The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

ackup utility provides exceptional flexibility as can be evidenced by the following
command examples:
BACKUP :4 TO :2
will copy all files from logical drive 4 to logical drive 2. If both drives are floppies
having the same physical configuration (i.e. both 40-track 2-headed with the same
density), then the backup will automatically be performed track by track called "mirror
image".
BACKUP /TXT:3 TO :5 (OLD)
will copy all files with a file extension of "TXT" from logical drive 3 to logical drive
5 but only if the file already exists on logical drive 5. The use of the "OLD" parameter
permits organization of archival copies.
BACKUP R$S/BAS:4 TO :2 (MOD,DATE="11/09/82-11/15/82")
will make copies of all files from logical drive 4 with a filename starting with the
character "R", the third character "S", with any character acceptable in all other file
name character positions. Also, files must have been last modified between the dates of
November 9, 1982 through November 15, 1982 inclusive in order to be included in the
backup. In addition, the file must not have been backed up since it was last modified.
These examples illustrate the extreme flexibility of managing archival copies of working
files. When used in a hard drive environment, large capacity floppy diskettes can be used
to store selected "classes" of files with working files backed up in a structured manor
only if they have been modified. Daily "churning" of working files is minimal, thus a
procedure that enables a backup only if a modification has been done to a working file
within a class certainly lends itself to optimum file management techniques without the
need for expensive backup hardware. For those cases where a single file exceeds the
capacity of a single floppy, a separate utility provides diskette spanning capabilities
for the backup.
The command to obtain a directory display is used frequently in most machine environments.
The DOS directory command listing is sorted by file name/ext. When the length
of a listing exceeds the line capacity of the video display, paging is performed with a
pause at each page. The listing provides data on the protection level, logical record
length, file length (in kilobytes), date of last update, and whether a backup copy
exists, for each file in the directory. A partial file specification can be requested to
limit the listing to those files in the "class" similar to the BACKUP utility.
Disk files are supported with two types of access - Record I/O and character I/O. Logical
Records of from one to 256 bytes in length can be read or written using the @READ or
@WRITE SVC requests. Record I/O can be random access (by position SVC requests prior to
READ/WRITE) or sequential access using repetitive READs or WRITEs. Character I/O is
accomplished by @GET and @PUT SVC requests and is essentially the same as record I/O with
a Logical Record Length (LRL) equal to one. However, if GET and PUT are used to implement
sequential access, then a file can be considered a character I/O device just like a
printer, a serial port, or a video display device. A byte I/O request is therefore
independent of the physical device "connected" to the control block which is requesting
the I/O. This makes the system "device independent".
Routing, filtering, and linking is 100% - devices may be routed to files and subsequently
filtered and linked. A priority level hierarchy is established according to bit assignments
in the DCB: file, NIL, route, link, and filter (file being the highest). Filters
are assigned control blocks in the DCB table area which supports up to 31 entries. Each
device driver and filter has its own entry. The establishment of a LINK also uses a DCB
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