[Info-vax] Backup TK50 tapes

[Johnny Billquist]

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.

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

-- 
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