Clocks and time stamps

In the descriptions of CICS monitoring data, the term clock is distinguished from the term time stamp.

A time stamp is an 8-byte copy of the output of a local store clock (STCK) instruction.

A clock consists of three components, arranged in order:
  1. Timer component. This is a value giving the accumulated time recorded by the clock, expressed in local store clock (STCK) units. For performance class data, the timer component is a 64-bit value. For transaction resource class data, the timer component is a 32-bit value, expressed in units of 16 microseconds. For exception class data, there are no clocks. For more information about timer components, see the TOD clock information in z/Architecture Principles of Operation.
  2. 8 reserved bits.
  3. Period count. The time recorded by the timer component is accumulated during one or more measurement periods. The period count is a 24-bit value giving the number of measurement periods. The period count runs to 16 777 216.
Neither the timer component of a clock nor its period count are protected against wraparound. The capacity of the clock depends on the class of monitoring data to which the clock applies:
  • For performance class data, the clock capacity is only bounded by the capacity of the local store clock, which is several years.
  • For transaction resource class data, the clock capacity is about 18 hours.
The 8 reserved bits have the following significance:
Bits 0, 1, 2 and 3
Used for online control of the clock when it is running, and should always be zeros on output.
Bits 4 and 7
Not used.
Bits 5 and 6
Used to indicate, when set to 1, that the clock has suffered at least one out-of-phase start (bit 5) or stop (bit 6).

All times produced in the offline reports are in GMT (Greenwich Mean Time), not local time. Times produced by online reporting can be expressed either in GMT, or in local time, by means of the local date and time offset values from the SMF product section of CICS monitoring SMF type 110 records. The CICS-supplied sample program DFH$MOLS shows an example of this.