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Tidy documentation

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Zoë Blade 2015-04-05 19:51:18 +01:00
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docs/floppy.txt
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@ -83,11 +83,11 @@ is usually behind the term "density".
A sensor detects when the head is on track 0 and the controller is not 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 supposed to try to go past it. In addition physical blocks prevent
the head from going out of the correct track range. Some systems 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 (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 just wham the head against the track 0 physical block, giving a
well-known crash noise and eventually damaging the head alignment. well-known crash noise and eventually damaging the head alignment.
Also, some systems (apple 2 and c64 again) have direct access to the 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 phases of the head positioning motor, allowing to trick the head into
going between tracks, in middle or even quarter positions. That was going between tracks, in middle or even quarter positions. That was
not usable to write more tracks, since the head width did not change, not usable to write more tracks, since the head width did not change,
@ -95,15 +95,15 @@ but since reliable reading was only possible with the correct position
it was used for some copy protection systems. it was used for some copy protection systems.
The disk rotates at a fixed speed for a given track. The most usual 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" 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 90rpm floppies and most 8" ones, and a number of different values like 90 rpm
for the earlier floppies or 150rpm for an HD floppy in an amiga. 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 Having a fixed rotational speed for the whole disk is called Constant
Angular Velocity (CAV, almost everybody) or Zoned Constant Angular Angular Velocity (CAV, almost everybody) or Zoned Constant Angular
Velocity (ZCAV, C64) depending on whether the read/write bitrate is Velocity (ZCAV, C64) depending on whether the read/write bitrate is
constant or track-dependant. Some systems (apple 2, mac) varies the constant or track-dependant. Some systems (Apple II, Mac) vary the
rotational speed depending on the track (something like 394rpm up to rotational speed depending on the track (something like 394 rpm up to
590rpm) to end up with a Constant Linear Velocity (CLV). The idea 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 behind ZCAV/CLV is to get more bits out of the media by keeping the
minimal spacing between magnetic orientation transitions close to the minimal spacing between magnetic orientation transitions close to the
best the support can do. It seems that the complexity was not deemed best the support can do. It seems that the complexity was not deemed
@ -158,10 +158,10 @@ is never known.
2.3 Floppy controller 2.3 Floppy controller
The task of the floppy controller is to turn the signals to/from the 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 floppy drive into something the main CPU can digest. The level of
support actually done by the controller is extremely variable from one support actually done by the controller is extremely variable from one
device to the other, from pretty much nothing (apple2, c64) through device to the other, from pretty much nothing (Apple II, C64) through
minimal (amiga) to complete (western digital chips, upd765 family). minimal (Amiga) to complete (Western Digital chips, uPD765 family).
Usual functions include drive selection, motor control, track seeking Usual functions include drive selection, motor control, track seeking
and of course reading and writing data. Of these only the last two and of course reading and writing data. Of these only the last two
need to be described, the rest is obvious. need to be described, the rest is obvious.
@ -170,21 +170,21 @@ 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 or bytes) are encoded on the surface, and how these are grouped in
individually-addressable sectors. Two standards exist for these, individually-addressable sectors. Two standards exist for these,
called FM and MFM, and in addition a number of systems use their 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 home-grown variants. Moreover, some systems such as the Amiga use a
standard bit-level encoding (MFM) but an homegrown sector-level standard bit-level encoding (MFM) but a homegrown sector-level
organisation. organisation.
2.3.1 Bit-level encodings 2.3.1 Bit-level encodings
2.3.1.1 Cell organization 2.3.1.1 Cell organization
All floppy controllers, even the wonkiest like the apple 2 one, start All floppy controllers, even the wonkiest like the Apple II one, start
by dividing the track in equally-sized cells. They're angular by dividing the track in equally-sized cells. They're angular
sections in the middle of which a magnetic orientation inversion may sections in the middle of which a magnetic orientation inversion may
be present. From an hardware point of view the cells are seen as be present. From a hardware point of view the cells are seen as
durations, which combined with the floppy rotation give the section. durations, which combined with the floppy rotation give the section.
