VDM-10 Language Manual

Chapter 6. Expressions
• #active(operation name). The number of operation name operations that are currently
active. Thus: #active(operation name) = #act(operation name) - #fin(operation
name).
• #req(operation name). The number of requests that has been issued for the operation
name operation.
• #waiting(operation name). The number of outstanding requests for the operation
name operation. Thus: #waiting(operation name) = #req(operation name) -
#act(operation name).
For all of these operators, the name list version #history op(op1,. . .,opN) is simply shorthand
for #history op(op1) + · · · +#history op(opN).
Examples: Suppose at a point in the execution of a particular thread, three operations, A, B and
C may be executed. A sequence of requests, activations and completions occur during this
thread. This is shown graphically in figure 6.1.
Figure 6.1: History Expressions
Here we use the notation rA to indicate a request for an execution of operation A, aA indicates
an activation of A, fA indicates completion of an execution of operation A, and likewise for
operations B and C. The respective history expressions have the following values for the
interval [S,T]:
#act(A) = 1 #act(B) = 1 #act(C) = 1 #act(A,B,C) = 3
#fin(A) = 1 #fin(B) = 1 #fin(C) = 0 #fin(A,B,C) = 2
#active(A) = 0 #active(B) = 0 #active(C) = 1 #active(A,B,C) = 1
#req(A) = 2 #req(B) = 1 #req(C) = 3 #req(A,B,C) = 6
#waiting(A) = 1 #waiting(B) = 0 #waiting(C) = 2 #waiting(A,B,C) = 3
6.23 The Time Expression (VDM-RT)
Syntax: time expression = ‘time’ ;
Semantics: This is simply an easy way to refer to the current time on a given CPU. The time is
provided as a natural number.
Examples: If for example one would like to log when a certain operation takes place one can
create an operation such as logEnvToSys below.
67