Path: utzoo!attcan!sobmips!roe
From: roe@sobmips.UUCP (r.peterson)
Newsgroups: comp.sys.ncr
Subject: Re: Copying from tape and byte swapping
Message-ID: <1990Jan23.054353.9018@sobmips.UUCP>
Date: 23 Jan 90 05:43:53 GMT
References: <2053@cooper.cooper.EDU>
Organization: Sobeco Group - Montreal, Canada
Lines: 33

From article <2053@cooper.cooper.EDU>, by mayer@cooper.cooper.EDU (Mayer Ilovitz ):
> Hi,
> 
> 	The SCSI tape controllers used on our Tower 650 has a register
> that determines whether or not the bytes are swapped by the controller.

This is incorrect.  It implies that the controller has byte-swap
capability _in hardware_.  There are several problems with this - the
"three-byte block" is the least of them.

The hardware does not have this capability.

> By changing the value of this register you can avoid having to do a
> dd conv=swab.  In Release 2.01.00 there is an undocumented command which can
> change this register's value. The command is
> 
> 	/menu/obj/mtswap.o TAPEDEVICE -set VALUE
> 

What this command does is set an external device-driver variable that
convinces the driver to swap the bytes for you.

I realize the point may be moot - the user process sees the bytes in
the correct order.

However, you will pay a performance penalty.  Don't expect things to run
as fast, *especially* on write, as they might otherwise.


-- 
One makes strong assumptions delving	       Roe Peterson
into the beginning of the universe...	       {uunet,mcgill-vision}!sobeco!roe
	- Stephen Hawking, Cambridge