That is because Amiga uses GCR recording rather then FM or MFM.
On Thu, 18 May 2023, Peter Corlett via cctalk wrote:
Nope. You may have gotten confused with the Commodore 64 drives, which
were very Special, or perhaps early Apple gear.
The Amiga's disk controller supports both GCR and MFM, but MFM was used by
default because it is higher-density and the blitter can be used to perform
MFM decoding. It can read and write PC disks just fine using a third-party
block device driver (one was later bought-in and shipped with Workbench),
but the native format uses a different sector scheme which gets 880kiB on a
DD disk instead of the usual 720kiB of the PC.
Said third-party device drivers are flexible enough that they'll handle
disks from other platforms which use PC-compatible disk controllers such as
the Atari ST, Acorn Archimedes, Sun workstations, and later Apple Macs.
The Amiga pretty much died before HD disks became standard, although some
Amiga-compatible HD drives exist and gave 1760kiB per disk. These were like
hen's teeth even back then. I have one and have never seen another.
It's an easy, and common, mistake to make, . . .
Almost all other MFM formats use a track and sector layout based on what
was developed by IBM, long before the PC. Traditionally, it was called
"IBM format", but for the last 42 years, that name has only added to the
confusion, because most people who were NOT in computers more than 40
years ago will mistakenly assume that the name is referring to IBM PC.
To avoid that confusion, I call it "IBM/WD" format, which is less
ambiguous, since although IBM developed the style of format, Western
Digital had a major part in making it ubiquitous.
The Amiga is MFM, but does not follow the IBM/WD format "standard".
The NEC 765 FDC has trouble with it, because the NEC is looking for IBM/WD
sectors and tracks. The NEC FDC can be TRICKED into reading it. Part of
that involves changing drive select in mid-read.
In the relatively early days, Amiga came out with a 5.25" drive and
software to emulate PC! It was close enough in compatability to be able
to run XenoCopy (once called "the acid test of compatability"!) and read
SOME formats. But, people were displeased with the performance; it was as
slow as an AT. They got the elephant to fly!, but people complained about
the speed and payload.
--
Grumpy Ol' Fred ci...@xenosoft.com
