[Simh] RP/RM differences in header commands (for ITS, salvager)

[Clem Cole]

FWIW:   When Ted and I wrote fcsk in the mid 1970s, (after some arguing
about assuming perfect media) we added the notion of a bad block list to
the assign to the bad block file (it was not in the original code), but we
knew nothing about BAD144 at the time since we both were students.   Ultrix
[and Tru64] was later fixed (early 80s) to obey BAD144 (thank you, Fred
Canter).   IIRC, that part of the Ultrix code got sent back to UCB and
showed up in some later versions, but I've forgotten.   As I recall, the
Unix driver was modified after getting a bad block error from the
controller, to mark the block as bad and pick a replacement from BAD144.
 The problem was that the caused stray seeks under the covers after the
reformat.

I remember having a conversation with Fred at one time approx 1981-2 ish,
about not doing the replacement at all if the block was in the free list,
then assigning to the bad file so the seeks never occurred (*i.e.* creating
perfect media - which is what UNIX assumed).  I've forgotten the result of
that conversation.
ᐧ

On Fri, Jun 28, 2019 at 9:46 AM Paul Koning <paulkoning at comcast.net> wrote:

>
>
> > On Jun 28, 2019, at 3:07 AM, Johnny Billquist <bqt at softjar.se> wrote:
> >
> >> From a hardware point of view, sector header "Word 3" and "Word 4" have
> >> no particular meaning.  Just for completeness, can you point me to a
> >> reference where "pack number" is defined or used by the software?
> >
> > Search for DEC standard 144. If nothing else, I think there is a
> description in the RL11 documentation. (Me trusting my memory once more...)
>
> That's different, though.  It describes the bad block table, which
> includes a pack serial number in its header.  Some device types use 144,
> some do not (in particular, on RP/RM there is a mix -- RP04/5/6 do not,
> RP07 and RM02/3/5/80 do).  The 144 table is purely a software construct, a
> bit of data placed by convention in the last few sectors of the drive and
> interpreted by the OS.  For example, in RSTS the bad block table is read to
> mark those blocks as unavailable, and the serial number is read and
> displayed during formatting but not otherwise used for anything.
>
>         paul
>
>
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