[Simh] Write sector headers to disk

[David Brownlee]

On Tue, 25 Jun 2019 at 07:31, Lars Brinkhoff <lars at nocrew.org> wrote:
>
> Hello,
>
> Some of DEC's disks allow reading and writing the sector headers.  ITS'
> disk formatting program, SALV (the Salvager), uses this to make a new
> file structure on a pack, and to verify its integrity.
>
> This isn't fully supported by SIMH.  The file PDP10/pdp10_rp.c has the
> comment "23-Aug-01  RMS  Added read/write header stubs for ITS", and the
> stub is in "case FNC_WRITEH" in rp_svc.  It's a no-op that just sets the
> DONE flag.
>
> What is supposed to happen is that SALV wants to write two header words
> with geometry information and then the sector data with a particular bit
> pattern.  The verification phase reads back and checks the bit pattern.
> Since those bits aren't written, the verification fails.
>
> This isn't so bad since SALV offers to skip the verification.  But now
> this has come up again using Rich Cornwell's upcoming KA10 addition to
> SIMH.  The KA10 version of SALV does the same with an RP04 disk, but
> does not have the option to skip the verification.  SALV also writes
> some information to the key fields in the headers, which is later read
> back.
>
> I'm wondering what your opinions are reagarding this?  I see some options:
>
> 1. Do not change SIMH.  Patch SALV to skip the verification.  Obviously
> this is quite quick and painless, but leaves me with a slightly
> dissatisfied feeling.  Some historical software isn't running properly.
>
> 2. Add header words to the disk image file.  An incompatible change, but
> should of course be optional.
>
> 3. Store header words out of band somehow.  Last in the image file?
> In another file?
>
> I understand the case for making changes to SIMH is quite weak if
> running SALV is the one and only use case.  Are there any other programs
> that reads or writes the headers?

How about storing them only in memory - still an incomplete
implementation, but should be enough to pass the SALV tests...

Could always be extended to persist them later

David