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From: binder@dosadi.DEC (Wherever you go, there you are.)
Newsgroups: net.micro
Subject: Re: Reading Apple II disks
Message-ID: <1535@decwrl.UUCP>
Date: Mon, 8-Apr-85 10:57:59 EST
Article-I.D.: decwrl.1535
Posted: Mon Apr  8 10:57:59 1985
Date-Received: Wed, 10-Apr-85 06:12:50 EST
Sender: daemon@decwrl.UUCP
Organization: DEC Engineering Network
Lines: 54

You can't read an Apple disk with any system whose controller was not 
explicitly designed to read the Apple format.  That includes Apple, various 
third parties, and one IBM plug-in that emulates the Apple.  The only way to 
get data to or from an Apple is via serial communication lines and either a
terminal emulator like ASCII Express or Softerm, or a file transfer utility 
like Kermit.

For anyone interested in the technicalities of the Apple recording scheme and 
why it is so different, here's a brief explanation of it. 

1.  Apple uses soft sectoring, and even soft indexing - ALL format information
    is written on the disk at INIT time.  Any 5-1/4 floppy, hard-sectored, or
    soft-sectored, or even without an index hole, works just fine.

2.  Where all other controllers today use Modified Frequency Modulation (MFM)
    recording, Apple's recording scheme is group-coded recording (GCR).

    The sector headers are written in a 4+4 scheme, in which each two bytes of
    header contain one usable byte of information.  The data byte is broken
    into two nibbles, and each nibble is translated into a byte in which the
    two most significant bits are both ones.  These translated bytes are then
    written to the disk.  The start of a header or data field is marked by a
    specific pattern of bytes that include GCR violations such as a byte value
    of AA hex.

    DOS 3.3, 16 sectors:

    Data fields are written in a 6+2 scheme, in which every disk byte contains
    six meaningful bits - four disk bytes have three useful bytes of data.
    Each set of 24 bits (3 bytes) is broken into four 6-bit groups, which
    are translated into bytes in which one or the other of the upper two bits
    may be a zero, but not both, and certain patterns, like the AA, are still
    reserved.  342 disk bytes are required to record 256 data bytes. 

    DOS 3.2, 13 sectors:

    Data fields are written in the same 4+4 scheme as headers.  512 disk bytes
    are required to record 256 bytes of data.

3.  In MFM, synchronisation is done by detecting MFM violations coupled with a
    specific bit pattern.  In the Apple, synchronisation is done by the use of
    special 10-bit patterns called self-synch bytes, in which the two end bits
    are zeroes, and the centre eight bits are ones.  A preamble consists of
    five self-synch bytes in a row, and the controller's state machine uses
    them so that a start anywhere in the first 40 bits of the 50-bit field
    will result in synchronisation before its end. 

Cheers,
Dick Binder   (The Stainless Steel Rat)

UUCP:  { decvax, allegra, ucbvax... }!decwrl!dec-rhea!dec-dosadi!binder
ARPA:  binder%dosadi.DEC@decwrl.ARPA

Mon 8-Apr-1985 10:51 Eastern Standard Time