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──────────────────────────────────────────────────────────────────────────── Chapter 9 Volumes and Directories Each file in an MS-DOS system is uniquely identified by its name and its location. The location, in turn, has two components: the logical drive that contains the file and the directory on that drive where the filename can be found. Logical drives are specified by a single letter followed by a colon (for example, A:). The number of logical drives in a system is not necessarily the same as the number of physical drives; for example, it is common for large fixed-disk drives to be divided into two or more logical drives. The key aspect of a logical drive is that it contains a self-sufficient file system; that is, it contains one or more directories, zero or more complete files, and all the information needed to locate the files and directories and to determine which disk space is free and which is already in use. Directories are simply lists or catalogs. Each entry in a directory consists of the name, size, starting location, attributes, and last modification date and time of a file or another directory that the disk contains. The detailed information about the location of every block of data assigned to a file or directory is in a separate control area on the disk called the file allocation table (FAT). (See Chapter 10 for a detailed discussion of the internal format of directories and the FAT.) Every disk potentially has two distinct kinds of directories: the root directory and all other directories. The root directory is always present and has a maximum number of entries, determined when the disk is formatted; this number cannot be changed. The subdirectories of the root directory, which may or may not be present on a given disk, can be nested to any level and can grow to any size (Figure 9-1). This is the hierarchical, or tree, directory structure referred to in earlier chapters. Every directory has a name, except for the root directory, which is designated by a single backslash (\) character. MS-DOS keeps track of a "current drive" for the system and uses this drive when a file specification does not include an explicit drive code. Similarly, MS-DOS maintains a "current directory" for each logical drive. You can select any particular directory on a drive by naming in order── either from the root directory or relative to the current directory──the directories that lead to its location in the tree structure. Such a list of directories, separated by backslash delimiters, is called a path. When a complete path from the root directory is prefixed by a logical drive code and followed by a filename and extension, the resulting string is a fully qualified filename and unambiguously specifies a file. ┌────────────┐ │ Drive │ │ identifier │ └─────┬──────┘ │ ┌───────┴────────┐ │ Root directory │ │ (volume label) │ └─┬──┬──┬───┬──┬─┘ ┌───────────────────┘ │ │ │ └───────────────────┐ │ ┌───────────┘ │ └───────────┐ │ ┌────┴───┐ ┌────┴──────┐ ┌───┴────┐ ┌──────┴────┐ ┌───┴────┐ │ File A │ │ Directory │ │ File B │ │ Directory │ │ File C │ └────────┘ └─┬───────┬─┘ └────────┘ └─┬─────────┘ └─┬──────┘ │ │ │ │ │ │ │ │ ┌─────┘ │ │ │ │ │ │ │ ┌────┴──────┐ ┌──┴─────┐ ┌─────┴──┐ ┌───┴────┐ │ Directory │ │ File D │ │ File E │ │ File F │ └───────────┘ └────────┘ └────────┘ └────────┘ Figure 9-1. An MS-DOS file-system structure. Drive and Directory Control You can examine, select, create, and delete disk directories interactively with the DIR, CHDIR (CD), MKDIR (MD), and RMDIR (RD) commands. You can select a new current drive by entering the letter of the desired drive, followed by a colon. MS-DOS provides the following Int 21H functions to give application programs similar control over drives and directories: Function Action ────────────────────────────────────────────────────────────────────────── 0EH Select current drive. 19H Get current drive. 39H Create directory. 3AH Remove directory. 3BH Select current directory. 47H Get current directory. ────────────────────────────────────────────────────────────────────────── The two functions that deal with disk drives accept or return a binary drive code──0 represents drive A, 1 represents drive B, and so on. This differs from most other MS-DOS functions, which use 0 to indicate the current drive, 1 for drive A, and so on. The first three directory functions in the preceding list require an ASCIIZ string that describes the path to the desired directory. As with the handle-based file open and create functions, the address of the ASCIIZ string is passed in the DS:DX registers. On return, the carry flag is clear if the function succeeds or set if the function failed, with an error code in the AX register. The directory functions can fail for a variety of reasons, but the most common cause of an error is that some element of the indicated path does not exist. The last function in the preceding list, Int 21H Function 47H, allows you to obtain an ASCIIZ path for the current directory on the specified or default drive. MS-DOS supplies the path