For instance the standard MFM cell size for a 3" double-density floppy For instance the standard MFM cell size for a 3" double-density floppy
is 2us, which combined with the also standard 300rpm rotational speed 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 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. it is that there are 100K cells in a 3" DD track.
@ -234,7 +234,7 @@ Modulation encoding, which can cram exactly twice as much data on the
same surface, hence its other name of "double density". The cell size 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. 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 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: 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 - the first cell, called the clock bit, is '1' if both the previous
and current data bits are 0, '0' otherwise and current data bits are 0, '0' otherwise
@ -252,7 +252,7 @@ maximum of three zeroes.
Group Coded Recording, or GCR, encodings are a class of 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 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 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 Apple II, the Mac and the C64, and each system has its own table, or
tables. tables.
2.3.1.5 Other encodings 2.3.1.5 Other encodings
@ -342,7 +342,7 @@ usually start at 1 and size code is 0 for 128 bytes, 1 for 256, 2 for
The crc is a cyclic redundancy check of the data bits starting with The crc is a cyclic redundancy check of the data bits starting with
the mark just after the pulse train using polynom 0x11021. the mark just after the pulse train using polynom 0x11021.
The western digital-based controllers usually get rid of everything The Western Digital-based controllers usually get rid of everything
but some 0xff before the first sector and allow a better use of space but some 0xff before the first sector and allow a better use of space
as a result. as a result.
@ -365,19 +365,19 @@ Then for each sector:
- MFM-encoded 0xfb, sector data followed by two bytes of crc - MFM-encoded 0xfb, sector data followed by two bytes of crc
- A number of MFM-encoded 0x4e (usually 84, very variable) - A number of MFM-encoded 0x4e (usually 84, very variable)
The the track is finished with a stream of MFM-encoded 0x4e. The track is finished with a stream of MFM-encoded 0x4e.
The 250KHz pulse trains are used to lock the PLL to the signal The 250KHz pulse trains are used to lock the PLL to the signal
correctly. The cell pattern 4489 does not appear in normal correctly. The cell pattern 4489 does not appear in normal
MFM-encoded data and is used for clock/data separation. MFM-encoded data and is used for clock/data separation.
As for FM, the western digital-based controllers usually get rid of As for FM, the Western Digital-based controllers usually get rid of
everything but some 0x4e before the first sector and allow a better everything but some 0x4e before the first sector and allow a better
use of space as a result. use of space as a result.
2.3.2.3 Formatting and write splices 2.3.2.3 Formatting and write splices
To be usable a floppy must have the sector headers and default sector To be usable, a floppy must have the sector headers and default sector
data written on every track before using it. The controller starts data written on every track before using it. The controller starts
writing at a given place, often the index pulse but on some systems 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 whenever the command is sent, and writes until a complete turn is
@ -417,7 +417,7 @@ position of the start of the cell (not the size), and bits
- 3, MG_D -> Damaged zone, reads as neutral but cannot be changed by writing - 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 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. corresponds to one nanosecond when the drive rotates at 300 rpm.
The last cell implicit end position is of course 200,000,000. The last cell implicit end position is of course 200,000,000.
@ -523,7 +523,7 @@ extract_sectors_from_bitstream_fm_pc(const UINT8 *cell stream,
int sectdata_size) int sectdata_size)
Extract standard mfm or fm sectors from a regenerated Extract standard mfm or fm sectors from a regenerated
cell stream. Sectors must point to an array of 256 desc_xs. cell stream. Sectors must point to an array of 256 desc_xs.
An existing sector is recognizable by having ->data non-null. An existing sector is recognizable by having ->data non-null.
Sector data is written in sectdata up to sectdata_size bytes. Sector data is written in sectdata up to sectdata_size bytes.
@ -563,8 +563,8 @@ the current time is the same for all devices.
3.3.1 Control signals 3.3.1 Control signals
Due to the way they're usually connected to cpus (e.g. directly on an 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 I/O port), the control signals work with physical instead of logical
values. Which means than in general 0 means active, 1 means inactive. values. Which means than in general 0 means active, 1 means inactive.
Some signals also have a callback associated called when they change. Some signals also have a callback associated called when they change.