Disks of space dust (debris disks) surround many stars. Images were stored on video floppy disks. Lastly, DOS allocates letters for optical disc drives, RAM disks, and other hardware. Early PCs had to be booted from floppy disks. Installation media consisted of eight 1.2M floppy disks. Regardless of type, the file system of all floppy disks (under DOS) was FAT12. The source code itself was available on a set of floppy disks. In 1996, there were an estimated five billion standard floppy disks in use. This class was introduced with the emergence of floppy disks. SpeedFan also monitors S.M.A.R.T. readings for EIDE, SATA and SCSI hard disks. This drive was for use with industry standard double-sided -inch 360-kilobyte formatted flexible disks. The name derives from IBM's habit of calling hard drives "fixed disks". They were archived into 4TB hard disks. It is used to test RAM, hard disks and printers. Access to data stored on disks is a central feature of all operating systems.