VDM-10 Language Manual

Chapter 13. Statements
✞
op1 : nat ==> nat
op1(m) ==
duration (20) return m + 1;
op2 : () ==> nat
op2() ==
( dcl n : nat := 3;
duration(10) n := op1(1);
return n)
✡✝
✆
When executing op2, if the call to op1 is executed, the duration statement in op1 will be
overridden by the duration statement in the environment of the call. Thus in op2 following
execution of the statement n := op1(1); the internal clock is incremented by 10 time
units only.
13.17 The Cycles Statement (VDM-RT)
Syntax: statement = . . .
| cycles statement ;
cycles statement = ‘cycles’, ‘(’, numeral, ‘)’,
statement ;
Semantics: The cycles statement is a runtime directive to the Toolbox interpreter telling it that
when incrementing the internal clock for the enclosed statement, the value (a natural number)
given in the cycles statement should be used as an indication of how many clock cycles
that the enclosed statement should be incremented by instead of the increment which would
normally be computed for that statement. Thus the cycles statement provides a mechanism to
override the Toolbox’s default execution time computation similar to the duration statement
but in a way that is relative to the speed of the CPU that the computation is carried out on.
Example: First a simple example:
✞
✡✝
while n < 10 do
cycles(1000) n := n + 1;
✆
In this example, assuming that this loop is not executed in the context of an enclosing cycles
statement, on each iteration of the loop the Toolbox will increment its internal clock by the
time it will take to process 1000 instructions on the given CPU (relative to its capacity), rather
than computing the amount of time required to execute the statement n := n + 1.
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