Some documentation of the new unified decoder in cw2dmk 4.1
-----------------------------------------------------------

In cw2dmk 4.1, there is a single decoder that handles standard
IBM-style FM, standard IBM-style MFM, and DEC RX02 MFM.  The basic
idea of the decoder is to always translate the sample stream into bits
at the MFM data rate, simultaneously scan the bit stream for all three
types of address marks, and when a mark is recognized, switch
instantly and cleanly to decoding the bit stream into data according
to the type of encoding the mark corresponds to.

There are of course many other pesky details to flesh this idea out
into a complete decoder.

* The bit stream passes through a 64-bit shift register ("accum"),
with new bits shifted in at the low-order end.  Marks are detected in
the low-order 32 bits, and data bytes are decoded from the high order
16 (MFM) or 32 bits (FM).  The early detection of marks allows the
decoder to switch earlier, so that the entire mark is decoded cleanly
without a byte of misaligned garbage preceding it.  In general,
density switches and write splices don't occur immediately before
marks, and indeed a mark generally is supposed to have several bytes
of 00 preceding it.  Thus switching to the correct encoding several
bytes before a mark would be still better, but 32 bits is enough to be
useful.  (Note that there is extra cleanup code in dmk2cw that inserts
the needed 00 bytes when writing a DMK image to a real disk.)

* When decoding FM, the decoder uses only every other bit, and checks
that the other bits are 0.  There is a heuristic to add/drop bits if
doing so creates a correct clock pattern and correctly leaves 0's in
the unused bits.

* While decoding the body of a sector ID block or data block in
standard MFM, the decoder does not look for FM or RX02 marks, because
some legal standard MFM data patterns look the same as FM/RX02 marks.

* In addition to address marks, within gaps, the decoder also looks
for the repeated 4e4e MFM pattern (with normal clocking) that is a
standard gap fill in MFM.  This pattern is not a legal sequence in FM
at half the clock rate, so it helps shift the decoder into MFM and get
it byte-aligned early.

* When the decoder is in MFM mode and in a gap, it looks for MFM 0000
as well and will drop 1 bit to align with it if it appears shifted by
1 bit.  This pattern is ambiguous -- in a different alignment, it
could be FM ffff or MFM ffff, but MFM 0000 is more likely in an MFM
gap.  I'm not certain this heuristic is worthwhile, but it seems
harmless, at least.  It usually helps get a string of 00 (rather than
ff) bytes to precede MFM marks, as should happen.

* The decoder looks for patterns that correspond to marks read
backward but that cannot appear in legal data written forward.  This
gives it the ability to sense when a disk has "flippy" data on the
back or has been inserted into the drive upside down.

* The FM data address mark 0xf9 is ambiguous.  It could be a
nonstandard data address mark that the WD1771 FDC supports for
ordinary FM and which is used on some TRS-80 copy-protected disks, or
it could be the deleted data address mark for an RX02-MFM sector.  The
decoder chooses the 1771 interpretation if (1) it is in -e0
(autodetect) mode and no normal RX02-MFM (0xfd DAM) sectors have
appeared on the disk so far, or (2) it is in -e1 (standard FM) mode.
It chooses the RX02 interpretation if (3) it is in autodetect mode and
at least one 0xfd sector has been seen, or (4) it is in -e3 (RX02
only) mode.

* The decoder avoids looking for marks for 32 bit times in places
where it expects to see a write splice.  Without this feature, it
could spuriously detect marks there on occasion.

Standard FM
-----------

Standard FM encodes data using one clock bit and one data bit per
input data bit.  The clock bit is always 1 for normal data, and the
data bit is the input data.  Certain bytes in which some clock bits
are 0 are used as address marks.  An address mark is always preceded
by several normally encoded 00 bytes (not important for decoding
normal FM, but important for RX02, discussed later).

Here are the address marks that are used.

