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(mess): Floppy subsystem documentation checkpoint. Incomplete, but

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there's too many people looking at it not to push it. [O. Galibert]
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The new floppy subsystem
------------------------
1. Introduction
The new floppy subsystem aims at emulating the behaviour of floppies
and floppy controllers at a level low enough that protections work as
a matter of course. It reaches its goal by following the real
hardware configuration:
- a floppy image class keeps in memory the magnetic state of the
floppy surface and its physical characteristics
- an image handler class talks with the floppy image class to simulate
the floppy drive, providing all the signals you have on a floppy drive
connector
- floppy controller devices talk with the image handler and provide
the register interfaces to the host we all know and love
- format handling classes are given the task of statelessly converting
to and from an on-disk image format to the in-memory magnetic state
format the floppy image class manages
2. Floppy storage 101
2.1. Floppy disk
A floppy disk is a disc that stores magnetic orientations on their
surface disposed in a series on concentric circles called tracks or
cylinders[1]. Its main characteristics are its size (goes from a
diameter of around 2.8" to 8") , its number of writable sides (1 or 2)
and its magnetic resistivity. The magnetic resistivity indicates how
close magnetic orientation changes can happen and the information
kept. That's one third of what defines the term "density" that is so
often used for floppies (the other two are floppy drive head size and
bit-level encoding).
The magnetic orientations are always binary, e.g. they're one way or
the opposite, there's no intermediate state. Their direction can
either be tengentially to the track, e.g in the direction or opposite
to the rotation, or in the case of perpendicular recording the
direction is perpendicular to the disc surface (hence the name).
Perpendicular recording allows for closer orientation changes by
writing the magnetic information more deeply, but arrived late in the
technology lifetime. 2.88Mb disks and the floppy children (Zip
drives, etc) used perpendicular recording. For simulation purposes
the direction is not important, only the fact that only two
orientations are possible is. Two more states are possible though: a
portion of a track can be demagnetized (no orientation) or damaged (no
orientation and can't be written to).
A specific position in the disk rotation triggers an index pulse.
That position can be detected through a hole in the surface (very
visible in 5.25" and 3" floppies for instance) or through a specific
position of the rotating center (3.5" floppies, perhaps others). This
index pulse is used to designate the beginning of the track, but is
not used by every system. Older 8" floppies have multiple index holes
used to mark the beginning of sectors (called hard sectoring) but one
of them is positioned differently to be recognized as the track start,
and the others are at fixed positions relative to the origin one.
2.2 Floppy drive
A floppy drive is what reads and writes a floppy disk. It includes an
assembly capable of rotating the disk at a fixed speed and one or two
magnetic heads tied to a positioning motor to access the tracks.
The head width and positioning motor step size decides how many tracks
are written on the floppy. Total number of tracks goes from 32 to 84
depending on the floppy and drive, with the track 0 being the most
exterior (longer) one of the concentric circles, and the highest
numbered the smallest interior circle. As a result the tracks with
the lowest numbers have the lowest physical magnetic orientation
density, hence the best reliability. Which is why important and/or
often changed structures like the boot block or the fat allocation
table are at track 0. That is also where the terminology "stepping
in" to increase the track number and "stepping out" to decrease it
comes from. The number of tracks available is the second part of what
is usually behind the term "density".
A sensor detects when the head is on track 0 and the controller is not
supposed to try to go past it. In addition physical blocks prevent
the head from going out of the correct track range. Some systems
(apple 2, some c64) do not take the track 0 sensor into account and
just wham the head against the track 0 physical block, giving a
well-known crash noise and eventually damaging the head alignment.
Also, some systems (apple 2 and c64 again) have direct access to the
phases of the head positioning motor, allowing to trick the head into
going between tracks, in middle or even quarter positions. That was
not usable to write more tracks, since the head width did not change,
but since reliable reading was only possible with the correct position
it was used for some copy protection systems.
The disk rotates at a fixed speed for a given track. The most usual
speed is 300rpm for every track, with 360rpm found for HD 5.25"
floppies and most 8" ones, and a number of different values like 90rpm
for the earlier floppies or 150rpm for an HD floppy in an amiga.
Having a fixed rotational speed for the whole disk is called Constant
Angular Velocity (CAV, almost everybody) or Zoned Constant Angular
Velocity (ZCAV, C64) depending on whether the read/write bitrate is
constant or track-dependant. Some systems (apple 2, mac) varies the
rotational speed depending on the track (something like 394rpm up to
590rpm) to end up with a Constant Linear Velocity (CLV). The idea
behind ZCAV/CLV is to get more bits out of the media by keeping the
minimal spacing between magnetic orientation transitions close to the
best the support can do. It seems that the complexity was not deemed
worth it since almost no system does it.
