[Simh] Write sector headers to disk

[Lars Brinkhoff]

Johnny Billquist wrote:
> The fourth option is to treat this as a normal write. Essentially,
> unless I remember wrong, that is pretty much the end result on real
> hardware.

Agreed.  If SIMH was updated to do this, I think SALV would be 99%
happy.

> I've played with a few disks back in the day that could write the
> headers as well as read them, but there were no ability to store
> arbitrary data there, so what would be returned on a read headers
> could be computed when needed.

I'm using the term "key field" as documented here:
http://www.bitsavers.org/www.computer.museum.uq.edu.au/pdf/EK-RP04-MM-001%20RP04%20Device%20Control%20Logic%20Maintenance%20Manual.pdf

See e.g. page 2-37.  The header consists of five 16-bit words: cylinder
and format, sector and track, key field #1, key field #2, and CRC.  The
key fields are arbitrary data under software control.

I don't know if this applies to the RP06.

SALV does write information to the key fields, but it seems the data
isn't very important.