This driver provides access to floppy disk drives. Floppy disks using either FM (single-density) or MFM (double or high-density) recording can be handled.
Floppy disk controllers can connect up to four drives each. The fdc driver can currently handle up to two drives per controller (or four drives on ACPI). Upon driver initialization, an attempt is made to find out the type of the floppy controller in use. The known controller types are either the original NE765 or i8272 chips, or alternatively enhanced controllers that are compatible with the NE72065 or i82077 chips. These enhanced controllers (among other enhancements) implement a FIFO for floppy data transfers that will automatically be enabled once an enhanced chip has been detected. This FIFO activation can be disabled using the per-controller flags value of 0x1.
By default, this driver creates a single device node /dev/fd N for each attached drive with number N. For historical reasons, device nodes that use a trailing UFS-style partition letter (ranging from 'a' through 'h') can also be accessed, which will be implemented as symbolic links to the main device node.
Accessing the main device node will attempt to autodetect the density of the available medium for multi-density devices. Thus it is possible to use either a 720 KB medium or a 1440 KB medium in a high-density 3.5 inch standard floppy drive. Normally, this autodetection will only happen once at the first call to open(2) for the device after inserting the medium. This assumes the drive offers proper changeline support so media changes can be detected by the driver. To indicate a drive that does not have the changeline support, this can be overridden using the per-drive device flags value of 0x10 (causing each call to open(2) to perform the autodetection).
When trying to use a floppy device with special-density media, other device nodes can be created, of the form /dev/fd N. MMMM, where N is the drive number, and MMMM is a number between one and four digits describing the device density. Up to 15 additional subdevices per drive can be created that way. The administrator is free to decide on a policy how to assign these numbers. The two common policies are to either implement subdevices numbered 1 through 15, or to use a number that describes the medium density in kilobytes. Initially, each of those devices will be configured to the maximal density that is possible for the drive type (like 1200 KB for 5.25 inch HD drives or 1440 KB for 3.5 inch HD drives). The desired density to be used on that subdevice needs to be configured using fdcontrol(8).
Drive types are configured using the lower four bits of the per-drive device flags. The following values can be specified: