[sebhc] h17 and h8d disk images
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
> 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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