Index address mark
0xfc with 0xd7 clock pattern:

        ------f ------c
data:   1 1 1 1 1 1 0 0
clock: 1 1 0 1 0 1 1 1 
       ---f---7---7---a

At the MFM double data rate, preceded by one extra 0 bit, this looks
like:

             ------------f   ------------c
data:    0   1   1   1   1   1   1   0   0
clock: 1   1   1   0   1   0   1   1   1
space:  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
       ---8---a---a---2---a---2---a---8---8


ID address mark
0xfe with 0xc7 clock pattern:

        ------f ------e
data:   1 1 1 1 1 1 1 0
clock: 1 1 0 0 0 1 1 1 
       ---f---5---7---e

             ------------f   ------------e
data:    0   1   1   1   1   1   1   1   0
clock: 1   1   1   0   0   0   1   1   1
space:  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
       ---8---a---a---2---2---2---a---a---8


Standard deleted data address mark
0xf8 with 0xc7 clock pattern:

        ------f ------8
data:   1 1 1 1 1 0 0 0
clock: 1 1 0 0 0 1 1 1 
       ---f---5---5---a

             ------------f   ------------8
data:    0   1   1   1   1   1   0   0   0
clock: 1   1   1   0   0   0   1   1   1
space:  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
       ---8---a---a---2---2---2---8---8---8


Nonstandard data address mark available with WD1771 FDCs
Also used as RX02 deleted data address mark (see below)
0xf9 with 0xc7 clock pattern:

        ------f ------9
data:   1 1 1 1 1 0 0 1
clock: 1 1 0 0 0 1 1 1 
       ---f---5---5---b

             ------------f   ------------9
data:    0   1   1   1   1   1   0   0   1
clock: 1   1   1   0   0   0   1   1   1
space:  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
       ---8---a---a---2---2---2---8---8---a


Nonstandard data address mark available with WD1771 FDCs
0xfa with 0xc7 clock pattern:

        ------f ------a
data:   1 1 1 1 1 0 1 0
clock: 1 1 0 0 0 1 1 1 
       ---f---5---5---e

             ------------f   ------------a
data:    0   1   1   1   1   1   0   1   0
clock: 1   1   1   0   0   0   1   1   1
space:  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
       ---8---a---a---2---2---2---8---a---8


Standard normal data address mark
0xfb with 0xc7 clock pattern:

        ------f ------b
data:   1 1 1 1 1 0 1 1
clock: 1 1 0 0 0 1 1 1 
       ---f---5---5---f

             ------------f   ------------b
data:    0   1   1   1   1   1   0   1   1
clock: 1   1   1   0   0   0   1   1   1
space:  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
       ---8---a---a---2---2---2---8---a---a


RX02 normal data address mark (see below)
0xfd with 0xc7 clock pattern:

        ------f ------d
data:   1 1 1 1 1 1 0 1
clock: 1 1 0 0 0 1 1 1 
       ---f---5---7---b

             ------------f   ------------d         
data:    0   1   1   1   1   1   1   0   1
clock: 1   1   1   0   0   0   1   1   1
space:  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
       ---8---a---a---2---2---2---a---8---a


Backward FM
-----------

Reading any of the data address marks (0xf8, 0xf9, 0xfa, 0xfb, or 0xfd
with 0xc7 clock) backward, starting from just before the three data
bits that distinguish them from each other and continuing into the
preceding 00 byte, happens to yield a unique pattern that is also
missing clock bits but does not match any of the FM address marks read
forward.  Like forward FM address marks, the pattern is legal within
MFM data, so the decoder doesn't look for it within MFM ID or sector
data.

That is:

        X-----< f-----<
data:   X X X 1 1 1 1 1 0 0 0 0
         7-----< c-----<
clock: 1 1 1 1 0 0 0 1 1 1 1 1
             ---d---5---e---a

         X-----------<   f-----------<
data:    X   X   X   1   1   1   1   1   0   0   0   0
clock: 1   1   1   1   0   0   0   1   1   1   1   1
space:  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
                   ---a---2---2---2---a---8---8---8---8


Standard MFM
------------

Standard MFM is a modified version of FM, in which a clock bit is
normally 0 unless the data bits on both sides of it are 1.  With this
scheme, two 1 bits never appear consecutively (so it can be recorded
on the same media as FM at twice the bit rate), and at most three 0
bits appear consecutively (so it's not difficult for a decoder to stay
in sync with the signal).

As in FM, sequences that would otherwise be illegal due to missing
clocks are used as address marks.  Fewer such sequences are available;
in fact, only two are used: a 0xc2 data byte with a missing clock
between bits 3 and 4 (using 0-origin big-endian bit numbering!)  a
0xa1 data byte with a missing clock between bits 4 and 5 (again, using
0-origin big-endian bit numbering).  Since IBM-style formats require
more than two different marks, marks are formed by repeating one of
these specially encoded bytes (which I call a "premark") three times
and following it with another byte of normally encoded data that
specifies the type of mark.  An index address mark is 00c2c2c2fc, an
ID address mark is 00a1a1a1fe, a normal DAM is 00a1a1a1fb, and a
deleted DAM is 00a1a1a1f8.  Many other marks would be possible with
this scheme, but I don't know of any others being used.

