Notes on using the as simulation routines: • CMS files are physically blocked in aDO-byte blocks, and logically
blocked according to a record length. If the file.ode of the
file is not 4, the logical record length is equal to the DCBLRECL and
the file .ust always be referenced with the same DCBLRECL, whetber or
not the file is blocked. If tbe filemode of the file is 4, tbe
logical record length is equal to the DCBBLKSI and the file aust always be referenced witb tbe same DCBBLKSI. • When writing CMS files with a filemode number other than four, the OS simulation routines deblock the output and write it on a disk in
unblocked records. The simulation routines delete each 4-byte block
descriptor word (BDW) and each 4-byte record descriptor word (RDW) of
variable length records. This makes tbe OS-created files compatible
with CMS-created files and CMS utilities. When CMS reads a eMS file
with a filemode number otber than four, CMS blocks the record input
as specifies and restores the BDW and RDi control words of variable
length records. • If the CMS filemode number is four, CMS does not unblock or delete tiuws or HDWs on output. CMS assumes on input that the file is
blocked as specified and that variable length records contain block
descriptor words and record descriptor words.
To set the READ/WRITE
FILEDEF command must
option.
pointers for a file at tbe end of the file, a
be issued for the file specifying the MeD • A file is erased and a new one created if the file is opened and all
the following conditions exist:
The OUTPUT or OUTIN option of OPEN is specified.
The TYPE option of OPEN is not J.
The dataset organization option of tbe DCE is not direct access or
partiticned.
A FILEDEF command has not been issued for data set specifying the MOD option. • The results are unpredictable if two DCBs read and write to the same
data set at the same time. fLO!: The module DMSACC gets control first wben you
invoke tbe ACCESS command. DMSACC verifies parameter list validity and
sets the necessary internal flags for latpr use. If the disk you access
specifies a target mode of anotber disk currently accessed, DMSACC calls DMSALU to clear all pertinent information in the old active disk table. DMSACC then calls DMSACF to bring in the user file directory of the
disk. As soon as DMSACF gets control, DMSACF calls DMSACM to read in
the master file directory of the disk. Once DMSACM reads the label cf
the disk, and determines that it is an OS disk, DMSACM calls DMSROS (ROSACC) to complete the access of the OS disk. Upon returning from DMSROS, DMSACM returns immediately to DMSACF, bypassing the master file
directory logic for CMS disks. DMSACF then cbecks to determine if tbe
accessed disk is an OS disk. If it is an as disk, DMSACF returns
immediately to DMSACC, bypassing all the user file directory logiC for OS disks. DMSACC checks to determine if the accessed disk is an OS 2-130 IBM VM/370 System Logic and Program Determination--Volume 2

disk; if it is, another check determines if the accessed disk replaces another disk to issue an information message tc that effect. Another
check determines if you specified any options or fileid and, if you did,
a warning message appears On the terminal. Control now returns to the
calling routine. DMSFLD gets control first when you issue a FILEDEF command. DMSFLD adds, changes, or deletes a FILEDEF control
block and returns control to the calling routine. LISTDS COMMAND FLOW: The module DMSLDS gets control first when you Invoke the-IISTDS-comaand. DMSLDS verifies parameter list validity and
calls module DESLAD to get the active disk tatle associated with the
specified mode. DMSLDS reads all format 1 DSCB and if you specified the PBS option and the data set is partitioned, DMSLDS calls DMSRCS (ROSFIND) to get the members of the data set. After displaying the DSCB (or DSCB) on you console, DMSLDS returns to the calling routine. I1Q!: The module DMSMVE gets control first when you
issue a CMS MOVEFILE command. DMSMVE calls DMSFLD to get an input and
output CMSCB and, if the input DMSCB is for a disk file, DMSftVE calls DMSSTT to verify the existence of the input file and get default DCB parameters in absence of CMSCB DCB parameters. DMSMVE uses OS OPEN, FIND, GET, PUT, and CLOSE macros to move data from the input file to the
output file. After moving the specified data, control returns to the
calling routine. 2QER! £QMMA!Q I1Q!: The module DMSQRY gets control first when you invoke
the QUERY command. DMSQRY verifies parameter list validity and calls DMSLAD to get the active disk table associated with the specified mode. DMSQRY displays all the information that you requested on your console. When DMSQRY finishes, control returns to the calling routine. FLQ!: The module DMSARE gets control first when you
invoke the RELEASE command. DMSARE verifies parameter list validity and
checks to determine if the disk you want to release is accessed. If the
disk you want to release is currently active, DMSARE calls D8SALU to
clear all pertinent information associated with the active disk. DMSALU first checks the active disk table for any existing CMS tables kept in
free storage. If the disk you want to release is an OS disk, DMSALU does not find any tables associated with a CMS disk. If the disk is an OS disk, D5SALU releases the os PST blocks (if any) and clears any OS PST pointers in the OS file control blocks. DMSALU then clears the
active disk table and returns to DMSARE. DMSIRE then clears the device
table address for the specified disk and returns to the calling routine. STATE fLO!: The module DMSSTT gets control first when you invoke
the STATE command. DMSSTT verifies the parameter list validity and
calls module DMSLAD to get the active disk table associated with the
specified mode. Upon return from DMSLAD, DMSSTT calls DMSLFS to find
the file status table (FST) associated with the file you specified. Once DMSLFS finds the associated FST, it checks to determine if the file
resides on an OS disk. If it does, DMSLFS calls DMSROS (ROSSTT) to read
the extents of the data set. Upon return from D"SROS, DMSLFS returns to DMSSTT. DMSSTT then copies the FST (or OS FST) to the FST copy in
statefst and returns to the calling routine. DMSACC MODULE: Once DMSACC determines that the disk you want to access Is-an-oS-disk, it bypasses the routines that perfor. LOGIN UFD and LOGIN ERASE. CMS Method of Operation and Program Organization 2-131