On Friday, October 14, 2022 at 2:34:52 PM UTC-6, The Horny Goat wrote:
> >And the 5.25" format would, no doubt, have turned out to be
> >Apple-specific. Which isn't unusual; my first computer (a NS Horizon)
> >had its own format (well, two of them if you added CP/M).
> Didn't know that about the North Star but the Apple II and CP/M used
> different formats though Apple made a CP/M add on card that made the
> II a "CP/M box". So far as I know I only used one CP/M application and
> trashed it when Appleworks 1.0 came out.
It's interesting that you mention the Apple _and_ North Star when discussing
disk format incompatibilities. Because, while in those days, nearly every
computer's disk format was incompatible with every other computer's disk
format, *those* two were extreme outliers.
Let's take the Apple II first. Most 5 1/4" floppies used a specialized integrated
circuit that translated eight bits of data from the computer into ten bits which
satisfied certain conditions to directly record on the disk.
The Apple, having been made in the early days of microcomputers, didn't
use that circuit - it was too expensive or hard to get. Instead, Wozniak's
genius let them use standard TTL. Software split the data up into units
of five or six bits (there were two distinct schemes used by Apple, the
original one with five, and the improved one with six) and those were
translated into units of eight bits, which could be recorded on the disk
with a standard MSI part doing the work.
North Star was an outlier in a completely different way. It was one of
the few computers that used "hard sectored" disks. To some of those
who read this, that phrase will bring a flood of circa 1977 memories
flooding back.
A normal floppy disk, in addition to the big hole in the center for the
spindle, has one tiny hole in the magnetic surface, which is sensed
photoelectrically, to indicate where the first sector of each track should
start.
A hard sectored disk, on the other hand, might have eight little holes,
one for each sector, plus one extra hole to indicate which sector was
first.
So those were perhaps the two most incompatible disk formats in
all computerdom!
John Savard