April 27, 1981
If you have a disk accessed as an extension of another disk, the
extension disk is automatically read-only, and you cannot write on it. You miqht access a disk as its own extension, therefore, to protect the
files on it, so that you do not accidentally write on it. For example:
access 235 bib
Another use of extensions is to extend the CMS search order. If you
issue a command requesting to read a file, for example:
type alpha plan CMS searches your A-disk for the file named ALPHA PLAN and if it does
not find it, searches any extensions that your A-disk may have. If you
have a file named ALPHA PLAN on your B-disk but have not accessed it as
an extension of your A-disk, CMS will not find the file, and you will
have to reenter the command:
type alpha plan b
Additionally, if you issue a CMS command that reads and writes a
file, and the file to be read is on an extension of a read/write disk,
the output file is written to the parent read/write disk. The EDIT
command is a qood example of this type of command. If you have a file
named FINAL LIST on a B-disk extension of a read/write A-disk, and if
you invoke the editor to modify the file with the command:
edit final list
after you have made modifications to the file, the changed file is
written onto your A-disk. The file on the B-disk remains unchanged. When you access a disk as a read-only extension, it remains an extension
of the parent disk as long as both disks are still accessed. If either
disk is released, the relationship of parent disk/extension is
terminat ed • If the parent disk is released, the extension remains accessed and
you may still read files on it. If you access another disk at the mode
letter of the oriqinal parent disk, the parent/extension relationship
remains in effect.
If you release a read-only extension and access another disk with the
same mode letter, it is not an extension of the original parent disk
unless you access it as such. For example, if you enter:
access 198 cia
release c
access 199 c
the C-disk at virtual address 199 is not an extension of your A-disk. WHEN TO SPECIFY FIIEMODE LETTERS: READING FILES When you request CMS to access a file, you have to identify it so that CMS can locate it for you. The commands that expect files of particular
52 IBM VM/370 eMS User's Guide

April 1, 1981 filetvpes (reserved filetyues) allow you to enter only the filename of
the file when you issue the command. When you execute any of these or execute a MODULE or FrEe file, searches all of your disks (usinq the standard search order) to locate the file.
The that perform this type of search are: DOSLIE EXEC Some commands
identify file. Yoa specify the filemode,
when it looks the
disk yOJ specify and vou use this
EDIT FILEDEF PRINT GLOBAL LOAD KODKAP RUN TXTLIB require you to enter the and filetype to
may specify the filemode letter; if you do not searches only your A=disk and its extensions
file. If you do specify a filemode letter, the
its extensions are searched for the file. The
way are: STATE SYNONYM TAPE DUMP TYPE UPDATE There two CMS commands that do not search extensions of disks
when for files. They are: DISK LISTPItE You must explicitly enter the filemode if you want to use these commands
to list oc dump files that are on extensions. For some commands, if you specify the filemode of a file as an
asterisk, it indicates that you either do not know or do not care what
disk the file is on and you want eMS to locate it for you. For example,
if YJU enter: listfile myfile test *
the LIsrFILE command responds by listing all files on your accessed disks MVFTLE TEST. When you specify an asterisk for the filemode
of the CDPYFILE, ERASE, or PENAME commands, CMS locates all copies of
the specified file. For example:
rename temp sort * qood sort
renames all files named TEMP SORT to GOOD SORT on all of your accessed
read/write An equal sign (=) is valid in output fileids for the RENAME ana COPYFILF commands.
For some commands, when you specify an asterisk for the filemode of a
file, stops searching as soon as it finds the first copy of the
file. example: type myfile assemble * Section The File System 53