Commodore originally manufactured the VIC 1540 disk drive for use with the VIC20 computer. This drive is a single sided, 35 track, GCR format drive. When the C64 was first being developed, Commodore had a bug which caused them to not be able to refresh the display and access the disk drive at the same time. As a work around, they slowed the serial bus speed so that disk access would not interfear with display refreshes. At this time, they introduced the VIC 1541 disk drive. The drive is identical to the 1540, except that it defaults to the slower speed at power-up. Both drives can be issued commands to change their speeds. The "VIC" was later dropped from the 1541's name, and the color was changed to match that of the C64, but all changes were cosmetic only. Commodore later sold 1570 and 1571 disk drives when the C128 was introduced. Both drives are 40 track and able to read GCR formatted disks as well as industry standard MFM formated disks. This was done to allow software interchange between the C128 and other machines running the CP/M operating system. The only difference between the drives is that the 1570 is single sided, and the 1571 is double sided. Commodore's final drive made for the 8 bit line of machines is the 1581. This is an 80 track 3.5" disk drive (while all others are 5.25") which uses only the industry standard MFM format.