📄 ch02.6.htm
字号:
<HTML><HEAD> <META NAME="GENERATOR" CONTENT="Adobe PageMill 2.0 Mac"> <LINK REL="STYLESHEET" HREF="ch02.css"> <TITLE> 2.6 Strings </TITLE></HEAD><BODY BGCOLOR="#ffffff"><P><HR ALIGN=LEFT></P><P><A HREF="ch02.htm">Chapter start</A> <A HREF="ch02.5.htm">Previous page</A> <A HREF="ch02.7.htm">Next page</A></P><H1><A NAME="pgfId=238"></A>2.6 Strings</H1><P><P CLASS="Body"><A NAME="pgfId=239"></A>A <B>string</B> is a sequenceof characters enclosed by double quotes (<CODE> ""</CODE> ) andcontained on a single line. Strings used as operands in expressions andassignments shall be treated as unsigned integer constants represented bya sequence of eight-bit ASCII values, with one eight bit ASCII value representingone character.</P><P><P CLASS="SubSection"><A NAME="pgfId=243"></A>String variable declaration</P><P><P CLASS="Body"><A NAME="pgfId=184"></A>String variables are variablesof register type (see section 3.2) with width equal to the number ofcharacters in the string multiplied by 8.</P><P><P CLASS="Body"><A NAME="pgfId=231"></A>Example:</P><P><P CLASS="Body"><A NAME="pgfId=235"></A>To store the twelve charactersstring <CODE>"Hello world!"</CODE> requires a register 8*12, or96 bits wide</P><P><P CLASS="CodeText"><A NAME="pgfId=5329"></A><B>reg </B>[8*12:1] stringvar;<BR><B>initial</B> <B>begin<BR></B> stringvar = "Hello world!";<BR><B>end</B></P><P><P CLASS="SubSection"><A NAME="pgfId=240"></A>String manipulation</P><P><P CLASS="Body"><A NAME="pgfId=251"></A>Strings can be manipulated usingthe Verilog HDL operators. The value being manipulated by the operator isthe sequence of 8-bit ASCII values.</P><P><P CLASS="Body"><A NAME="pgfId=168"></A>Example:</P><PRE><A NAME="pgfId=5336"></A> <B>module</B> string_test;<B>reg</B> [8*14:1] stringvar;<B>initial</B><B> </B> <B>begin</B> stringvar = "Hello world"; <B>$display</B>("%s is stored as %h", stringvar,stringvar); stringvar = {stringvar,"!!!"}; <B>$display</B>("%s is stored as %h", stringvar,stringvar);<B>end</B><B>endmodule</B></PRE><P><P CLASS="Body"><A NAME="pgfId=255"></A>The output is:</P><PRE><A NAME="pgfId=177"></A> Hello world is stored as 00000048656c6c6f20776f726c64Hello world!!! is stored as 48656c6c6f20776f726c64212121</PRE><P><P CLASS="Note"><A NAME="pgfId=245"></A>NOTE--When a variable is largerthan required to hold a value being assigned, the contents on the left arepadded with zeros after the assignment. This is consistent with the paddingthat occurs during assignment of non-string values. If a string is largerthan the destination string variable, the string is truncated to the leftand the left most characters will be lost.</P><P><P CLASS="SubSection"><A NAME="pgfId=258"></A>Special characters in strings</P><P><P CLASS="Body"><A NAME="pgfId=259"></A>Certain characters can only beused in strings when preceded by an introductory character called an escapecharacter. Table 2-1 lists these characters in the right-hand columnwith the escape sequence that represents the character in the left-handcolumn.</P><P><TABLE BORDER="1" CELLSPACING="2" CELLPADDING="0"><CAPTION ALIGN="TOP"><P CLASS="TableTitle"><A NAME="pgfId=323"></A>Table 2-1: Specifyingspecial characters in string</CAPTION><TR><TD><P CLASS="CellHeading"><A NAME="pgfId=330"></A>Escape <BR>String</TD><TD><P CLASS="CellHeading"><A NAME="pgfId=331"></A>Character Produced by<BR>Escape String</TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=333"></A>\n</TD><TD><P CLASS="CellBody"><A NAME="pgfId=334"></A>new line character</TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=335"></A>\t</TD><TD><P CLASS="CellBody"><A NAME="pgfId=336"></A>tab character</TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=337"></A>\\</TD><TD><P CLASS="CellBody"><A NAME="pgfId=338"></A>\ character</TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=339"></A>\"</TD><TD><P CLASS="CellBody"><A NAME="pgfId=340"></A>" character</TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=341"></A>\ddd</TD><TD><P CLASS="CellBody"><A NAME="pgfId=332"></A>a character specified in 1-3octal digits<BR>(0 <= d <= 7)</TD></TR></TABLE><HR ALIGN=LEFT></P><P><A HREF="ch02.htm">Chapter start</A> <A HREF="ch02.5.htm">Previous page</A> <A HREF="ch02.7.htm">Next page</A></BODY></HTML>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -