[Info-vax] 9 track end of tape (Was: Copying VMS SaveSet Under Windows)

[glen herrmannsfeldt]

Alan Frisbie <Usenet02_REMOVE at flying-disk.com> wrote:

(snip, I wrote) 
>> The usual S/360 tape drives have a "Read Backwards" channel
>> command that will read the block while moving the tape backwards.
>> The program supplies the address of the end of the buffer,
>> and the drive fills the buffer from the end toward the beginning,
>> such that the data ends up in the right order.  There are sort
>> algorithms optimized for "Read Backwards" to avoid the rewind time.
 
> Wow, I never knew that.   I wish I could go back 40 years and
> show this to the programmer.   It would have made life a lot
> easier!

It is harder in high-level languages, though.  It might be
that PL/I supports it, but others don't.  It would have been
done through an assembly routine callable from another language.

Helical scan drives, like DDS and Exabyte/8mm, can't read
backwards well at all, but I believe that 3480 and 3490 can.
I am not sure about Ultrium, though.

-- glen