Alternate Character Sets

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.

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.

Supported codepages

`B` - US ASCII (default)
`A` - UK ASCII: # replaced with £
`0` - Symbols and basic line drawing (standard DEC alternate character set)
`1` - Symbols and advanced line drawing (based on DOS codepage 437, ESPTerm specific)
`2` - Block characters and thick line drawing (ESPTerm specific)
`3` - Extra line drawing (ESPTerm specific)

All codepages use codes 32-127, 32 being space. A character with no entry in the active codepage stays unchanged.

$cp) { echo "

Codepage `$name`

\n"; echo '

'; $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 '

'; } ?>

Codepage switching commands

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.

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 (\ec).

The following commands are used:

Code	Meaning
`\e(x`	Set G0 = codepage x
`\e)x`	Set G1 = codepage x
_SO_ (14)	Activate G0
_SI_ (15)	Activate G1