Hardware Reference
In-Depth Information
thatadiskhasbeeneitherinsertedorejected.Thisregisterissettoindicatethatadiskhas
been inserted or removed (changed) by default.
The register is cleared when the controller sends a step pulse to the drive and the drive re-
sponds, acknowledging that the heads have moved. At this point, the system knows that a
specificdiskisinthedrive.IftheDCsignalisnotreceivedbeforethenextaccess,thesys-
tem can assume that the same disk is still in the drive. Any information read into memory
during the previous access can therefore be reused without rereading the disk.
A problem occurs if a drive or cable fault prevents the system from seeing the DC signal
on pin 34. If a system is told (through BIOS setup) that the drive is any type other than
a 360KB (which can never send the DC signal), the system expects the drive to send DC
wheneveradiskhasbeenejected.Ifthesystemdoesn'treceivethesignal,thesystemnev-
er recognizes that a disk has been changed. Therefore, even if you do change the disk, the
system still acts as though the first disk is in the drive and holds the first disk's directory
and FAT information in RAM. This can be dangerous because the FAT and directory in-
formation from the first disk can be partially written to any subsequent disks written to in
the drive.
Floppy Disk Media Types and Specifications
Table 10.6
shows the technical specifications for different floppy disk media.
Table 10.6 Floppy Disk Media Specifications
Density
Disks have two types of densities: longitudinal density and linear density.
Longitudinal
density
isindicatedbyhowmanytrackscanberecordedonthediskandisoftenexpressed
asanumberoftracksperinch(TPI).
Linear density
isthecapabilityofanindividualtrack
