[sv-cc] VPI time related callbacks: specified time should be relative or absolute?

Hello,

I have question about treating time during registration of time related
callbacks (Verilog LRM 2005, p. 27.33.2): should it be absolute or
relative?

What is the distinction between cbAtStartOfSimTime and cbAfterDelay in
the meaning of time? If cbAtStartOfSimTime has absolute time does it
mean that registering this callback for past time rises an error?

What about other callbacks in this section?
IMHO all callbacks except cbAfterDelay should interpret time as
absolute.

The worts is that every simulator I could check (asking our customers or
looking on discussion lists) behaves in different way.

Example:
At simulation time 10 I register two callbacks:

void f() {
  PLI_INT32 cb( p_cb_data ) { return 0; }

  s_vpi_time time = { vpiSimTime, 0, 11 };

  vpi_register_cb( cbAtStartOfSimulationTime, cb, 0 , &time };
  vpi_register_cb( cbAfterDelay,              cb, 0 , &time };
}

I know that cbAfterDelay should execute @21 but what about
cbAtStartOfSimulationTime? Should it be @11 or @21, too? If the latter
is true then what is the distinction between them?
Below there are aligned definitions for these callbacks from Verilog LRM
2005:

cbAtStartOfSimulationTime: Callback shall occur
before execution of events in a specified time queue. A callback can be
set for any time, even if no event is present.
cbAfterDelay             : Callback shall occur after a specified amount
of time, before execution of events in a specified time queue. A
callback can be set for any time, even if no event is present.

This also applies to cbNBASynch, cbReadWriteSynch, cbAtEndOfSimTime,
cbReadOnlySynch?

From this LRM exceprt

cb_data_p->[time->low,time->high,time->real]

    These fields shall contain the requested time of the callback or the
delay before the callback.

it seems to me that there is a distinction for relative and absolute
time but it is not clear which time interpretation should be applied to
a particular callback.

Best Regards,

Krzysztof Konopko
Leader Application Engineer
Aldec


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