A diskette (or floppy disk, short floppy) is a magnetic recordable and changeable storage medium, which consists of a with metal particles coated plastic disk, which usually is put inside an angled plastic casing.
Diskette formats of the Commodore home computer (GCR)
The specification of the diskette capacity of Commodore computers is done according to the GCR storage format (Group Coded Recording) in blocks. As one block corresponds the size of 256 bytes and the maximum storage capacity of a 5.25" SS-disk is 174,848 bytes, there are as result 683 blocks, of which 664 blocks (approx. 166 Kbyte) are usable for saving. Furthermore a maximum of 144 files can be put on a disk side in the GCR format.
The data transfer rate of the disk drives was approx. 300 byte/s and could be raised to up to 10 Kbyte/s by corresponding floppy speeders.
With the C64 (also usable for the VC-20, C128 and Plus/4) the format 5.25" DD (although the 1S and HD formats are applicable, too) are used for the disk drives Commodore 1541, 1570 and 1571. Here the front and the reverse side could be formatted. With the help of a disk puncher the reverse side could be made readable.
The disk drives 1570/1571 and 1581 (usable with a VIC-20, C16/64/116/128,Plus/4) can next to the GCR storage format additionally handle the MFM format (Modified Frequency Modulation; used with disk drives for PC systems). In the MFM format saving is done in bytes. The maximum storage capacity is here 500.000 bytes per disk side (with the drive 1581: 1.000.000 bytes).
As the disk drive 1571 has two reading and writing heads, it can read on both sides without turning over the disk. So a total of 349.696 bytes are available in the GCR storage format, this corresponds to 1366 blocks, of which 1328 (approx. 332 KB) are usable for data. This operating mode only works with the C64 and the C128. The disk drive 1571 is compatible with the formats of the disk drives VC-1541, 1551, 1570, 4040 and 2031.
With the disk drive 1581 also 3.5" disks could be used on the C64/128. Here were 3160 blocks (approx. 790 KB) available. Moreover it was the only Commodore drive that allowed it to build subdirectories.
The forerunner of the VC-1541 was the VIC-1540, which is only usable at the VC 20 and to the disks could be saved about 166 KB.
The 5.25" disk drive 1551 can only be used with the home computers C16/116 and Plus/4.
The thin 5.25" disks are poorly protected, because the casing can be easily folded and a part of the storage coating in the area of the reading and writing head is exposed. As a dust catcher there is a fleece between the magnetic disk and the plastic casing.
5.25" disks should be treated with caution, i.e. to store them inside the paper case when they are not in the drive, not to bend and fold them and not to touch them in the exposed reading and writing area.
The still used and more robust 3.5" disks have a thicker and less flexible plastic case and a metal protection which automatically covers the reading and writing area, when the disk is taken out of the drive. The hardware writing protection can be modified through a plasic slide.
For all disks it is essential to store them in a dry place between 10 and 52°C and not to expose them to direct sun and magnetic fields.
Overview of all disk sizes
- 2.00" (disk format since 1985; max. 720 KB)
- 2.50" Mini Disk Data (since 1995, 140-180 MByte)
- 3.00" (disk format since 1982 for Schneider CPC; 360, max. 720 KB)
- 3.50" (third disk format since 1981)
- 3.50" DD (Double Density; max. 1024 KB
- With IBM-PC and Atari ST max. 720KB; Apple Macintosh max. 800KB; AMIGA max. 880KB or 950KB)
- 3.50" HD (High Density; max. 1440 KB - with AMIGA max. 1760 KB)
- 3.50" ED (Extra High Density; since 1991; max. 2880 KB)
- 5.25" (Second disk format since 1978; storage capacities 110, 160, 170, 180, 320, 360, max. 1244 KB)
- 5.25" 1D (Single Density; max. 170 KB)
- 5.25" SS or 1S (Single Sided*)
- 5.25" DS or 2S (Double Sided)
- 5.25" DD (max. 320 KB)
- 5.25" QD (Quadruple Density; 720 KB, max. 1200 KB)
- 5.25" HD (max. 1244 KB)
- 8.00" First disk format in the 1970s (since 1971; storage capacities 80, 180, 250, 256, 500, 800, max. 1000 KB).
- There are also special disk formats for the computer systems MAC and PC e.g.:
- *ZIP drives (since 1995; 100, 250 and 750 MByte)
- *3.5" LS drives (since 1996; 120 and 240 MByte)
- *JAZ drives (since 1996; 1 and 2 GByte)
- *Supper-Floppy (seit 1997; 1,5 GByte)
- *3.5" HiFD drives (since 1998; 100 and 200 MByte)
- *REV drives (since 2001; 35 GByte)
/* The manufacturer of the disks often only guarantees that the disks are faultlessly usable on one side (front side), although the reverse side is also magnetically coated.
Furthermore the recording type SS,1S,DS,2S is often combined with the recording density 1D,DD,HD, as in 5,25" DD-DS.
Common disk sizes
The disk sizes 8", 5.25" and 3.5" (today still used) are utilised, as well as partially the ZIP, JAZ or REV drives in companies for backing up data.