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<a name="1204414"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">9 Real-time clocks</font>
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<a name="1204419"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Time is very important for real-time systems. OS20 provides some basic functions for manipulating quantities of time:</font>
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<a name="1204420"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The ST20 traditionally regards time as circular. That is, the counters which represent time can wrap round, with half the time period being in the future, and half of it in the past. This behavior means that clock values should only be manipulated using time functions. OS20 provides functions to:</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204424"> </a>add and subtract quantities of time,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204425"> </a>determine if one time is after another,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204426"> </a>return the current time.</font></p>
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