Finally, after the disc rotates and the head is over the proper track
reading happens. The reading is done through an inductive head, which
gives it the interesting characteristic of not reading the magnetic
orientation directly but instead of being sensitive to orientation
inversions, called flux transitions. This detection is weak and
somewhat uncalibrated, so an amplifier with Automatic Gain Calibration
(AGC) and a peak detector are put behind the head to deliver clean
pulses. The AGC slowly increases the amplification level until a
signal goes over the threshold, then modulates its gain so that said
signal is at a fixed position over the threshold. Afterwards the
increase happens again. This makes the amplifier calibrate itself to
the signals read from the floppy as long as flux transitions happen
often enough. Too long and the amplification level will reach a point
where the random noise the head picks from the environment is
amplified over the threshold, creating a pulse where none should be.
Too long in our case happens to be around 16-20us with no transitions.
That means a long enough zone with a fixed magnetic orientation or no
orientation at all (demagnetized or damaged) is going to be read as a
series of random pulses after a brief delay. This is used by
protections and is known as "weak bits", which read differently each
time they're accessed.
A second level of filtering happens after the peak detector. When two
transitions are a little close (but still over the media threshold) a
bouncing effect happens between them giving two very close pulses in
the middle in addition to the two normal pulses. The floppy drive
detects when pulses are too close and filter them out, leaving the
normal ones. As a result, if one writes a train of high-frequency
pulses to the floppy they will be read back as a train of too close
pulses (weak because they're over the media tolerance, but picked up
by the AGC anyway, only somewhat unreliably) they will be all filtered
out, giving a large amount of time without any pulse in the output
signal. This is used by some protections since it's not writable with
a normally clocked controller.
Writing is symmetrical, with a series of pulses sent which make the
write head invert the magnetic field orientation each time a pulse is
received.
So, in conclusion, the floppy drive provides inputs to control disk
rotation and head position (and choice when double-sided), and the
data goes both way as a train of pulses representing magnetic
orientation inversions. The absolute value of the orientation itself
is never known.
2.3 Floppy controller
The task of the floppy controller is to turn the signals to/from the
floppy drive into something the main cpu can digest. The level of
support actually done by the controller is extremely variable from one
device to the other, from pretty much nothing (apple2, c64) through
minimal (amiga) to complete (western digital chips, upd765 family).
Usual functions include drive selection, motor control, track seeking
and of course reading and writing data. Of these only the last two
need to be described, the rest is obvious.
The data is structured at two levels: how individual bits (or nibbles,
or bytes) are encoded on the surface, and how these are grouped in
individually-addressable sectors. Two standards exist for these,
called FM and MFM, and in addition a number of systems use their
home-grown variants. Moreover, some systems such as the amiga use a
standard bit-level encoding (MFM) but an homegrown sector-level
organisation.
2.3.1 Bit-level encodings
2.3.1.1 Cell organization
All floppy controllers, even the wonkiest like the apple 2 one, start
by dividing the track in equally-sized cells. They're angular
sections in the middle of which a magnetic orientation inversion may
be present. From an hardware point of view the cells are seen as
durations, which combined with the floppy rotation give the section.
For instance the standard MFM cell size for a 3" double-density floppy
is 2us, which combined with the also standard 300rpm rotational speed
gives an angular size of 1/100000th of a turn. Another way of saying
it is that there are 100K cells in a 3" DD track.
In every cell there may or may not be a magnetic orientation
transition, e.g. a pulse coming from (reading) or going to (writing)
the floppy drive. A cell with a pulse is traditionally noted '1', and
one without '0'. Two constraints apply to the cell contents though.
First, pulses must not be too close together or they'll blur
each-other and/or be filtered out. The limit is slightly better than
1/50000th of a turn for single and double density floppies, half that
for HD floppys, and half that again for ED floppies with perpendicular
recording. Second, they must not be too away from each other or
either the AGC is going to get wonky and introduce phantom pulses or
the controller is going to lose sync and get a wrong timing on the
cells on reading. Conservative rule of thumb is not to have more than
three consecutive '0' cells.
Of course protections play with that to make formats not reproducible
by the system controller, either breaking the three-zeroes rule or
playing with the cells durations/sizes.
Bit endocing is then the art of transforming raw data into a cell 0/1
configuration that respects the two constraints.