string without the drive identifier or a leading backslash. Int 21H Function 47H is most commonly used with Int 21H Function 19H to build fully qualified filenames. Such filenames are desirable because they remain valid if the user changes the current drive or directory. Section 2 of this book, "MS-DOS Functions Reference," gives detailed information on the drive and directory control functions. Searching Directories When you request an open operation on a file, you are implicitly performing a search of a directory. MS-DOS examines each entry of the directory to find a match for the filename you have given as an argument; if the file is found, MS-DOS copies certain information from the directory into a data structure that it can use to control subsequent read or write operations to the file. Thus, if you wish to test for the existence of a specific file, you need only perform an open operation and observe whether it is successful. (If it is, you should, of course, perform a subsequent close operation to avoid needless expenditure of handles.) Sometimes you may need to perform more elaborate searches of a disk directory. Perhaps you wish to find all the files with a certain extension, a file with a particular attribute, or the names of the subdirectories of a certain directory. Although the locations of a disk's directories and the specifics of the entries that are found in them are of necessity hardware dependent (for example, interpretation of the field describing the starting location of a file depends upon the physical disk format), MS-DOS does provide functions that will allow examination of a disk directory in a hardware-independent fashion. In order to search a disk directory successfully, you must understand two types of MS-DOS search services. The first type is the "search for first" function, which accepts a file specification──possibly including wildcard characters──and looks for the first matching file in the directory of interest. If it finds a match, the function fills a buffer owned by the requesting program with information about the file; if it does not find a match, it returns an error flag. A program can call the second type of search service, called "search for next," only after a successful "search for first." If the file specification that was originally passed to "search for first" included wildcard characters and at least one matching file was present, the program can call "search for next" as many times as necessary to find all additional matching files. Like "search for first," "search for next" returns information about the matched files in a buffer designated by the requesting program. When it can find no more matching files, "search for next" returns an error flag. As with nearly every other operation, MS-DOS provides two parallel sets of directory-searching services: Action FCB function Handle function ────────────────────────────────────────────────────────────────────────── Search for first 11H 4EH Search for next 12H 4FH ────────────────────────────────────────────────────────────────────────── The FCB directory functions allow searches to match a filename and extension, both possibly containing wildcard characters, within the current directory for the specified or current drive. The handle directory functions, on the other hand, allow a program to perform searches within any directory on any drive, regardless of the current directory. Searches that use normal FCBs find only normal files. Searches that use extended FCBs, or the handle-type functions, can be qualified with file attributes. The attribute bits relevant to searches are as follows: Bit Significance ────────────────────────────────────────────────────────────────────────── 0 Read-only file 1 Hidden file 2 System file 3 Volume label 4 Directory 5 Archive needed (set when file modified) ────────────────────────────────────────────────────────────────────────── The remaining bits of a search function's attribute parameter should be zero. When any of the preceding attribute bits are set, the search function returns all normal files plus any files with the specified attributes, except in the case of the volume-label attribute bit, which receives special treatment as described later in this chapter. Note that by setting bit 4 you can include directories in a search, exactly as though they were files. Both the FCB and handle directory-searching functions require that the disk transfer area address be set (with Int 21H Function 1AH), before the call to "search for first," to point to a working buffer for use by MS-DOS. The DTA address should not be changed between calls to "search for first" and "search for next." When it finds a matching file, MS-DOS places the information about the file in the buffer and then inspects the buffer on the next "search for next" call, to determine where to resume the search. The format of the data returned in the buffer is different for the FCB and handle functions, so read the detailed descriptions in Section 2 of this book, "MS-DOS Functions Reference," before attempting to interpret the buffer contents. Figures 