Premark 0xc2 with missing clock (indicated as "o"):

        ------c ------2 ------c ------2
data:   1 1 0 0 0 0 1 0 1 1 0 0 0 0 1 0
clock: 0 0 0 1 o 1 0 0 0 0 0 1 o 1 0 0 
       ---5---2---2---4---5---2---2---4

Premark 0xa1 with missing clock (indicated as "o"):

        ------a ------1 ------a ------1
data:   1 0 1 0 0 0 0 1 1 0 1 0 0 0 0 1
clock: 0 0 0 0 1 o 1 0 0 0 0 0 1 o 1 0
       ---4---4---8---9---4---4---8---9


Backward MFM
------------

A backward 0xa1a1a1 premark preceded by a 00 byte looks like a forward
0xc2c2 premark followed by an 0x80.  This sequence is not used as a
normal mark, so the decoder is able to recognize it.  This would also
work for premarks consisting of only two 0xa1's, as long as the next
byte had the high bit set; for example, 0x00a1a1fb.

          1-----< a-----< 1-----< a-----< 0-----< 0-----<
data:   1 1 0 0 0 0 1 0 1 1 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0
clock: 0 0 0 1 o 1 0 0 0 0 0 1 o 1 0 0 0 0 0 1 1 1 1 1 1 1
       ---5---2---2---4---5---2---2---4---4---2---a---a
        ------c ------2 ------c ------2 ------8 ------0


RX02 DEC-modified MFM
---------------------

The DEC RX02 floppy disk drive uses a peculiar format.  It is similar
to standard FM format, except that if the DAM on a sector is 0xf9
(deleted data) or 0xfd (data), there are twice as many data bytes, and
the sector data and sector CRC are in a variant of MFM.  The MFM
encoding is modified to prevent the address mark detector from
sometimes firing inside the MFM data, since a standard MFM stream can
contain sequences that look just an an FM address mark.  (This
RX02-modified MFM should not be confused with MMFM, which is completely
different.)

The address marks used in this format are all shown in the "FM"
section above.  As in standard FM, 0xfc with 0xd7 clock is used as an
index address mark.  As just mentioned, 0xf9 and 0xfd with 0xc7 clock
are used as deleted and normal data address marks.  It is important
for this scheme that a mark be preceded by at least one bit of
FM-encoded 0; otherwise it can still match a legal RX02-modified MFM
pattern.

The pattern 1000101010100 occurs in each of the marks used in RX02
format, with the final 0 being one of the missing clock bits that
characterize them as marks.  In standard MFM data encoding, this
pattern can't be generated with the first bit being a clock bit,
because it would have a missing clock when decoded in that
registration.  It could be generated with the first 1 as a data bit,
however, by the data stream 1011110.  The RX02 data encoding prevents
this by encoding that data stream a different way.

Whenever the data stream contains 011110, instead of encoding it in
the normal way as:

data:   0 1 1 1 1 0
clock: x 0 0 0 0 0

RX02 transforms it to:

data:   0 0 0 0 0 0
clock: x 1 0 1 0 1

This MFM sequence is otherwise illegal (i.e., unused), since it has
missing clocks, so using it for this purpose does not create a
collision with the encoding of any other data sequence.

As far as I know, a decoder could undo this transform by looking
specifically for the 11-bit sequence above, or it could apply the
simpler rule that when we see a missing clock between two 0 data bits,
we change both data bits to 1's.  The simpler rule will cause some
illegal MFM sequences to be silently accepted, which is perhaps a
drawback.  It also leaves some apparent clock violations in the
translated data that must be ignored, also a drawback.  However, the
other rule doesn't seem to work on a test disk I have; this might be
due to the boundary conditions described next, or perhaps I made a
mistake in the implementation.  See the code under #if WINDOW == 4
vs. WINDOW == 12 in cw2dmk.c.

What happens at the start and end of the data area?  Looking at a
sample disk, it appears that the encoding is done as if the data field
were preceded by 00 and followed by ff.  In other words, 11110 at the
start is specially encoded, but 01111 at the end (second CRC byte) is
not.  The following ff (or is it fe?) may actually be written as a
lead-out; I'm not sure.

cw2dmk's decoder always does the untransform in a shift register that
runs in parallel to accum, called taccum.  Data is decoded from this
register only inside the sector data area of an RX02-encoded sector.


Backward RX02
-------------

See "Backward FM" above for backward versions of the address marks
used in RX02 format.

