Re: 1.4MB Disk Image

limtc <thyechean@xxxxxxxxx> wrote:

Mmm.. what's the difference? What's a high density 3.5" disk? I have
read the comments twice and can't figure out. :) You mean there is a
1.4MB high density format and there is a double density disk?

There are two physical types of 3.5" disk.

The double density disk was the original one, used on the Lisa, Mac,
Apple IIe/IIc with UniDisk 3.5, Apple IIgs, built into the Apple IIc+,
and also used on DOS/Windows PCs and other platforms.

On the Lisa and Mac, Apple originally used single sided drives, allowing
400KB per disk. They never used this format on the Apple II, though the
drives can support it and some third party Apple II software was
distributed on 400K disks.

The double sided 3.5" drive was introduced with the Mac Plus and 512KE,
and used on the Apple II (UniDisk 3.5 and Apple 3.5 Drive). It supports
800K per disk.

There are several different drive models, which have different system
compatibility but the same media compatibility. Early Macs used a single
sided (400K) or double sided (800K) drive which doesn't work on an Apple
II (except with the third party Universal Disk Controller). These drives
don't have an eject button, requiring a software eject mechanism.

Apple introduced the UniDisk 3.5 for use with the Apple II, which
supports double sided double density (800K). The UniDisk 3.5 has a
special interface board in the drive, which uses the SmartPort protocol
to communicate with an Apple II. It isn't compatible with any other
platform, and requires a SmartPort disk controller ("Liron" card,
"SuperDrive" card, or IIc/IIc+/IIgs disk port).

The Apple 3.5 Drive was released for the IIgs, and also used on the
Apple IIc+ and the Mac SE and II. It has the same physical drive
mechanism as the UniDisk 3.5, but a different interface board which
supports the Apple IIgs, IIc+ and Mac SE. It doesn't work on an Apple
IIc, or the "Liron" card in a IIe. (It might work on the Mac Plus but
doesn't work on earlier Mac models.) This drive also works with the
"SuperDrive" card in a IIe or IIgs, and with the third-party Universal
Disk Controller. It can be used by a PC Transporter (but not very
reliably).

Apple's 400K and 800K double density 3.5" disks use a physical format
(GCR) which is derived from the format Apple used on the 5.25" disk,
with some enhancements. It is proprietary to Apple and is very difficult
to support on other platforms, because Apple used a special drive with
variable rotation speed.

The Lisa and Mac use the MFS file system on 400K disks, HFS on 800K. The
Apple II typically uses ProDOS on all 3.5" disks, but the IIgs can read
and write HFS with System 6, and there is third party software to read
MFS and HFS in ProDOS-8.

On MS-DOS, Windows and other platforms, the standard format for a 3.5"
double density disk uses MFM encoding (the same as MS-DOS uses for
5.25"), with fixed rotation speed and 720KB per disk. In principle you
can also get a single sided double density 360KB disk, but I never saw
one. Later Mac models can also support this physical format.

The high density 3.5" disk and drive were introduced by Apple around
1988. The new drive was standard on the Mac IIx and later models, and
could be retrofitted to the Mac II and SE (a firmware update was
required). It was also available as an external drive, and Apple
released a disk controller card to allow it to be connected to the IIgs
and enhanced IIe. The floppy drive is called the "SuperDrive".

The high density media looks the same as double density, except for one
detail: a cutout hole in the plastic on the side opposite to the write
protect hole. This extra hole is used by high density drives to detect
whether the disk is double density (hole covered) or high density (hole
open). You should avoid using high density disks in double density
drives because it causes compatibility problems and might have
reliability problems.

For the high density disk format, Apple decided to ditch their
proprietary GCR format and stick to the industry standard MFM. This has
twice the storage capacity of MFM double density, so 720KB becomes
1440KB (counting "binary" kilobytes).

The SuperDrive supports all of these formats:

- 400K or 800K GCR (double density), with variable rotation speed.
- 720K MFM (double density), with fixed rotation speed.
- 1440K MFM (high density), with fixed rotation speed.

With the right software, an Apple IIgs with a SuperDrive can read Mac
400K disks (MFS), read/write Mac 720K, 800K and 1440K disks (HFS),
read/write Apple II 400K, 800K and 1440K disks (ProDOS), and read/write
MS-DOS 720K and 1440K disks (FAT).

Not to mention other file systems such as Pascal or CP/M.

There are even some hacked versions of DOS 3.3 which allow the use of
two 400K volumes on a double density 3.5" disk, but I've never heard of
an extension for high density (which would require four volumes on a
single disk - the maximum volume size for DOS 3.3 is 400K).

Applied Engineering also released a high density 3.5" drive which
supports a GCR variable speed mode that allows 1.6 MB on a high density
3.5" disk. I've never seen one. It could apparently work on the IIgs
built-in disk port.

--
David Empson
dempson@xxxxxxxxxxxxx
.

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