9-2 and 9-3 provide equivalent examples of searches for all files in a given directory that have the .ASM extension, one example using the FCB directory functions (Int 21H Functions 11H and 12H) and the other using the handle functions (Int 21H Functions 4EH and 4FH). (Both programs use the handle write function with the standard output handle to display the matched filenames, to avoid introducing tangential differences in the listings.) ────────────────────────────────────────────────────────────────────────── start: ; set DTA address for buffer ; used by search functions mov dx,seg buff ; DS:DX = buffer address mov ds,dx mov dx,offset buff mov ah,1ah ; function 1ah = search for first int 21h ; transfer to MS-DOS ; search for first match... mov dx,offset fcb ; DS:DX = FCB address mov ah,11h ; function 11h = search for first int 21h ; transfer to MS-DOS or al,al ; any matches at all? jnz exit ; no, quit disp: ; go to a new line... mov dx,offset crlf ; DS:DX = CR-LF string mov cx,2 ; CX = string length mov bx,1 ; BX = standard output handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS ; display matching file mov dx,offset buff+1 ; DS:DX = filename mov cx,11 ; CX = length mov bx,1 ; BX = standard output handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS ; search for next match... mov dx,offset fcb ; DS:DX = FCB address mov ah,12h ; function 12h = search for next int 21h ; transfer to MS-DOS or al,al ; any more matches? jz disp ; yes, go show filename exit: ; final exit point mov ax,4c00h ; function 4ch = terminate, ; return code = 0 int 21h ; transfer to MS-DOS . . . crlf db 0dh,0ah ; ASCII carriage return- ; linefeed string fcb db 0 ; drive = current db 8 dup ('?') ; filename = wildcard db 'ASM' ; extension = ASM db 25 dup (0) ; remainder of FCB = zero buff db 64 dup (0) ; receives search results ────────────────────────────────────────────────────────────────────────── Figure 9-2. Example of an FCB-type directory search using Int 21H Functions 11H and 12H. This routine displays the names of all files in the current directory that have the .ASM extension. ────────────────────────────────────────────────────────────────────────── start: ; set DTA address for buffer ; used by search functions mov dx,seg buff ; DS:DX = buffer address mov ds,dx mov dx,offset buff mov ah,1ah ; function 1ah = search for first int 21h ; transfer to MS-DOS ; search for first match... mov dx,offset fname ; DS:DX = wildcard filename mov cx,0 ; CX = normal file attribute mov ah,4eh ; function 4eh = search for first int 21h ; transfer to MS-DOS jc exit ; quit if no matches at all disp: ; go to a new line... mov dx,offset crlf ; DS:DX = CR-LF string mov cx,2 ; CX = string length mov bx,1 ; BX = standard output handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS ; find length of filename... mov cx,0 ; CX will be char count ; DS:SI = start of name mov si,offset buff+30 disp1: lodsb ; get next character or al,al ; is it null character? jz disp2 ; yes, found end of string inc cx ; else count characters jmp disp1 ; and get another disp2: ; display matching file... ; CX already contains length ; DS:DX = filename mov dx,offset buff+30 mov bx,1 ; BX = standard output handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS ; find next matching file... mov ah,4fh ; function 4fh = search for next int 21h ; transfer to MS-DOS jnc disp ; jump if another match found exit: ; final exit point mov ax,4c00h ; function 4ch = terminate, ; return code = 0 int 21h ; transfer to MS-DOS . . . crlf db 0dh,0ah ; ASCII carriage return- ; linefeed string fname db '*.ASM',0 ; ASCIIZ filename to ; be matched buff db 64 dup (0) ; receives search results ────────────────────────────────────────────────────────────────────────── Figure 9-3. Example of a handle-type directory search using Int 21H Functions 4EH and 4FH. This routine also displays the names of all files in the current directory that have a .ASM extension. Moving Files The rename file function that was added in MS-DOS version 2.0, Int 21H Function 56H, has the little-advertised capability to move a file from one directory to another. The function has two ASCIIZ parameters: the "old" and "new" names for the file. If the old and new paths differ, MS-DOS moves the file; if the filename or extension components differ, MS-DOS renames the file. MS-DOS can carry out both of these actions in the same function call. Of course, the old and new directories must be on the same drive, because the file's actual data is not moved at all; only the information that describes the file is removed from one directory and placed in another directory. Function 56H fails if the two ASCIIZ strings include different logical-drive codes, if the file is read-only, or if a file with the same name and location as the "new" filename already exists. The FCB-based rename file service, Int 21H Function 17H, works only on the current directory and cannot be used to move files. Volume Labels Support for volume labels was first added to MS-DOS in version 2.0. A volume label is an optional name of from 1 to 11 characters that the