IEC501E

Explanation
Note: For Automated Tape Library dataservers
(including Virtual Tape Servers), messages IEC501A and IEC501E are
not issued to any operator console.  They are only issued to the System
Log.
M indicates that a volume is to be mounted on the device: If ser is a 6-digit serial number, the volume
with that serial number is to be mounted on device dev.
If ser is SCRTCH, a public scratch volume is
to be mounted. SCRTCH is used when the dataset being created on
the non-specific volume is temporary [DISP=(NEW,DELETE) or DSN=&&tempname].
If ser is PRIVAT, a private scratch volume
is to be mounted. PRIVAT is used for all other cases of non-specific
volumes.
If ser begins with L, the volume to be mounted
is unlabeled; the number after the first character is an internal
serial number assigned by the system to an unlabeled volume. The internal
serial is of the form Lxxxyy where xxx is the data set number (0-999)
and yy is the volume sequence number (0-99). The numbers are recycled
after they reach the limit.

IEC501A is issued by Open when Allocation has not issued
a mount message (that is, when no IEF233A or IEF233D has been issued
-- for example, mounts with DEFER coded) and the volume is needed
immediately.
IEC501A is issued by EOV when the end of a volume
has been reached and the next volume to be processed is needed immediately.
IEC501E
is issued by Open or EOV when the volume  to be mounted is not needed
immediately (this is referred to as a "look-ahead" mount).  The volume
indicated by ser is the next volume to be processed after the system
finishes processing the current volume.
In the message text: labtyp
Indicates the label type of the mounted volume (either SL,
NL, AL, NSL, or BLP) and will appear for tape volumes only.
den
Indicates the density of the volume to be mounted and will appear
for tape volumes only.
trtch
Indicates the recording mode as COMP (compaction) or NOCOMP (non-compaction)
and will appear for tape volumes only.
jjj
Indicates the job requesting the volume.
sss
Indicates the job step requesting the volume.
dev
The device number of the specified device.
ser
The volume serial number.
dsname
If a MONITOR DSNAME command is active, the dsname is
of a nontemporary data set requiring the volumes. The data set name
will not be specified for data sets being deleted. If the data set
name causes the message to exceed 70 characters, the data set name
will appear on the second line of the message text.
mediatype
The media type.

System action
If the message ID is IEC501A, the job step waits
for the volume to be mounted or until the task reaches time-out. 
This is independent of whether the original allocation was batch (JCL)
or dynamic.
If the message ID is IEC501E, the job step does
not wait for the volume to be mounted.  If the data set is multi-volume
and more than one unit is allocated, but fewer units are allocated
than volumes, then all volumes after the first may be requested by
EOV via message IEC501E.  For example, if a data set consists of three
volumes, and two tape drives are requested by UNIT=(TAPE,2),  then
as soon as End of Volume is reached on volume 1, message IEC502E is
issued for volume 1, message IEC501E is issued for volume 3 on that
same drive, and the job then starts processing volume 2 immediately,
without waiting for the mount for volume 3 to be satisfied. This is
independent of whether the original allocation was batch (that is,
JCL) or dynamic.

Operator response
If ser is SCRTCH or PRIVAT
and the device is a tape unit, make sure that the file protection
ring has been inserted in the volume.
Mount the volume on the
device.  If ser is SCRTCH or PRIVAT and a scratch volume is already
on the device, mount another scratch volume. Then ready the device.

Source
DFSMSdfp

Module
 IFG0193D, IFG0193E, IFG0194A, IFG0194F, IFG0194K, IFG0195H,
IFG0196N, IFG0196T, IFG0196U, IFG0552B, IFG0552F