Graphics Programs Reference
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p
local ready
p
ext req
t
start
T
ext
p
not local exec
p
not ext req
p
exec
T
end
t
req start
p
ext acc
p
req exe
p
ext
T
ext acc
T
req end
Figure 3.12: Timed PN system with age memory policy
This behaviour can be modelled by means of a transition T
watch−dog
enabled
when a request from processor 2 is pending and processor 1 is using the
resource. The PN system in Fig.
3.11
describes this behaviour by means of
the transition T
watch−dog
which is timed with enabling memory policy. This
transition models the time-out associated with the preemption privilege of
processor 2 and the timer associated with the transition is set to τ
watch−dog
whenever it becomes enabled. In fact, if the activity of processor 1 using the
common resource ends while the timer associated with T
watch−dog
has not
yet reached zero, transition T
end 1
fires moving a token to p
idle
(the common
resource is naturally released). This new marking enables the immediate
transition t
start 2
and disables T
watch−dog
. When T
watch−dog
will be enabled
again, its associated timer will again be set to τ
watch−dog
. It must be ob-
served that the memory policies of the other timed transitions of the same
PN system need not be the same.
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