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Retidy, removing align=left from headings and empty paragraphs

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James Gregory 2013-12-30 15:37:13 +11:00
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<p><br /></p>
<p>In fact, the Zen timer shown in Listing 3.1 can only time intervals of up to about 54 ms in length, since that is the period of time that can be measured by timer 0 before its count turns over and repeats. fifty-four ms may not seem like a very long time, but even a CPU as slow as the 8088 can perform more than 1,000 divides in 54 ms, and division is the single instruction that the 8088 performs most slowly. If a measured period turns out to be longer than 54 ms (that is, if timer 0 has counted down and turned over), the Zen timer will display a message to that effect. A long-period Zen timer for use in such cases will be presented later in this chapter.</p>
<p>The Zen timer determines whether timer 0 has turned over by checking to see whether an IRQ0 interrupt is pending. (Remember, interrupts are off while the Zen timer runs, so the timer interrupt cannot be recognized until the Zen timer stops and enables interrupts.) If an IRQ0 interrupt is pending, then timer 0 has turned over and generated a timer interrupt. Recall that <b>ZTimerOn</b> initially sets timer 0 to 0, in order to allow for the longest possible period&mdash;about 54 ms&mdash;before timer 0 reaches 0 and generates the timer interrupt.</p>
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<p>You may well want to devise still other approaches better suited to your needs than those I&rsquo;ve presented. Go to it! I&rsquo;ve just thrown out a few possibilities to get you started.</p>
<p><br /></p>
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