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James Gregory 2013-12-30 15:11:04 +11:00
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<meta name="vspubdate" content="07/01/97" />
<meta name="vscategory" content="Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development" />
<title>Michael Abrash's Graphics Programming Black Book Special Edition: Assume Nothing</title><!-- HEADER -->
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<title>Michael Abrash's Graphics Programming Black Book Special Edition: Assume Nothing</title>
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<meta name="pages" content="045-048" />
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<p>Nonetheless, it&rsquo;s a good idea to reboot your computer at the end of each session with the Zen timer in order to make sure that the system clock is correct.</p>
<h3><a id="Heading8"></a>Stopping the Zen Timer</h3>
<h3 id="Heading8">Stopping the Zen Timer</h3>
<p>At some point after <b>ZTimerOn</b> is called, <b>ZTimerOff</b> must always be called to mark the end of the timing interval. <b>ZTimerOff</b> saves the context of the calling program, latches and reads the timer 0 count, converts that count from the countdown value that the timer maintains to the number of counts elapsed since <b>ZTimerOn</b> was called, and stores the result. Immediately after latching the timer 0 count&mdash;and before enabling interrupts&mdash;<b>ZTimerOff</b> checks the 8259 interrupt controller to see if there is a pending timer interrupt, setting a flag to mark that the timer overflowed if there is indeed a pending timer interrupt.</p>
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<p>One interesting aspect of <b>ZTimerOff</b> is the manner in which timer 0 is stopped in order to read the timer count. We don&rsquo;t actually have to stop timer 0 to read the count; the 8253 provides a special latched read feature for the specific purpose of reading the count while a time is running. (That&rsquo;s a good thing, too; we&rsquo;ve no documented way to stop timer 0 if we wanted to, since its gate input isn&rsquo;t connected. Later in this chapter, though, we&rsquo;ll see that timer 0 can be stopped after all.) We simply tell the 8253 to latch the current count, and the 8253 does so without breaking stride.</p>
<h3><a id="Heading9"></a>Reporting Timing Results</h3>
<h3 id="Heading9">Reporting Timing Results</h3>
<p><b>ZTimerReport</b> may be called to display timing results at any time after both <b>ZTimerOn</b> and <b>ZTimerOff</b> have been called. <b>ZTimerReport</b> first checks to see whether the timer overflowed (counted down to 0 and turned over) before <b>ZTimerOff</b> was called; if overflow did occur, <b>ZTimerOff</b> prints a message to that effect and returns. Otherwise, <b>ZTimerReport</b> subtracts the reference count (representing the overhead of the Zen timer) from the count measured between the calls to <b>ZTimerOn</b> and <b>ZTimerOff</b>, converts the result from timer counts to microseconds, and prints the resulting time in microseconds to the standard output.</p>
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<div align="center">
Graphics Programming Black Book &copy; 2001 Michael Abrash
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