Model III TRSDOS (culminating in version 1.3) supported 40 track disks formatted in double density (using modified frequency modulation). Model Is retrofitted with double density controllers and Models I/III equipped with 80 track drives or double-sided drives could not use TRSDOS; RadioShack sold Logical System's LDOS operating system which could control these types of drives. The Model 4's TRSDOS 6 is a development of LDOS and has the same capabilities. Hard disk drives (then also known as winchester drives) required custom driver software supplied by their manufacturers. These drivers permitted any TRSDOS installation to access them with up to eight possible drive partitions, each assigned to drive numbers zero through seven. Actually, a large hard drive could be formatted with more than eight partitions, but TRSDOS can only access eight during any one session. Hard drives could have some partitions formatted under TRSDOS and others under the CP/M OS. Each floppy drive in the system would also take up one drive number assignment. The Model 4, with its ability to set up a ramdisk (Memdisk), also required a drive number assignment for this. All versions of TRSDOS use overlays to satisfy most system requests and disk directories are not maintained in memory. This has two implications for system performance. First, upon initial file access the DOS always references the disk directory to obtain information giving the physical mapping of disk space allocated to the file (including its extents, if any). After the initial access this information is maintained in a File Control Block, the memory space for which is supplied by the calling application. Further references do not need to read the disk directory (unless the file is written to and more disk space needs to be allocated). For this reason system performance depends greatly on how close a file's allocated disk space(s) is/are to the directory cylinder, and how fragmented (extents located in non-contiguous space(s)) the file is as a whole. The farther away the directory cylinder is, the more the drive's read/write head will need to move, which slows disk access and produces more mechanical wear on the drive. TRSDOS has commands permitting the user to optimize placement of particular files on the disk's physical space, and the command to display a map of a file's physical placement on a drive. The second implication of the overlay-based architecture is that a disk containing TRSDOS system files (file extension /SYS) must always be present in whichever drive is assigned as logical drive number zero.