ESPTerm web interface submodule, separated to make testing and development easier
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espterm-front-end/pages/help/charsets.php

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<div class="Box fold">
<h2>Alternate Character Sets</h2>
<div class="Row v">
<p>
ESPTerm implements Alternate Character Sets as a way to print box drawing characters
and special symbols. A character set can change what each received ASCII character
is printed as on the screen (eg. "{" is "π" in codepage `0`). The implementation is based
on the original VT devices.
</p>
<p>
Since ESPTerm also supports UTF-8, this feature is the most useful for applications
which can't print UTF-8 or already use alternate character sets for historical reasons.
</p>
<h3>Supported codepages</h3>
<ul>
<li>`B` - US ASCII (default)</li>
<li>`A` - UK ASCII: # replaced with £</li>
<li>`0` - Symbols and basic line drawing (standard DEC alternate character set)</li>
<li>`1` - Symbols and advanced line drawing (based on DOS codepage 437, ESPTerm specific)</li>
<li>`2` - Block characters and thick line drawing (ESPTerm specific)</li>
<li>`3` - Extra line drawing (ESPTerm specific)</li>
</ul>
<p>
All codepages use codes 32-127, 32 being space. A character with no entry in the active codepage
stays unchanged.
</p>
<script>
function bchst(start, str) {
var ar = str.split(' ');
for(var i=0;i<ar.length;i++) {
var a = String.fromCharCode(start+i);
var r = ar[i];
document.write('<div'+(r===a?' class="none"':'')+'><span>'+(start+i)+'</span><span>'+$.htmlEscape(a)+'</span><span>'+$.htmlEscape(r)+'</span></div>');
}
}
</script>
<?php
$codepages = load_esp_charsets();
foreach($codepages as $name => $cp) {
echo "<h4>Codepage `$name`</h4>\n";
echo '<div class="charset">';
$t = implode("\x01", $cp['chars']);
$t = json_encode($t, JSON_UNESCAPED_UNICODE|JSON_UNESCAPED_SLASHES);
$t = str_replace('\u0001', " ", $t); // space is never included
$t = htmlspecialchars($t,ENT_HTML5);
echo '<script>bchst('.$cp['start'].','.$t.')';
echo '</script></div>';
}
?>
<h3>Codepage switching commands</h3>
<p>
There are two character set slots, G0 and G1.
Those slots are selected as active using ASCII codes Shift In and Shift Out (those originally served for shifting
a red-black typewriter tape). Often only G0 is used for simplicity.
</p>
<p>
Each slot (G0 and G1) can have a different codepage assigned. G0 and G1 and the active slot number are
saved and restored with the cursor and cleared with a screen reset (<code>\ec</code>).
</p>
<p>The following commands are used:</p>
<table class="ansiref w100">
<thead><tr><th>Code</th><th>Meaning</th></tr></thead>
<tbody>
<tr>
<td>`\e(<i>x</i>`</td>
<td>Set G0 = codepage <i>x</i></td>
</tr>
<tr>
<td>`\e)<i>x</i>`</td>
<td>Set G1 = codepage <i>x</i></td>
</tr>
<tr>
<td>_SO_ (14)</td>
<td>Activate G0</td>
</tr>
<tr>
<td>_SI_ (15)</td>
<td>Activate G1</td>
</tr>
</table>
</div>
</div>