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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML EXPERIMENTAL 970324//EN"><HTML><HEAD><META NAME="GENERATOR" CONTENT="Adobe FrameMaker 5.5/HTML Export Filter"><LINK REL="STYLESHEET" HREF="ch10.css"><TITLE> 10.1 Distinctions between tasks and functions</TITLE></HEAD><BODY BGCOLOR="#ffffff"><DIV><HR><P><A HREF="ch10.htm">Chapter start</A> <A HREF="ch10.htm">Previous page</A> <A HREF="ch10.2.htm">Next page</A></P></DIV><H1 CLASS="Section"><A NAME="pgfId=145"> </A>10.1 Distinctions between tasks and functions</H1><P CLASS="Body"><A NAME="pgfId=146"> </A>The following rules distinguish tasks from functions:</P><UL><LI CLASS="DashedList"><A NAME="pgfId=147"> </A>A function shall execute in one simulation time unit; a task can contain time-controlling statements.</LI><LI CLASS="DashedList"><A NAME="pgfId=148"> </A>A function cannot enable a task; a task can enable other tasks and functions.</LI><LI CLASS="DashedList"><A NAME="pgfId=149"> </A>A function shall have at least one <B CLASS="Keyword">input</B> type argument and shall not have an <B CLASS="Keyword">output</B> or <B CLASS="Keyword">inout</B> type argument; a task can have zero or more arguments of any type.</LI><LI CLASS="DashedList"><A NAME="pgfId=150"> </A>A function shall return a single value; a task shall not return a value. </LI></UL><P CLASS="Body"><A NAME="pgfId=151"> </A>The purpose of a function is to respond to an input value by returning a single value. A task can support multiple goals and can calculate multiple result values. However, only the <B CLASS="Keyword">output</B> or <B CLASS="Keyword">inout</B> type arguments pass result values back from the invocation of a task. A function is used as an operand in an expression; the value of that operand is the value returned by the function. </P><P CLASS="Body"><A NAME="pgfId=144"> </A>Example:</P><P CLASS="Body"><A NAME="pgfId=121"> </A>Either a task or a function can be defined to switch bytes in a 16-bit word. The task would return the switched word in an output argument, so the source code to enable a task called <CODE CLASS="code">switch_bytes</CODE> could look like the following example:</P><PRE CLASS="CodeIndent"><A NAME="pgfId=152"> </A>switch_bytes (old_word, new_word);</PRE><P CLASS="Body"><A NAME="pgfId=153"> </A>The task <CODE CLASS="code">switch_bytes</CODE> would take the bytes in <CODE CLASS="code">old_word</CODE>, reverse their order, and place the reversed bytes in <CODE CLASS="code">new_word</CODE>. </P><P CLASS="Body"><A NAME="pgfId=122"> </A>A word-switching function would return the switched word as the return value of the function. Thus, the function call for the function <CODE CLASS="code">switch_bytes</CODE> could look like the following example:</P><PRE CLASS="CodeIndent"><A NAME="pgfId=155"> </A>new_word = switch_bytes (old_word);</PRE><HR><P><A HREF="ch10.htm">Chapter start</A> <A HREF="ch10.htm">Previous page</A> <A HREF="ch10.2.htm">Next page</A></P></BODY></HTML>?? 快捷鍵說明
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