Managing and synchronizing clocks

The control program clock software uses three hardware clock components:
  • The time-of-day (TOD) clock
  • The TOD clock comparator
  • The CPU timer.

Timekeeping in this system is performed using the TOD clock to drive a set of software clocks. Whenever the system is above 1052 state, TOD clock comparator interrupts increment a control program seconds clock, a systems clock, and a set of subsystem clocks. Software clocks are not incremented when the system is not above 1052 state. The CPU timer is used to verify that ECB-controlled programs release control of the CPU, in a half-second.

Note: Base systems (those that do not include HPO/MDBF) are treated as MDBF systems with a basic subsystem only. For systems that include MDBF, all subsystem clock values, including the basic subsystem, are derived from the system clock.
The system clocks maintained in this system are:
  • System seconds clock
  • System perpetual minutes clock
  • System local standard time (LST) clock
  • System last midnight value clock.
The subsystem clocks are:
  • Subsystem perpetual minutes clock
  • Subsystem perpetual minutes clock without daylight saving time (DST)
  • Subsystem local standard time clock
  • Subsystem last midnight value clock
  • Subsystem Greenwich mean clock
  • Subsystem Greenwich mean time midnight value clock.