VDM-10 Language Manual

VDM-10 Language Manual
Semantics: Each system description has the following parts:
• A system header with the system name.
• An optional system body.
• A system tail.
The system name as given in the system header is the defining occurrence of the name of
the class. A system name is globally visible, i.e. visible in all other classes/systems in the
specification.
The system name in the class header must be the same as the system name in the system tail.
Furthermore, defining system names must be unique throughout the specification.
The special thing about the system is that it can make use of special implicitly defined classes
called CPU and BUS. It is not possible to create instances of the system, but instances made
of CPU and BUS will be created at initialisation time. Note that CPU and BUS cannot be used
outside the system definition.
The instances of CPU and BUS must be made as instance variables and the definition must use
constructors. The constructor for the CPU class takes two parameters: the first one indicate
the primary scheduling policy used for the CPU whereas the second parameter provides the
capacity of the CPU (indicated as Million Instructions Per Second or MIPS). The constructor
for the BUS class takes three parameters. The first one indicates the kind of bus, the second
one the capacity of the bus (its band width) and finally the third parameter gives a set of CPU
instances connected together by the given BUS instance.
The currently supported primary scheduling policies for the CPU are:
: Fixed Priority
: First Come First Served
The currently supported primary scheduling policy for the BUS is:
: First Come First Served
The CPU class have member operations called deploy and setPriority. The deploy
operation takes one significant parameter which must be an object that is declared as a static
instance variable inside the system 1 . The semantics of the deploy operation is that execution
of all functionality inside this object will take place on the CPU that it has been deployed
to. The setPriority operation takes two parameters where the first must be the name
of a public operation that has been deployed to the CPU and the second parameter is a
natural number. The semantics of the setPriority operation is that the given operation
is assigned the given priority (the second parameter). This will be used when fixed priority
scheduling is used on the given CPU. Per default operations that are not explicitly assigned
a priority using the setPriority operation are assigned a default priority of 1.
1 It is also allowed to take a string as a second parameter for future extensions but that is ignored at the moment.
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