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432
System
o
Procesa
O.
Process
running
on
GDP
needs
I/O
service
1.
Process
formulates
message
describing
service,
sends
it
to
device
task
2.
Device
task
receives
service
order,
interprets
it
Service
Order
Messaqe
Service
Reply
Message
KEY
CONCEP'IS
Peripheral
Subayatem
-------------------
Device
Task
(:;'\
-V
3.
Device
task
transfers
data
according
to
service
order
parameters
4.
Device
task
formulates
reply
message
containing
result
of
transfer
operation,
sends
it
back
to
originating
process.
5.
Originating
process
receives
reply,
interprets
it,
executes
accordingly
Figure
1-2
Basic
I/O
Service
Cycle
1-5