user assigns to a disk during a FORMAT operation. You can display a volume label with the DIR, TREE, CHKDSK, or VOL command. Beginning with MS-DOS version 3.0, you can use the LABEL command to add, display, or alter the label after formatting. In MS-DOS version 4, the FORMAT program also assigns a semi-random 32-bit binary ID to each disk it formats; you can display this value, but you cannot change it. The distinction between volumes and drives is important. A volume label is associated with a specific storage medium. A drive identifier (such as A) is associated with a physical device that a storage medium can be mounted on. In the case of fixed-disk drives, the medium associated with a drive identifier does not change (hence the name). In the case of floppy disks or other removable media, the disk accessed with a given drive identifier might have any volume label or none at all. Hence, volume labels do not take the place of the logical-drive identifier and cannot be used as part of a pathname to identify a file. In fact, in MS-DOS version 2, the system does not use volume labels internally at all. In MS-DOS versions 3.0 and later, a disk driver can use volume labels to detect whether the user has replaced a disk while a file is open; this use is optional, however, and is not implemented in all systems. MS-DOS volume labels are implemented as a special type of entry in a disk's root directory. The entry contains a time-and-date stamp and has an attribute value of 8 (i.e., bit 3 set). Except for the attribute, a volume label is identical to the directory entry for a file that was created but never had any data written into it, and you can manipulate volume labels with Int 21H functions much as you manipulate files. However, a volume label receives special handling at several levels: ■ When you create a volume label after a disk is formatted, MS-DOS always places it in the root directory, regardless of the current directory. ■ A disk can contain only one volume label; attempts to create additional volume labels (even with different names) will fail. ■ MS-DOS always carries out searches for volume labels in the root directory, regardless of the current directory, and does not also return all normal files. In MS-DOS version 2, support for volume labels is not completely integrated into the handle file functions, and you must use extended FCBs instead to manipulate volume labels. For example, the code in Figure 9-4 searches for the volume label in the root directory of the current drive. You can also change volume labels with extended FCBs and the rename file function (Int 21H Function 17H), but you should not attempt to remove an existing volume label with Int 21H Function 13H under MS-DOS version 2, because this operation can damage the disk's FAT in an unpredictable manner. In MS-DOS versions 3.0 and later, you can create a volume label in the expected manner, using Int 21H Function 3CH and an attribute of 8, and you can use the handle-type "search for first" function (4EH) to obtain an existing volume label for a logical drive (Figure 9-5). However, you still must use extended FCBs to change a volume label. ────────────────────────────────────────────────────────────────────────── buff db 64 dup (?) ; receives search results xfcb db 0ffh ; flag signifying extended FCB db 5 dup (0) ; reserved db 8 ; volume attribute byte db 0 ; drive code (0 = current) db 11 dup ('?') ; wildcard filename and extension db 25 dup (0) ; remainder of FCB (not used) . . . ; set DTA address for buffer ; used by search functions mov dx,seg buff ; DS:DX = buffer address mov ds,dx mov dx,offset buff mov ah,1ah ; function 1ah = set DTA int 21h ; transfer to MS-DOS ; now search for label... ; DS:DX = extended FCB mov dx,offset xfcb mov ah,11h ; function 11h = search for first int 21h ; transfer to MS-DOS cmp al,0ffh ; search successful? je no_label ; jump if no volume label . . . ────────────────────────────────────────────────────────────────────────── Figure 9-4. A volume-label search under MS-DOS version 2, using an extended file control block. If the search is successful, the volume label is returned in buff, formatted in the filename and extension fields of an extended FCB. ────────────────────────────────────────────────────────────────────────── buff db 64 dup (?) ; receives search results wildcd db '*.*',0 ; wildcard ASCIIZ filename . . . ; set DTA address for buffer ; used by search functions mov dx,seg buff ; DS:DX = buffer address mov ds,dx mov dx,offset buff mov ah,1ah ; function 1ah = set DTA int 21h ; transfer to MS-DOS ; now search for label... ; DS:DX = ASCIIZ string mov dx,offset wildcd mov cx,8 ; CX = volume attribute mov ah,4eh ; function 4eh = search for first int 21h ; transfer to MS-DOS jc no_label ; jump if no volume label . . . ────────────────────────────────────────────────────────────────────────── Figure 9-5. A volume-label search under MS-DOS version 3, using the handle-type file functions. If the search is successful (carry flag returned clear), the volume name is placed at location buff+1EH in the form of an ASCIIZ string.