VDM-10 Language Manual

Chapter 14. Top-level Specification (VDM++ and VDM-RT)
The system “class” is limited in the way that it can only contain:
Instance variables: The only instances that can be declared in the system “class” is of the
special classes CPU and BUS as well as static instances of the different system components
that one wish to allocate to different CPU’s.
Constructor: The actual deployment of instances to CPU’s and setting of priorities for the
different operations is set inside the constructor which is the only operation that can be
placed in the system “class”. The only kind of statements that can be used inside this
constructor is a block statement with a sequence of invocations of the special deploy
and setPriority operations.
In addition there are limitations with respect to the use of static declarations for instances
that are deployed to different CPU’s. Basically the user should ensure that only one instance
is deployed to a CPU if the class the instance comes from contains any static operations
or functions. In case a static instance variable is used it is accessed directly (without any
communication over the busses, so this in essence not proper from a distribution standpoint.
Thus, all instance variables of instances to be deployed should only be accessed through the
use of operations.
Example: The system class could for example be defined as:
✞
system Simple
instance variables
static public a : A := new A();
static public b : B := new B();
-- define the first CPU with fixed priority scheduling
-- and 22E6 MIPS
CPU1 : CPU := new CPU (, 22E6);
static public c : C := new C();
-- define the second CPU with fixed priority scheduling
-- and 11E6 MIPS
CPU2 : CPU := new CPU (, 11E6);
-- create a communication bus that links the three
-- CPU’s together
BUS1 : BUS := new BUS (, 72E3, {CPU1, CPU2})
operations
public Simple: () ==> Simple
Simple () ==
( -- deploy a on CPU1
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