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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 II, 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 II 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 300 rpm for every track, with 360 rpm found for HD 5.25"
-floppies and most 8" ones, and a number of different values like 90 rpm
-for the earlier floppies or 150 rpm 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 II, Mac) vary the
-rotational speed depending on the track (something like 394 rpm up to
-590 rpm) 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 (Apple II, 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 a 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 II 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 a 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 300 rpm 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 II, 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 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 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 300 rpm.
-
-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 control 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.