[Info-vax] Backup TK50 tapes

[Paul Sture]

In article <kgjof8$e0l$1 at Iltempo.Update.UU.SE>,
 Johnny Billquist <bqt at softjar.se> wrote:

> On 2013-02-26 23:15, Bob Koehler wrote:
> > In article <kgj03k$2ke$1 at Iltempo.Update.UU.SE>, Johnny Billquist 
> > <bqt at softjar.se> writes:
> >>
> >> Anyway, the problem with bootable tapes, at least on a PDP-11, is that
> >> your tape starts with a boot block. That block is read into memory, and
> >> the boot rom jumps into the code read in. This boot block on the tape is
> >> outside of what is visible as files on an ANSI-labelled tape. So you
> >> cannot see, or access this block except by doing low level reads from
> >> the tape. Any tool like COPY is unable to see or access this data.
> >
> >     Interesting.  On VAXen, bootable tapes tended to keep all thier files
> >     in ANSI labeled files.  Thus the earlier post the the TK started with
> >     VMB.EXE.
> 
> But then the boot roms needs to understand the ANSI tape format, read 
> through tape headers, parse stuff, locate the right file, read it into 
> memory, and then execute it. Even to the point of possibly understanding 
> the format of the executable.
> That's a heck of a lot to have in the rom image. And what if you want to 
> boot something that use a different tape format? Unix bootable tape 
> distributions have never been ANSI labelled tapes, for instance.

And IIRC 11/780 boot floppies were written in RT-11 format, which was a 
lot simpler.

> But as I said previously, early VAXen could not even boot from tape. 
> That capability didn't arrive until the uVAX. (That much I do know, as I 
> had the MtXinu distribution for VAX on bootable tape, and it was quite 
> some work to boot those tapes, as you had to write the tape boot program 
> by hand into memory and then execute it. Manuals have the boot programs 
> for all the old VAXen to boot from tape, as DEC didn't provide that 
> ability.)
> Apart from the VAX-11/750, VAXen didn't even use boot blocks on disks in 
> the early days.
> 
> So for a PDP-11, a bootable tape is really also an ANSI labeled tape (or 
> rather - can be). That don't mean you can mount it and copy all the 
> files from it, and expect to get a bootable tape.
> 
> 	Johnny

-- 
Paul Sture