[Simh] RP vs RM header operations

[Lars Brinkhoff]

Rob Doyle wrote:
> As a clarification, it looks to me like the sector header data "words"
> are always 16-bit, regardless if the disk pack is formatted for 16-bit
> data or 18-bit data.

That's how I understood it too.

> 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?

Here's where the pack number is written to the header:
https://github.com/PDP-10/its/blob/master/src/kshack/nsalv.261#L5399-L5401

And here's where it's read back:
https://github.com/PDP-10/its/blob/master/src/kshack/nsalv.261#L5107-L5109

I don't know why it's done this way.  It seems to go back to the Systems
Concepts DC10 disk controller for the AI lab KA10.