2.3.1.2 FM encoding
The very first encoding method developed for floppies is called
Frequency Modulation, or FM. The cell size is set at slighly over the
physical limit, e.g. 4us. That means it is possible to reliably have
consecutive '1' cells. Each bit is encoded on two cells:
- the first cell, called the clock bit, is '1'
- the second cell, called data bit, is the bit
Since every other cell at least is '1' there is no risk of going over
three zeroes.
The name Frequency Modulation simply derives from the fact that a 0 is
encoded with one period of a 125Khz pulse train while a 1 is two
periods of a 250Khz pulse train.
2.3.1.3 MFM encoding
The FM encoding has been superseded by the Modified Frequency
Modulation encoding, which can cram exactly twice as much data on the
same surface, hence its other name of "double density". The cell size
is set at slightly over half the physical limit, e.g. 2us usually.
The constraint means that two '1' cells must be separated by at least
one '0' cell. Each bit is once again encoded on two cells:
- the first cell, called the clock bit, is '1' if both the previous
and current data bits are 0, '0' otherwise
- the second cell, called data bit, is the bit
The minimum space rule is respected since a '1' clock bit is by
definition surrounded by two '0' data bits, and a '1' data bit is
surrounded by two '0' clock bits. The longest '0'-cell string
possible is when encoding 101 which gives x10001, respecting the
maximum of three zeroes.
2.3.1.4 GCR encodings
Group Coded Recording, or GCR, encodings are a class of encodings
where strings of bits at least nibble-size are encoded into a given
cell stream given by a table. It has been used in particular by the
apple 2, the mac and the c64, and each system has its own table, or
tables.
2.3.1.5 Other encodings
Other encodings exist, like M2FM, but they're very rare and
system-specific.
2.3.1.6 Reading back encoded data
Writing encoded data is easy, you only need a clock at the appropriate
frequency and send or not a pulse on the clock edges. Reading back
the data is where the fun is. Cells are a logical construct and not a
physical measurable entity. Rotational speeds very around the defined
one (+/- 2% is not rare) and local perturbations (air turbulence,
surface distance...) make the instant speed very variable in general.
So to extract the cell values stream the controller must dynamically
synchronize with the pulse train that the floppy head picks up. The
principle is simple: a cell-sized duration window is build within
which the presence of at least one pulse indicates the cell is a '1',
and the absence of any a '0'. After reaching the end of the window
the starting time is moved appropriately to try to keep the observed
pulse at the exact middle of the window. This allows to correct the
phase on every '1' cell, making the synchronization work if the
rotational speed is not too off. Subsequent generations of
controllers used a Phase-Locked Loop (PLL) which vary both phase and
window duration to adapt better to wrong rotational speeds, with
usually a tolerance of +/- 15%.
Once the cell data stream is extracted decoding depends on the
encoding. In the FM and MFM case the only question is to recognize
data bits from clock bits, while in GCR the start position of the
first group should be found. That second level of synchronization is
handled at a higher level using patterns not found in a normal stream.
2.3.2 Sector-level organization
Floppies have been designed for read/write random access to reasonably
sized blocks of data. Track selection allows for a first level of
random access and sizing, but the ~6K of a double density track would
be too big a block to handle. 256/512 bytes are considered a more
appropriate value. To that end data on a track is organized as a
series of (sector header, sector data) pairs where the sector header
indicates important information like the sector number and size, and
the sector data contains the data. Sectors have to be broken in two
parts because while reading is easy, read the header then read the
data if you want it, writing requires reading the header to find the
correct place then once that is done switching on the writing head for
the data. Starting writing is not instantaneous and will not be
perfectly phase-aligned with the read header, so space for
synchronization is required between header and data.
In addition somewhere in the sector header and in the sector data are
pretty much always added some kind of checksum allowing to know
whether the data was damaged or not.
FM and MFM have (not always used) standard sector layout methods.
2.3.2.1 FM sector layout
The standard "PC" track/sector layout for FM is as such:
- A number of FM-encoded 0xff (usually 40)
- 6 FM-encoded 0x00 (giving a steady 125KHz pulse train)
- The 16-cell stream 1111011101111010 (f77a, clock 0xd7, data 0xfc)
- A number of FM-encoded 0xff (usually 26, very variable)
Then for each sector:
- 6 FM-encoded 0x00 (giving a steady 125KHz pulse train)
- The 16-cell stream 1111010101111110 (f57e, clock 0xc7, data 0xfe)
- Sector header, e.g. FM encoded track, head, sector, size code and two bytes of crc
- 11 FM-encoded 0xff
- 6 FM-encoded 0x00 (giving a steady 125KHz pulse train)
- The 16-cell stream 1111010101101111 (f56f, clock 0xc7, data 0xfb)
- FM-encoded sector data followed by two bytes of crc
- A number of FM-encoded 0xff (usually 48, very variable)
The the track is finished with a stream of '1' cells.
