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17.12.3 时钟流</div>
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Throughout this subsection, c, d denote clocking event expressions and v, w, x, y, z denote sequences with no clocking events. </p>
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Clock flow allows the scope of a clocking event to extend in a natural way through various parts of multiplyclocked sequences and properties and reduces the number of places at which the same clocking event must be specified. </p>
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Intuitively, clock flow provides that in a multiply-clocked sequence or property the scope of a clocking event flows left-to-right across linear operators (e.g., repetition, concatenation, negation, implication) and distributes to the operands of branching operators (e.g., conjunction, disjunction, intersection, if...else) until it is replaced by a new clocking event. </p>
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For example, </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00718"><pre class="Element12">@(c) x |=> @(c) y ##1 @(d) z</pre></div></div>
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can be written more simply as </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00719"><pre class="Element12">@(c) x |=> y ##1 @(d) z</pre></div></div>
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because clock c is understood to flow across |=>. </p>
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Clock flow eliminates the need to write clocking events in positions where the clock is not allowed to change. For example, </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00720"><pre class="Element12">@(c) x |-> @(c) y ##1 @(d) z</pre></div></div>
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can be written as </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00721"><pre class="Element12">@(c) x |-> y ##1 @(d) z</pre></div></div>
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to reinforce the restriction that the clock not change across |->. Similarly, </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00722"><pre class="Element12">@(c) if (b) @(c) w ##1 @(d) x else @(c) y ##1 @(d) z</pre></div></div>
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can be written as </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00723"><pre class="Element12">@(c) if (b) w ##1 @(d) x else y ##1 @(d) z</pre></div></div>
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to reinforce the restriction that the clock not change from the boolean condition b to the beginnings of the if and else clause properties. </p>
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Clock flow also makes the adjointness relationships between concatenation and implication clean for multiplyclocked properties: </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00724"><pre class="Element12">@(c) x ##1 y |=> @(d) z</pre></div></div>
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is equivalent to </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00725"><pre class="Element12">@(c) x |=> y |=> @(d) z</pre></div></div>
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and </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00726"><pre class="Element12">@(c) x ##0 y |=> @(d) z</pre></div></div>
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is equivalent to </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00727"><pre class="Element12">@(c) x |-> y |=> @(d) z</pre></div></div>
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The scope of a clocking event flows into parenthesized subexpressions and, if the subexpression is a sequence, also flows left-to-right across the parenthesized subexpression. However, the scope of a clocking event does not flow out of enclosing parentheses. </p>
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For example, in </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00728"><pre class="Element12">@(c) w ##1 (x ##1 @(d) y) |=> z</pre></div></div>
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w, x, z are clocked at c and y is clocked at d. Clock c flows across ##1, across the parenthesized subsequence (x ##1 @(d) y), and across |=>. Clock c also flows into the parenthesized subsequence, but it does not flow through @(d). Clock d does not flow out of its enclosing parentheses. </p>
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</p>
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As another example, in </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00729"><pre class="Element12">@(c) v |=> (w ##1 @(d) x) and (y ##1 z)</pre></div></div>
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v, w, y, z are clocked at c and x is clocked at d. Clock c flows across |=>, distributes to both operands of the and (which is a property conjunction due to the multiple clocking), and flows into each of the parenthesized subexpressions. Within (w ##1 @(d) x), c flows across ##1 but does not flow through @(d). Clock d does not flow out of its enclosing parentheses. Within (y ##1 z), c flows across ##1. </p>
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Similarly, the scope of a clocking event flows into an instance of a named sequence or property, and, if the instance is a sequence, also flows left-to-right across the instance. However, a clocking event in the declaration of a sequence or property does not flow out of an instance of that sequence or property. </p>
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Note that juxtaposing two clocking events nullifies the first of them: </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00730"><pre class="Element12">@(d) @(c) x</pre></div></div>
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is equivalent to </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00731"><pre class="Element12">@(c) x</pre></div></div>
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because the flow of clock d is immediately overridden by clock c.</p></div>
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