It turns out these are the "VT100 graphics characters", and the xfonts-utils program ucs2any can optionally put specific Unicode code points in these spots:
/* DEC VT100 graphics characters in the range 1-31 (as expected by some old xterm versions and a few other applications) */
#define decmap_size 31
static int decmap[decmap_size] = {
0x25C6, /* BLACK DIAMOND */
0x2592, /* MEDIUM SHADE */
0x2409, /* SYMBOL FOR HORIZONTAL TABULATION */
0x240C, /* SYMBOL FOR FORM FEED */
0x240D, /* SYMBOL FOR CARRIAGE RETURN */
0x240A, /* SYMBOL FOR LINE FEED */
0x00B0, /* DEGREE SIGN */
0x00B1, /* PLUS-MINUS SIGN */
0x2424, /* SYMBOL FOR NEWLINE */
0x240B, /* SYMBOL FOR VERTICAL TABULATION */
0x2518, /* BOX DRAWINGS LIGHT UP AND LEFT */
0x2510, /* BOX DRAWINGS LIGHT DOWN AND LEFT */
0x250C, /* BOX DRAWINGS LIGHT DOWN AND RIGHT */
0x2514, /* BOX DRAWINGS LIGHT UP AND RIGHT */
0x253C, /* BOX DRAWINGS LIGHT VERTICAL AND HORIZONTAL */
0x23BA, /* HORIZONTAL SCAN LINE-1 (Unicode 3.2 draft) */
0x23BB, /* HORIZONTAL SCAN LINE-3 (Unicode 3.2 draft) */
0x2500, /* BOX DRAWINGS LIGHT HORIZONTAL */
0x23BC, /* HORIZONTAL SCAN LINE-7 (Unicode 3.2 draft) */
0x23BD, /* HORIZONTAL SCAN LINE-9 (Unicode 3.2 draft) */
0x251C, /* BOX DRAWINGS LIGHT VERTICAL AND RIGHT */
0x2524, /* BOX DRAWINGS LIGHT VERTICAL AND LEFT */
0x2534, /* BOX DRAWINGS LIGHT UP AND HORIZONTAL */
0x252C, /* BOX DRAWINGS LIGHT DOWN AND HORIZONTAL */
0x2502, /* BOX DRAWINGS LIGHT VERTICAL */
0x2264, /* LESS-THAN OR EQUAL TO */
0x2265, /* GREATER-THAN OR EQUAL TO */
0x03C0, /* GREEK SMALL LETTER PI */
0x2260, /* NOT EQUAL TO */
0x00A3, /* POUND SIGN */
0x00B7 /* MIDDLE DOT */
Apparently on a real VT100 these are printed by sending various escape codes that cause them to be mapped at character values 95..126.
various escape sequences.
Entry first conceived on 26 July 2024, 0:17 UTC, last modified on 26 July 2024, 13:18 UTC
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