[sebhc] h17 and h8d disk images

Dave Dunfield dave04a at dunfield.com
Wed Sep 1 16:34:30 CDT 2004

>>1) I had not-to-long ago just done the Altair simulator which has the
>>   NorthStar hard sectored single-density disk system in it - this system
>>   DOES NOT use headers, and sector numbering HAS TO BE the physical
>Hi Dave
> Actually, single-density and double-density both use headers,
>you just don't, usually, have as direct access to them as Dan's
>methods would have. Both can also have different physical and
>logical interleaving. I used to play with the physical sector
>interleaving on an original IBM PC to speed up transfers.

I guess I wasn't clear - NOT EVERY hard-sectored system in the world uses
headers - H8 uses headers, but NorthStar (which is the only HardSectored
system I had worked on before the H8) DOES NOT ** (I know because I ported
my own OS to it and wrote the low level disk system drivers) - that is what
1) is trying to explain - I made the decision NOT to use physical headers
in part because that is what I was used to at the time, and I saw no need
for them other than to satisfy the H8 disk drivers (which my "on the fly"
generated headers do) - this is an explaination of history, not an argument.

** The N* system has a register in the controller which tells you the current
   sector number - it does this by detecting the track index hole (shorter
   timing than sector holes) and then maintaining a hardware counter for each
   sector index pulse - As long as the drive remains selected this counter is
   accurate - that is why N* systems keep the drive selected for a fairly long
   period after the last access. When you deselect/select (including switch
   drives), you have to wait for the next track index to resync - this is all
   done with hardware timers and counters! - At least that is the way the Single
   Density system which which I am famier works - there are only 256 bytes of
   readable data recorded between the sector index pulses.

[This also means that you CANNOT HAVE INTERLEAVE on a N* controller, which is
 why the "Raw binary" dump of sequential sectors seemed natural to me]


dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Vintage computing equipment collector.

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