[Simh] VAX Tape Emulation?

[Johnny Billquist]

On 2018-01-26 01:25, Phil Budne wrote:
> Johnny Billquist <bqt at softjar.se> wrote:
>> This is obviously a limitation of the rmt protocol, which makes the
>> assumption that all records are of equal length. Works for the standard
>> Unix tools, which also tend to want to use tapes with a fixed record length.
> 
>  From the FreeBSD 10.3 rmt(8) man page:
> 
>       ..... The protocol is comprised of the following commands,
>       which are sent as indicated - no spaces are supplied between the command
>       and its arguments, or between its arguments, and `\n' indicates that a
>       newline should be supplied:
> ......
> 
>       Wcount\n
>               Write data onto the open device.  The rmt utility reads count
>               bytes from the connection, aborting if a premature end-of-file is
>               encountered.  The response value is that returned from the
>               write(2) call.
> 
> So each write to the tape can have a different record length.

Ah! Thanks. Then my muddy memory foiled me (I blame suggestions made me 
believe it).
So, rmt should really just work also from VMS, unless there are some 
other issues.

Still, tar and dump (and dd) under Unix do tend to use the same block 
size for all reads/writes, so they don't really test such things that much.

> All this reminds me how happy I am not to deal with tape drives any
> more!  I can't remember how many times I issued "mt fsf" without an
> "&" not remembering the operation made the process incorruptible, and
> resistant to CTRL-Z.
> 
> I always said tape drivers and boot roms always sucked because once
> you got them working, you didn't want to touch them again!

:-)
I don't actually find them so bad. But they are more of a headache under 
Unix than under some DEC OS, most of the time, I'd say. But 
occasionally, you can get stuck waiting for tapes for a long time.

All that said, we have one drive online at Update, connected to Magica. 
A TU81, which can do 1600 (PE) and 6250 (GCR) BPI. Just in case anyone 
ever stumbles upon a tape that needs to be read...

We might, at some point, try to get a TU77 up and running as well, so 
that we can deal with 800 (NRZI) BPI as well.

   Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol