Bernecky
Snake Island Research Inc.

PS: I'm new to this posting biz, so please accept my apologies if I 
got the wrong group. I can't even FIND the name of the damn comp.filk
group at the moment. Playing adventure is easier..