The 125KHz pulse trains are used to lock the PLL to the signal
correctly. The specific 16-cells streams allow to distinguish between
clock and data bits by providing a pattern that is not supposed to
happen in normal FM-encoded data. In the sector header track numbers
start at 0, heads are 0/1 depending on the size, sector numbers
usually start at 1 and size code is 0 for 128 bytes, 1 for 256, 2 for
512, etc.
The crc is a cyclic redundancy check of the data bits starting with
the mark just after the pulse train using polynom 0x11021.
The western digital-based controllers usually get rid of everything
but some 0xff before the first sector and allow a better use of space
as a result.
2.3.2.2 MFM sector layout
The standard "PC" track/sector layout for MFM is as such:
- A number of MFM-encoded 0x4e (usually 80)
- 12 FM-encoded 0x00 (giving a steady 250KHz pulse train)
- 3 times the 16-cell stream 0101001000100100 (5224, clock 0x14, data 0xc2)
- The MFM-encoded value 0xfc
- A number of MFM-encoded 0x4e (usually 50, very variable)
Then for each sector:
- 12 FM-encoded 0x00 (giving a steady 250KHz pulse train)
- 3 times the 16-cell stream 0100010010001001 (4489, clock 0x0a, data 0xa1)
- Sector header, e.g. MFM-encoded 0xfe, track, head, sector, size code and two bytes of crc
- 22 MFM-encoded 0x4e
- 12 MFM-encoded 0x00 (giving a steady 250KHz pulse train)
- 3 times the 16-cell stream 0100010010001001 (4489, clock 0x0a, data 0xa1)
- MFM-encoded 0xfb, sector data followed by two bytes of crc
- A number of MFM-encoded 0x4e (usually 84, very variable)
The the track is finished with a stream of MFM-encoded 0x4e.
The 250KHz pulse trains are used to lock the PLL to the signal
correctly. The cell pattern 4489 does not appear in normal
MFM-encoded data and is used for clock/data separation.
As for FM, the western digital-based controllers usually get rid of
everything but some 0x4e before the first sector and allow a better
use of space as a result.
2.3.2.3 Formatting and write splices
To be usable a floppy must have the sector headers and default sector
data written on every track before using it. The controller starts
writing at a given place, often the index pulse but on some systems
whenever the command is sent, and writes until a complete turn is
done. That's called formatting the floppy. At the point where the
writing stops there is a synchronization loss since there is no chance
the cell stream clock warps around perfectly. This brutal phase
change is called a write splice, specifically the track write splice.
It is the point where writing should start if one wants to raw copy
the track to a new floppy.
Similarly two write splices are created when a sector is written at
the start and end of the data block part. They're not supposed to
happen on a mastered disk though, even if there are some rare
exceptions.
3 The new implementation
3.1 Floppy disk representation
Th floppy disk contents are represented by the class floppy_image. It
contains information of the media type and a representation of the
magnetic state of the surface.
The media type is divided in two parts. The first half
indicates the physical form factor, i.e. all medias with that
form factor can be physically inserted in a reader that handles
it. The second half indicates the variants which are usually
detectable by the reader, such as density and number of sides.
Track data consists of a series of 32-bits lsb-first values
representing magnetic cells. Bits 0-27 indicate the absolute
position of the start of the cell (not the size), and bits
28-31 the type. Type can be:
- 0, MG_A -> Magnetic orientation A
- 1, MG_B -> Magnetic orientation B
- 2, MG_N -> Non-magnetized zone (neutral)
- 3, MG_D -> Damaged zone, reads as neutral but cannot be changed by writing
The position is in angular units of 1/200,000,000th of a turn. It
corresponds to one nanosecond when the drive rotates at 300rpm.
The last cell implicit end position is of course 200,000,000.
Unformatted tracks are encoded as zero-size.
The "track splice" information indicates where to start writing
if you try to rewrite a physical disk with the data. Some
preservation formats encode that information, it is guessed for
others. The write track function of fdcs should set it. The
representation is the angular position relative to the index.
3.2 Converting to and from the internal representation
3.2.1 Class and interface
We need to be able to convert on-disk formats of the floppy data to
and from the internal representation. This is done through classes
derived from floppy_image_format_t. The interface to be implemented
includes:
- name() gives the short name of the on-disk format
- description() gives a short description of the format
- extensions() gives a comma-separated list of file name extensions
found for that format
- supports_save() returns true is converting to that external format
is supported
- identify(file, form factor) gives a 0-100 score for the file to be
of that format:
- 0 = not that format
- 100 = certainly that format
- 50 = format identified from file size only
- load(file, form factor, floppy_image) loads an image and converts it
into the internal representation
- save(file, floppy_image) (if implemented) converts from the internal
representation and saves an image
All these methods are supposed to be stateless.
