fd.4

操作系统设计与实现源码

.TH FD 4
.SH NAME
fd \- floppy disk
.SH DESCRIPTION
The
.B fd*
devices refer to the Floppy disk driver using the NEC PD765 floppy disk
controller.  These diskettes are arrays of 512 byte sectors, although Minix
always works with two sectors at a time due to its 1024 byte block size.  You
can read or write any number of bytes however, Minix takes care of cutting
and pasting incomplete blocks together.
.PP
The driver is normally configured for two floppy disk devices
.B fd0
and
.BR fd1 .
It can handle two more, but it is unlikely that the average PC can.
.PP
On the first access to an
.B fd
device (by
.BR open (2)
or
.BR mount (2)),
the driver will execute a series of read tests to determine the floppy type.
This works ok for all floppy types except the true 360k type, because it
is indistinguishable from the 720k type.  This only means that the size of
the floppy is not estimated right.
.PP
Bits 2\-6 of the minor device number may be set to the floppy disk type
to make it known to the driver what type of diskette it is reading or
writing.  The non-auto devices should be used for formatting, or when one wants to
be absolutely sure that the device is accessed right.  These devices exist for
drive 0:
.sp
.nf
.ta +4n +7n +9n +8n
	type	device	minor	media
.ta +5n +7n +9n +7n
	0	fd0	0	autodetect
	1	pc0	4	360k, 5.25"
	2	at0	8	1.2M, 5.25"
	3	qd0	12	360k in a 720k, 5.25" drive
	4	ps0	16	720k, 3.5"
	5	pat0	20	360k in a 1.2M, 5.25" drive
	6	qh0	24	720k in a 1.2M, 5.25" drive
	7	PS0	28	1.44M, 3.5"
.fi
.DT
.PP
Type 4 may also be used for the rarely seen 720k, 5.25" floppies (type 2 made
them obsolete fast.)  Note that these "types" only describe the floppies from
a software point of view, type 1 and 4 drives use the same parameters.
.PP
If the format bit (bit 7) is set, then the driver interprets write commands
as track formatting requests.  This is used by the
.BR format (1)
command.
.PP
If the type bits are set to 28, 29, 30, or 31, then the driver uses a
partition table found in sector 0 to partition the floppy.  The partitions
of
.B fd0
may be accessed as
.B fd0a
through
.BR fd0d .
See
.BR hd (4)
for a description of the partition table, and associated ioctl commands.
.SH FILES
/dev/fd[0\-3], /dev/pc[0\-3], /dev/at[0\-3], /dev/qd[0\-3], /dev/ps[0\-3],
/dev/pat[0\-3], /dev/qh[0\-3], /dev/PS[0\-3], /dev/fd[0\-3][a\-d]
.SH "SEE ALSO"
.BR format (1),
.BR hd (4),
.BR part (8).
.SH BUGS
The driver does not know the size of a 360k diskette in a 360k 5.25"
drive, because it uses the 720k parameters for it.  So it will happily try
to read past the end making all kinds of interesting noises.  It's a good
thing these drives are practically obsolete.
.PP
The partition table is only read when the drive motor is off and only for
an auto or partition device.  The driver assumes that a floppy in a drive
with a running motor can't have been replaced all of a sudden.
.SH AUTHOR
Kees J. Bot ([e-mail])