[sebhc] h17 and h8d disk images

[Dwight K. Elvey]

>From: "Barry Watzman" <Watzman at neo.rr.com>
>
>
>There are two ways to interleave.
>
>In the 1st method, used by CP/M, you have an interleave table in the
>operating system itself.  The sectors are in physical order on the diskette,
>but are not used in physical sequence.  That is, CP/M (on an 8" disk) uses
>sector 1 then 7 then 13, then 19, etc.
>
>The other way is to have the OS use the sectors sequentially (1,2,3 etc.)
>but to do the interleaving when the disk is formatted.  So the OS will ask
>for sector 1, 2, 3 ...., but on the disk the sector after sector 1 is not 2
>but rather something else.  There is no requirement that the sectors on the
>disk be in sequential order.  In fact, there is no requirement that they be
>contiguous, or that they be "positive" numbers (if one wants to interpret
>them as signed numbers).  They are more like labels than actual numbers,
>they can be in any sequence, and there can be gaps (a fact on which may copy
>protection schemes rely).

Hi Barry
 On the HDOS hard sectored disk, the headers do have to have sector
numbers that need to match those that are requested. The order on
the disk doesn't matter for the H17 ROM. They can even be random order.
 How we got here was that I was concerned that if Dan's code read
the physical sectors and placed them sequentially in the image file,
any interleaving might cause loss of order of the sectors because
the headers are not saved. He assures me that his code looks both
at the sector order and the sector header for a match. As long as
he is not reading interleaved disk, there is no issue.
 He states that he'd never seen this on HDOS. I believe him. I state
that I'd seen it before I'd ever done it myself ( I believe myself ).
Dwight


--
Delivered by the SEBHC Mailing List