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Code Editor : jed020.html

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<h2 id="sec41" class="section">19&#X2003;Eight Bit Clean Issues</h2>
<h3 id="sec42" class="subsection">19.1&#X2003;Displaying Characters with the High Bit Set</h3>
There are several issues to consider here. The most important issue is how
to get jed to display 8 bit characters in a &#X201C;clean&#X201D; way. By
&#X201C;clean&#X201D; I mean any character with the high bit set is sent to the
display device as is. This is achieved by putting the line:<pre class="verbatim"> DISPLAY_EIGHT_BIT = 1;
</pre>in the <code class="verb">jed.rc</code> (<code class="verb">.jedrc</code>) startup file. European systems might
want to put this in the file <code class="verb">site.sl</code> for all users. The default is
1 so unless its value has been changed, this step may not be necessary.There is another issue. Suppose you want to display 8 bit characters with
extended Ascii codes greater than or equal to some value, say 160. This
is done by putting <code class="verb">DISPLAY_EIGHT_BIT = 160;</code>. I believe that ISO
Latin character sets assume this. This is the default value for Unix and
VMS systems.
<h3 id="sec43" class="subsection">19.2&#X2003;Inputting Characters with the hight bit Set</h3>

Inputting characters with the high bit set into jed is another issue.
How jed interprets this bit is controlled by the variable
<code class="verb">META_CHAR</code>. What happens is this: When jed reads a character from
the input device with the high bit set, it:
<ol class="enumerate" type=1><li class="li-enumerate">
Checks the value of <code class="verb">META_CHAR</code>. If this value is -1, jed simply
inserts the character into the buffer.</li><li class="li-enumerate">For any other value of <code class="verb">META_CHAR</code> in the range 0 to 255, jed
returns two 7-bit characters. The first character returned is
<code class="verb">META_CHAR</code> itself. The next character returned is the original
character but with the high bit stripped.
</li></ol>The default value of <code class="verb">META_CHAR</code> is -1 which means that when jed sees
a character with the high bit set, jed leaves it as is. Please note that
a character with the high bit set it cannot be the prefix character
of a keymap. It can be a part of the keymap but not the prefix.Some systems only handle 7-bit character sequences and as a result, jed
will only see 7-bit characters. jed is still able to insert any
character in the range 0-255 on a 7-bit system. This is done through the
use of the <code class="verb">quoted_insert</code> function which, by default, is bound to
the backquote key &#X2018;. If the <code class="verb">quoted_insert</code> function is called
with a digit argument (repeat argument), the character with the value of
the argument is inserted into the buffer. Operationally, one hits
Esc, enters the extended Ascii code and hits the backquote key. For
example, to insert character 255 into the buffer, simply press the
following five keys: Esc 2 5 5 &#X2018;.
<h3 id="sec44" class="subsection">19.3&#X2003;Upper Case - Lower Case Conversions</h3>
The above discussion centers around input and output of characters with
the high bit set. How jed treats them internally is another issue and new
questions arise. For example, what is the uppercase equivalent of a
character with ASCII code 231? This may vary from language to language.
Some languages even have characters whose uppercase equivalent correspond
to multiple characters. For jed, the following assumptions have been
made:
<ul class="itemize"><li class="li-itemize">
Each character is only 8 bits.
</li><li class="li-itemize">Each character has a unique uppercase equivalent.
</li><li class="li-itemize">Each character has a unique lowercase equivalent.
</li></ul> 
It would be nice if a fourth assumption could be made:
<ul class="itemize"><li class="li-itemize"> 
The value of the lowercase of a character is greater than or equal to
its uppercase counterpart.
</li></ul> 
However, apparently this is not possible since most IBMPC character sets
violate this assumption. Hence, jed does not assume it. Suppose X
is the upper case value of some character and suppose Y is its lower
case value. Then to make jed aware of this fact and use it case
conversions, it may be necessary to put a statement of the form:
<pre class="verbatim"> define_case (X, Y);
</pre>
in the startup file. For example, suppose 211 is the uppercase of 244.
Then, the line
<pre class="verbatim"> define_case (211, 244);
</pre>
will make jed use this fact in operations involving the case of a character.This has already been done for the ISO Latin 1 character set. See the file
<code class="verb">iso-latin.sl</code> for details. For MSDOS, this will not work. Instead
use the files <code class="verb">dos437.sl</code> and <code class="verb">dos850.sl</code>. By default, jed&#X2019;s
internal lookup tables are initialized to the ISO Latin set for Unix and
VMS systems and to the DOS 437 code page for the IBMPC. To change the
defaults, it is only necessary to load the appropriate file. For example,
to load <code class="verb">dos850.sl</code> definitions, put
<pre class="verbatim"> evalfile ("dos850"); pop ();
</pre>
in the startup file (e.g., <code class="verb">site.sl</code>). In addition to
uppercase/lowercase information, these files also contain word
definitions, i.e., which characters constitute a &#X201C;word&#X201D;.
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