Graphics Programs Reference
In-Depth Information
/
2
/
2
p
in
t
1
p
1
t
2
p
2
T
1−2
p
3
T
3
p
out
p
5
T
4
p
4
p
control
p
6
Figure 7.8: Age memory conflict
before an interruption.
The Erlang-3 representation of Fig.
7.7
corresponds to the specification of
an age memory policy (see Chapter
3)
since the tokens representing the
stages that are still to be executed before completion are unaffected by the
interruption. When the subnet is enabled again in the future the firing
delay will account only for the stages of service not executed yet. This can
be easily seen by examining Fig.
7.8
where the Erlang-3 subnet is in conflict
with the exponential transitions T
4
. Assuming that place p
in
is marked,
the firing of transition T
4
disables the Erlang-3 subnet that keeps memory
of its internal state and that will restart from it when a new token is later
deposited in place p
in
. Indeed, the firing of T
4
removes the token from place
p
in
disabling the transition of the Erlang subnet that is currently enabled.
The internal state of the subnet is represented by the marking of places p
1
,
p
2
, and p
3
which make sure that the work of the transition starts from where
it was interrupted at the exact moment of the arrival of a new token in place
p
in
.
If we want instead to represent an enabling memory policy, the subnet must
be reset by any interruption. This means that the subnet must lose track of
the stages of service completed before the interruption. To achieve this, we
must modify the internal structure of the subnet as depicted in Fig.
7.9.
In this case, removing a token from p
in
(because of the firing of T
4
) when
p
control
is not marked, causes those places that are non-empty among p
1
,
p
2
, and p
3
to be cleared from tokens, thus returning the subnet to its initial
“idle” state. Places p
1
, p
2
, and p
3
may contain several tokens because of the
multiplicity of the output arc of transition t
1
. The clearing of these places
is accomplished in two steps and with the help of place p
5
that signals the
condition “the firing of the subnet is in progress” (place p
5
is marked upon
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