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1

This clause specifies a high-resolution, monotonic
clock package.

2

The following language-defined
library package exists:

3

4

Time_First :

Time_Last :

Time_Unit :

5

Time_Span_First :

Time_Span_Last :

Time_Span_Zero :

Time_Span_Unit :

6

7

8

9

10

11/1

12

13

14

15

16

17

... --

18

In this Annex, *real time*
is defined to be the physical time as observed in the external environment.
The type Time is a *time type* as defined by 9.6;
values of this type may be used in a delay_until_statement.
Values of this type represent segments of an ideal time line. The set
of values of the type Time corresponds one-to-one with an implementation-defined
range of mathematical integers.

19

The Time
value I represents the half-open real time interval that starts with
E+I*Time_Unit and is limited by E+(I+1)*Time_Unit, where Time_Unit is
an implementation-defined real number and E is an unspecified origin
point, the *epoch*, that is the same for all values of the type
Time. It is not specified by the language whether the time values are
synchronized with any standard time reference. For example, E can correspond
to the time of system initialization or it can correspond to the epoch
of some time standard.

20

Values of the type Time_Span represent length
of real time duration. The set of values of this type corresponds one-to-one
with an implementation-defined range of mathematical integers. The Time_Span
value corresponding to the integer I represents the real-time duration
I*Time_Unit.

21

Time_First and Time_Last are the smallest and
largest values of the Time type, respectively. Similarly, Time_Span_First
and Time_Span_Last are the smallest and largest values of the Time_Span
type, respectively.

22

A value of type Seconds_Count represents an elapsed
time, measured in seconds, since the epoch.

23

Time_Unit is the smallest amount of real time
representable by the Time type; it is expressed in seconds. Time_Span_Unit
is the difference between two successive values of the Time type. It
is also the smallest positive value of type Time_Span. Time_Unit and
Time_Span_Unit represent the same real time duration. A
*clock tick* is a real time interval during which the clock value
(as observed by calling the Clock function) remains constant. Tick is
the average length of such intervals.

24

The function To_Duration converts the value TS
to a value of type Duration. Similarly, the function To_Time_Span converts
the value D to a value of type Time_Span. For both operations, the result
is rounded to the nearest exactly representable value (away from zero
if exactly halfway between two exactly representable values).

25

To_Duration(Time_Span_Zero) returns 0.0, and
To_Time_Span(0.0) returns Time_Span_Zero.

26

The functions Nanoseconds, Microseconds, and
Milliseconds convert the input parameter to a value of the type Time_Span.
NS, US, and MS are interpreted as a number of nanoseconds, microseconds,
and milliseconds respectively. The result is rounded to the nearest exactly
representable value (away from zero if exactly halfway between two exactly
representable values).

27

The effects of the operators on Time and Time_Span
are as for the operators defined for integer types.

28

The function Clock returns the amount of time
since the epoch.

29

The effects of the Split and Time_Of operations
are defined as follows, treating values of type Time, Time_Span, and
Seconds_Count as mathematical integers. The effect of Split(T,SC,TS)
is to set SC and TS to values such that T*Time_Unit = SC*1.0 + TS*Time_Unit,
and 0.0 <= TS*Time_Unit < 1.0. The value returned by Time_Of(SC,TS)
is the value T such that T*Time_Unit = SC*1.0 + TS*Time_Unit.

30

The range of Time values shall be sufficient
to uniquely represent the range of real times from program start-up to
50 years later. Tick shall be no greater than 1 millisecond. Time_Unit
shall be less than or equal to 20 microseconds.

31

Time_Span_First shall be no greater than -3600
seconds, and Time_Span_Last shall be no less than 3600 seconds.

32

A *clock jump* is the
difference between two successive distinct values of the clock (as observed
by calling the Clock function). There shall be no backward clock jumps.

33

The implementation shall document the values
of Time_First, Time_Last, Time_Span_First, Time_Span_Last, Time_Span_Unit,
and Tick.

34

The implementation shall document the properties
of the underlying time base used for the clock and for type Time, such
as the range of values supported and any relevant aspects of the underlying
hardware or operating system facilities used.

35

The implementation shall document whether or
not there is any synchronization with external time references, and if
such synchronization exists, the sources of synchronization information,
the frequency of synchronization, and the synchronization method applied.

36/1

The implementation shall document any aspects
of the external environment that could interfere with the clock behavior
as defined in this clause.

37

For the purpose of the metrics defined in this
clause, real time is defined to be the International Atomic Time (TAI).

38

The implementation
shall document the following metrics:

39

- An upper bound on the real-time duration
of a clock tick. This is a value D such that if t1 and t2 are any real
times such that t1 < t2 and Clock
_{t1}= Clock_{t2}then t2 - t1 <= D.

40

- An upper bound on the size of a clock jump.

41

- An upper bound
on the
*drift rate*of Clock with respect to real time. This is a real number D such that

42

E*(1-D) <= (Clock_{t+E} - Clock_{t}) <= E*(1+D)

provided that: Clock_{t} + E*(1+D) <= Time_Last.

provided that: Clock

43

- where Clock
_{t}is the value of Clock at time t, and E is a real time duration not less than 24 hours. The value of E used for this metric shall be reported.

44

- An upper bound on the execution time of a call to the Clock function, in processor clock cycles.

45

- Upper bounds on the execution times of the operators of the types Time and Time_Span, in processor clock cycles.

46

Implementations targeted to machines with word
size smaller than 32 bits need not support the full range and granularity
of the Time and Time_Span types.

47

When appropriate, implementations should provide
configuration mechanisms to change the value of Tick.

48

It is recommended that Calendar.Clock and Real_Time.Clock
be implemented as transformations of the same time base.

49

It is recommended that the ``best'' time base
which exists in the underlying system be available to the application
through Clock. ``Best'' may mean highest accuracy or largest range.

NOTES

50

30 The rules in this clause
do not imply that the implementation can protect the user from operator
or installation errors which could result in the clock being set incorrectly.

51

31 Time_Unit is the granularity
of the Time type. In contrast, Tick represents the granularity of Real_Time.Clock.
There is no requirement that these be the same.