Rick Balkins wrote:

> It is probably an RLL scheme. MFM is for FAT/FAT12 disks. 5.25"
& 3.5"
> floppy disks and very early hard drives. New hard drives uses an RLL scheme.
> If anything, for legacy sake, this would be an MFM/RLL style scheme versus a
> GCR/RLL scheme.

Only older hard disks with the ST-506 interface used RLL and MFM.

Modern hard disks use something different.  Anything made between the ST-506
era and roughly 2006 uses parallel recording of course, but not the RLL/MFM
schemes.  What exactly is used probably depends on the manufacturer.  From
2006 onwards, drives are starting to appear that use perpendicular recording,
where the magnetic particles are oriented vertically with respect to the
media.

On top of the recording scheme are something called PRML (which helps make
sure reads are consistent despite noisy media), and Reed-Solomon error
correction (same scheme as used on CD and DVD media).

On top of that is the interface layer, the bit we all know as IDE, SATA, or
SCSI.  The controller in the computer has no idea at all about the recording
scheme.

> FAT was built around and on top of MFM coding ...

It might have been designed with floppy disks in mind, but FAT is a filesystem
layer, independent of the underlying recording scheme.  These days, it is
most commonly found on memory cards, thumbdrives, mp3 players, and so on.

FAT is used so commonly because practically everything made in the last 20
years or so can read it.  Other filesystems can theoretically be used as
well, as long as the disk is low-level formatted properly and the
filesystem's metadata will actually fit into the 1.4 MB space available.

> If you take a look at Iomega SuperDisk project,

SuperDisk was a 3M/Imation technology, not Iomega.  That aside...

> it alludes to MFM and MFM/RLL encoding scehemes versus GCR because it
> doesn't read older GCR disks but MFM disks. So, this is probably the case.

You're talking about the floppy disk read capability.  The Imation SuperDisk
project is a variation on the older Floptical technology, which used RLL for
the recording scheme, with a laser to guide the head from track to track.

As Iomega disks are entirely magnetic, RLL probably also forms the basis of
the recording scheme, but it likely differs enough from the sort used on
floppy disks that it can no longer be called "RLL".

> GCR was Commodore, Apple II, and few IBM based systems:
> http://en.wikipedia.org/wiki/Group_Code_Recording

According to the above link, GCR can be used on top of RLL, which does make
sense.  Remember that RLL and MFM are the low-level recording schemes, while
GCR is really more of an encoding scheme, which is why we have raw
GCR-encoded 1541 disk images out there (.G64), in addition to the usual
unecoded version (.D64).

As to the ZIP drive, I would have to agree with Thomas - the low-level
recording scheme could be just about anything, since it is abstracted away by
the interface.

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