File system DOS

in dos, drives referred identifying letters. standard practice reserve , b floppy drives. on systems 1 floppy drive dos assigns both letters drive, prompting user swap disks programs alternate access between them. facilitates copying floppy floppy or having program run 1 floppy while accessing data on another. hard drives assigned letters c , d . dos support 1 active partition per drive. support more hard drives became available, developed first assigning drive letter each drive s active primary partition, making second pass on drives allocate letters logical drives in extended partition, third pass give other non-active primary partitions names (where such additional partitions existed , contained dos-supported file system). lastly, dos allocates letters optical disc drives, ram disks, , other hardware. letter assignments occur in order drivers loaded, drivers can instruct dos assign different letter; drivers network drives, example, typically assign letters nearer end of alphabet.

because dos applications use these drive letters directly (unlike /dev directory in unix-like systems), can disrupted adding new hardware needs drive letter. example addition of new hard drive having primary partition pre-existing hard drive contains logical drives in extended partitions; new drive assigned letter assigned 1 of extended partition logical drives. moreover, adding new hard drive having logical drives in extended partition still disrupt letters of ram disks , optical drives. problem persisted through microsoft s dos-based 9x versions of windows until replaced versions based on nt line, preserves letters of existing drives until user changes them. under dos, problem can worked around defining subst drive , installing dos program logical drive. assignment of drive changed in batch job whenever application starts. under versions of concurrent dos, under multiuser dos, system manager , real/32, reserved drive letter l: automatically assigned corresponding load drive whenever application starts.

reserved device names

there reserved device names in dos cannot used filenames regardless of extension occupied built-in character devices. these restrictions affect several windows versions, in cases causing crashes , security vulnerabilities.

the reserved names are: con (for console), aux (for auxiliary), prn (for printer) , lst (for lister), introduced 86-dos 0.74. 86-dos 1.10 , pc dos 1.0 added nul. except lst continued supported in versions of ms-dos, pc dos , dr-dos ever since. lst available in oem versions of ms-dos 1.25, whereas other oem versions of ms-dos 1.25 used lpt1 (first line printer) , com1 (first serial communication device) instead, introduced pc dos. in addition lpt1 , lpt2 com1 com3, hewlett-packard s ms-dos 2.11 hp portable plus supported lst alias lpt2 , 82164a alias com2; supported plt plotters. otherwise, com2, lpt2, lpt3 , clock$ (still named clock in issues of ms-dos 2.11) clock device introduced dos 2.0, , com3 , com4 added dos 3.3. multitasking ms-dos 4 supported keybd$ , screen$. dr dos 5.0 , higher , multiuser dos support $idle$ device dynamic idle detection saving power , improve multitasking. lpt4 optional built-in driver fourth line printer supported in versions of dr-dos since 7.02. config$ constitutes real mode pnp manager in ms-dos 7.0-8.0.

aux typically defaults com1, , prn lpt1 (lst), these defaults can changed in versions of dos point other serial or parallel devices. plt reconfigurable well.

filenames ended colon (:) such nul: conventionally indicate device names, colon not part of name of built-in device drivers. colons not necessary typed in cases, example:

it still possible create files or directories using these reserved device names, such through direct editing of directory data structures in disk sectors. such naming, such starting file name space, has been used viruses or hacking programs obscure files users not know how access these locations.