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CHKPT Macro
┌─────┬────────┬────────────────────────────────┐
│ │ │ res─ar─ address │
│ │ │ SYSnnn, (r1) │
│ │ │ , end address , ─poin─er │
│ VSE │ CHKPT │ (r2) (r3) │
│ │ │ dpoin─er , filename │
│ │ │ (r4) (r5) │
├─────┼────────┼────────────────────────────────┤
│ │ │ dcbaddress ,checkid address │
│ │ │ ,checkid leng─h │
│ MVS │ CHKPT │ ,′ S′ │
│ │ │ CANCEL │
└─────┴────────┴────────────────────────────────┘
The MVS CHKPT macro is similar to the VSE CHKPT macro with two minor
differences in the checkpoint logic. One difference is in the use of registers when
control returns to you after the CHKPT macro. In VSE, register 0 indicates
successful or unsuccessful checkpointing. In MVS, register 15 indicates not only
successful or unsuccessful checkpointing, but also successful restart. Another
difference is in the restart logic. In VSE, you specify a restart address as one of
the parameters in the CHKPT macro. MVS automatically restarts with the
instruction following the CHKPT macro.
Many VSE parameters are not necessary in MVS:.
•
The SYSnnn parameter in VSE specifies the logical unit on which the
checkpoint is stored. The dcbaddress parameter of the MVS CHKPT macro
gives the address of the user-coded data control block (equivalent to a DTF)
for the checkpoint data set, which must be a magnetic tape or direct access
volume.
•
A restart address is not given in MVS.
•
An end address used in VSE to specify the highest storage address to be
dumped during CHKPT is not needed, because MVS automatically dumps the
entire contents of the program′s virtual storage data areas.
•
tpointer
in VSE provides a list to the CHKPT macro of the tape files used in
the program. If this parameter was not specified, repositioning of tape files
would not be performed at restart. MVS automatically repositions tape files
without the use of a similar parameter.
•
dpointer
in VSE permits operator volume verification at restart time. The
facility is provided by MVS when it allocates devices for a particular job step.
•
Filename in VSE points to the DTF describing the disk CHKPT file. This file
must be opened prior to use, and label checking is performed at that time.
The MVS DCB is roughly equivalent. The data set may be opened by you or
by the checkpoint routine when the CHKPT macro is executed. It is
recommended that the user issue the OPEN rather than default to the
system, because the system opens every time a CHKPT is issued.
282 VSE to OS/390 Migration Workbook