A State-Based Programming Model for Wireless Sensor Networks

7.3. Memory-Efficient Programs 133
Program 7.4: Reimplementation of SURGE in OSM. A graphical representation
of the OSM state machine is depicted to the left.
SURGE
IDLE
BUSY
W4_DATA
W4_SEND
W4_ACK
timeout
dataReady
sendData
sendDone
1 state SURGE {
7.3 Memory-Efficient Programs
2 initial state IDLE {
3 timeout/ ADC_getData() -> BUSY;
4 }
5 state BUSY {
6 uint16_t sample;
7
8 initial state W4_DATA {
9 dataReady/
10 sample=dataReady.data,
11 post( sendData ) -> W4_SEND;
12 }
13 state W4_SEND {
14 sendData/ send(sample)->W4_ACK;
15 }
16 state W4_ACK {
17 // do nothing
18 }
19 sendDone/ -> IDLE;
20 } // end BUSY
21 } // end SURGE
In the previous section we have shown that partitioning sensor-node programs
in the time domain through explicitly modeled states notation benefits modularity
and structure of the program code. In this section we show that OSM’s
concept of state variables—variables the lifetime of which are associated with
states—can also benefit memory efficiency.
The basic idea of saving memory is to allocate memory only for those variables,
that are required in the currently active state of the program. When the
program state changes, the memory used by variables of the old program state
is automatically reused for the variables of the new state. In our state-based approach,
states not only serve as a structuring mechanism for the program, they
also serve as a scoping and lifetime qualifier for variables. Regular event-driven
programming models lack this abstraction and thus force programmers to use
global variables with global lifetime, as described in Sect. 4.1.
Using an example based on the program to calculate the average of remote
temperature sensors from Sect. 4.1, we now show how the state-based programming
model can be used in practice to save memory by storing temporary data
in state variables. Then we answer the question how this technique is generally
applicable to sensor-node programming and how much memory can be saved
in real-world programs.