3.2.2 Conversion helper methods
A number of methods are provided to simplify writing the converter
classes.
3.2.2.1 Load-oriented conversion methods
generate_track_from_bitstream(track number,
head number,
UINT8 *cell stream,
int cell count,
floppy image)
Takes a stream of cell types (0/1), MSB-first, converts it to the
internal format and stores it at the given track and head in the
given image.
generate_track_from_levels(track number,
head number,
UINT32 *cell levels,
int cell count,
splice position,
floppy image)
Takes a variant of the internal format where each value represents a
cell, the position part of the values is the size of the cell and
the level part is MG_0, MG_1 for normal cell types, MG_N, MG_D for
unformatted/damaged cells, and MG_W for Dungeon-Master style weak
bits. Converts it into the internal format. The sizes are
normalized so that they total to a full turn.
normalize_times(UINT32 *levels,
int level_count)
Takes an internal-format buffer where the position part represents
angle until the next change and turns it into a normal positional
stream, first ensuring that the total size is normalized to a full
turn.
3.2.2.2 Save-oriented conversion methods
generate_bitstream_from_track(track number,
head number,
base cell size,
UINT8 *cell stream,
int &cell_stream_size,
floppy image)
Extract a cell 0/1 stream from the internal format using a PLL setup
with an initial cell size set to 'base cell size' and a +/- 25%
tolerance.
struct desc_xs { int track, head, size; const UINT8 *data }
extract_sectors_from_bitstream_mfm_pc(...)
extract_sectors_from_bitstream_fm_pc(const UINT8 *cell stream,
int cell_stream_size,
desc_xs *sectors,
UINT8 *sectdata,
int sectdata_size)
Extract standard mfm or fm sectors from a regenerated
cell stream. Sectors must point to an array of 256 desc_xs.
An existing sector is recognizable by having ->data non-null.
Sector data is written in sectdata up to sectdata_size bytes.
get_geometry_mfm_pc(...)
get_geometry_fm_pc(floppy image,
base cell size,
int &track_count,
int &head_count,
int &sector_count)
Extract the geometry (heads, tracks, sectors) from a pc-ish floppy
image by checking track 20.
get_track_data_mfm_pc(...)
get_track_data_fm_pc(track number,
head number,
floppy image,
base cell size,
sector size,
sector count,
UINT8 *sector data)
Extract what you'd get by reading in order 'sector size'-sized
sectors from number 1 to sector count and put the result in sector
data.
3.3 Floppy drive
The class floppy_image_interface simulates the floppy drive. That
includes a number of control signals, reading, and writing. Control
signal changes must be synchronized, e.g. fired off a timer to ensure
the current time is the same for all devices.
3.3.1 Control signals
Due to the way they're usually connected to cpus (e.g. directly on an
i/o port) the controls signals work with physical instead of logical
values. Which means than in general 0 means active, 1 means inactive.
Some signals also have a callback associated called when they change.
mon_w(state) / mon_r()
Motor on signal, rotates on 0.
idx_r() / setup_index_pulse_cb(cb)
Index signal, goes 0 at start of track for about 2ms. Callback is
synchronized. Only happens when a disk is in and the motor is
running.
ready_r() / setup_ready_cb(cb)
Ready signal, goes to 1 when the disk is removed or the motor
stopped. Goes to 0 after two index pulses.
wpt_r() / setup_wpt_cb(cb)
Write protect signal (1 = readonly). Callback is unsynchronized.
dskchg_r()
Disk change signal, goes to 1 when a disk is change, goes to 0 on
track change.
dir_w(dir)
Selects track stepping direction (1 = out = decrease track number).
stp_w(state)
Step signal, moves by one track on 1->0 transistion.
trk00_r()
Track 0 sensor, returns 0 when on track 0.
ss_w(ss) / ss_r()
Side select
3.3.2 Read/write interface
The read/write interface is designed to work asynchronously,
e.g. somewhat independently of the current time.
[1] Cylinder is a hard-drive term somewhat improperly used for
floppies. It comes from the fact that hard-drives are similar to
floppies but include a series of stacked disks with a read/write head
on each. The heads are physically linked and all point to the same
circle on every disk at a given time, making the accessed area look
like a cylinder. Hence the name.