[pups] Making a TK50 2.11BSD boot tape for an 11/73

Johnny Billquist bqt at update.uu.se
Fri Jan 17 03:28:30 AEST 2003

On Thu, 16 Jan 2003, David Evans wrote:

> On Thu, Jan 16, 2003 at 07:53:02AM -0800, Andru Luvisi wrote:
> > maketape will pad each of "mtboot", "mtboot", and "boot" out to an even
> > length of 512 bytes.  The "cat | dd" method will not pad the first two
> > out.
>   Ahhh--of course.  One would have to do a series of "dd"s in order to
> get the padding happening.

Probably not. Repeated "dd"s will not produce the same result. This is the
typical error when not understanding that tapes actually are not a stream
of characters. (And also a good example when the Unix paradigm fails 
pretty bad).
Each write to the tape device will create a block. You cannot do two
writes after each other to extend the block. You will instead get two
different blocks.

Boot tapes for VAXen (and PDP-11s) are expected to have 512 byte
blocks. Actually, the first block should be repeated twice, and this is
the primary boot block. After that you can put whatever you want. Most
primary boot blocks however also expects to read 512 byte blocks, which
means that the next file (secondary booter or the system itself) also most
often is written with 512 byte blocks.

> > I discovered this while trying to create a bootable tape image for simh.
> > I needed to emulate the maketape behavior for it to work.
>   That's not a bad idea.  Perhaps I should just make a minimal bootable image
> using simh or such, dd it to a SCSI disk, and then attach that to my 11/73.
> Might be easier than endless futzing with the TK50.

Don't. The moment you copy a tape file to disk, you will loose meta
information unless you use a program specifically designed to preserve
that information.


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

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