B2te: The switch set by the RDJFCB is tested by the FORTRAN object-time
direct-access handler (DIOCS) to determine whether or not a referenced
disk file exists. If it does not, DIOCS initializes the direct access
file. SYNAD-SVC 68: Located in DMSSVT, SYNAD attempts to simulate the functIons-SYNADAF and SYNADRLS. SYNADAF expansion includes an SVC 68
and a high-order byte in register 15 denoting an access method. SYNAD prepares an error message line, swap save areas and register 13
pointers. The message buffer is 120 bytes: bytes 1-50, 84-119 blank;
bytes 51-120, 1205 INPUT/OUTPUT ERROR nnn ON FILE: "dsname"; where
nnn is the CMS RDBUF/WRBUF error code. All the options of SYNAD are
supported. SYNADRLS expansion includes SVC 68 and a
register 15. The save area is returned,
returned to free storage.
high order byte of X'FF' in
and the message buffer is BACKSPACE-SiC 69: Also in DMSSVT. For a tape, a BSH command is issued a direct access data set, the CMS write and read
pointers are decremented by Control is passed to BACKSPACE in DMSSiT when a BACKSPACE macro is issued. BACKSPACE decrements tlie read
write pointer by ene and returns control to the user. No physical tape or disk adjustments are made until the next READ or WRITE macro is
issued. All the options of BACKSPACE are supported. Located in DMSSiN, this routine receives control when
a TGET or TPUT macro is issued. It is provided to support TSO service
routines needed by program products. TGET reads a terminal line; TPUT writes a terminal line. The return code is zero if the operation vas
successful and a four if an error was encountered. TCLEARQ is located in DMSSiN and causes the terminal
input .queue to be cleared via a call to DESEUF. At completion a return
is made to the user. STAX-SiC 96: Located in DMSSiT, STAX gets and chains a CMSTAXE control block-for-each STAX SiC issued with an exit routine address specified.
The chain is anchored by TAXEADDR in DMSNUC. If no exit address is
specified the most recently added CMSTAXE is cleared from the chain. If
an error occurs during STAI SVC processing, a return code of eight is
placed in register 15. The only option of STAX which may be specified is EXIT ADDRESS. See the DMSSQS prolog for description. B]ADL!BITE: OS READ and WRITE macros branch and link to DMSSBS. DMSSES branches and links to DMSSEB and, if the disks is an as disk, DMSSEB branches and link to DMSROS. See DMSSBS for description. as NOTE, POINT, and FIND (type c) macros
branch and link to entry points in DMSSCT. If the disk is an as disk, DMSSCT branches and links to DMSROS. See DMSSCT for descriptions. CBEC!: See the DMSSCT prolog for description. CMS of Operation and program Organization 2-129

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