# D.14 Execution Time

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This clause describes a language-defined package to measure execution time.

#### Static Semantics

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The following language-defined library package exists:
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type CPU_Time is private;
CPU_Time_First : constant CPU_Time;
CPU_Time_Last  : constant CPU_Time;
CPU_Time_Unit  : constant := implementation-defined-real-number;
CPU_Tick : constant Time_Span;
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function Clock
return CPU_Time;
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function "+"  (Left : CPU_Time; Right : Time_Span) return CPU_Time;
function "+"  (Left : Time_Span; Right : CPU_Time) return CPU_Time;
function "-"  (Left : CPU_Time; Right : Time_Span) return CPU_Time;
function "-"  (Left : CPU_Time; Right : CPU_Time)  return Time_Span;
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function "<"  (Left, Right : CPU_Time) return Boolean;
function "<=" (Left, Right : CPU_Time) return Boolean;
function ">"  (Left, Right : CPU_Time) return Boolean;
function ">=" (Left, Right : CPU_Time) return Boolean;
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procedure Split
(T : in CPU_Time; SC : out Seconds_Count; TS : out Time_Span);
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function Time_Of (SC : Seconds_Count;
TS : Time_Span := Time_Span_Zero) return CPU_Time;
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private
... -- not specified by the language
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The execution time or CPU time of a given task is defined as the time spent by the system executing that task, including the time spent executing run-time or system services on its behalf. The mechanism used to measure execution time is implementation defined. It is implementation defined which task, if any, is charged the execution time that is consumed by interrupt handlers and run-time services on behalf of the system.
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The type CPU_Time represents the execution time of a task. The set of values of this type corresponds one-to-one with an implementation-defined range of mathematical integers.
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The CPU_Time value I represents the half-open execution-time interval that starts with I*CPU_Time_Unit and is limited by (I+1)*CPU_Time_Unit, where CPU_Time_Unit is an implementation-defined real number. For each task, the execution time value is set to zero at the creation of the task.
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CPU_Time_First and CPU_Time_Last are the smallest and largest values of the CPU_Time type, respectively.

#### Dynamic Semantics

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CPU_Time_Unit is the smallest amount of execution time representable by the CPU_Time type; it is expressed in seconds. A CPU clock tick is an execution time interval during which the clock value (as observed by calling the Clock function) remains constant. CPU_Tick is the average length of such intervals.
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The effects of the operators on CPU_Time and Time_Span are as for the operators defined for integer types.
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The function Clock returns the current execution time of the task identified by T; Tasking_Error is raised if that task has terminated; Program_Error is raised if the value of T is Task_Identification.Null_Task_Id.
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The effects of the Split and Time_Of operations are defined as follows, treating values of type CPU_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*CPU_Time_Unit = SC*1.0 + TS*CPU_Time_Unit, and 0.0 <= TS*CPU_Time_Unit < 1.0. The value returned by Time_Of(SC,TS) is the execution-time value T such that T*CPU_Time_Unit=SC*1.0 + TS*CPU_Time_Unit.

#### Erroneous Execution

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For a call of Clock, if the task identified by T no longer exists, the execution of the program is erroneous.

#### Implementation Requirements

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The range of CPU_Time values shall be sufficient to uniquely represent the range of execution times from the task start-up to 50 years of execution time later. CPU_Tick shall be no greater than 1 millisecond.

#### Documentation Requirements

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The implementation shall document the values of CPU_Time_First, CPU_Time_Last, CPU_Time_Unit, and CPU_Tick.
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The implementation shall document the properties of the underlying mechanism used to measure execution times, such as the range of values supported and any relevant aspects of the underlying hardware or operating system facilities used.

#### Metrics

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The implementation shall document the following metrics:
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• An upper bound on the execution-time duration of a clock tick. This is a value D such that if t1 and t2 are any execution times of a given task such that t1 < t2 and Clockt1 = Clockt2 then t2 – t1 <= D.
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• An upper bound on the size of a clock jump. A clock jump is the difference between two successive distinct values of an execution-time clock (as observed by calling the Clock function with the same Task_Id).
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• An upper bound on the execution time of a call to the Clock function, in processor clock cycles.
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• Upper bounds on the execution times of the operators of the type CPU_Time, in processor clock cycles.

#### Implementation Permissions

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