BBC escape codes

Every escape that can be typed in BBC BASIC source (Micro and Master share the notation), and the byte it stores. Escapes are recognised in string literals, REM and DATA bodies and *-command lines - the contexts where raw bytes live in a real program. On a MODE 7 screen the named teletext escapes are the colour/effect control bytes. Filters can be prefilled with ?q= and ?cat= query parameters.

See also the BBC BASIC reference and file formats.

MODE 7: turn graphics on first

A mosaic character is not a shape the teletext chip always draws. Every screen line starts in text mode, and the chip only has seven bits of each stored byte to work with — so a mosaic printed with nothing in front of it comes out as the character its low seven bits name. PRINT CHR$(161) on its own gives !, not a block.

What makes it a block is a graphics colour earlier on the same line:

10 MODE 7
20 PRINT "{GRAPHICS WHITE}🬂🬎🬂"
30 PRINT "{GRAPHICS YELLOW}🬀🬁🬂"

Three things follow from that, and all three bite:

• It resets every line. A graphics colour holds only to the end of the screen line it is on. The next line starts in text mode again and needs its own.
• The plain colours turn graphics off. {RED}, {WHITE} and the rest are text colours: they change the colour and go back to letters. Use {GRAPHICS RED} … {GRAPHICS WHITE} to change colour and stay in graphics.
• A control code takes a character cell, showing as a space. That is what {HOLD GRAPHICS} is for: it repeats the last mosaic under the next control code, so a colour change mid-picture does not punch a hole in it.

The on-screen keyboard's GRAPHICS palette offers the graphics colours ahead of the mosaics for this reason — take a colour first, then the shapes.

In the editor these control codes are drawn rather than spelled out: a small box in the colour the code selects, carrying a symbol for what it does — a mosaic cell for the graphics colours, an A for the text colours, and a matching mark for the rest. Hover one to see its name and character code. It is only the drawing that changes: the program still holds the escape exactly as written, it still exports byte-for-byte, and one delete removes the whole code.

		Description
{0xNN}	any	Any raw byte as two hex digits - VDU control codes 0x00–0x1F, delete (0x7F), the unnamed teletext slots (0x80, 0x8E–0x90, 0x9B), the blank mosaic (0xA0) and the top-bit capitals 0xC0–0xDF. The MODE 7 mosaics 0xA1–0xBF and 0xE0–0xFF are written as their block-graphics characters instead, though {0xNN} still loads. Recognised in strings, REM, DATA and *-command lines.
{RED}	0x81	Teletext: red text from here.
{GREEN}	0x82	Teletext: green text from here.
{YELLOW}	0x83	Teletext: yellow text from here.
{BLUE}	0x84	Teletext: blue text from here.
{MAGENTA}	0x85	Teletext: magenta text from here.
{CYAN}	0x86	Teletext: cyan text from here.
{WHITE}	0x87	Teletext: white text from here.
{FLASH}	0x88	Teletext: flashing text on.
{STEADY}	0x89	Teletext: flashing text off.
{END BOX}	0x8A	Teletext: end of boxed text.
{START BOX}	0x8B	Teletext: start of boxed text.
{NORMAL HEIGHT}	0x8C	Teletext: back to single-height text.
{DOUBLE HEIGHT}	0x8D	Teletext: double-height text (print the line twice). Outside a string 0x8D is the tokenized line-number marker - the context-aware importer keeps the two apart.
{GRAPHICS RED}	0x91	Teletext: red block graphics from here.
{GRAPHICS GREEN}	0x92	Teletext: green block graphics from here.
{GRAPHICS YELLOW}	0x93	Teletext: yellow block graphics from here.
{GRAPHICS BLUE}	0x94	Teletext: blue block graphics from here.
{GRAPHICS MAGENTA}	0x95	Teletext: magenta block graphics from here.
{GRAPHICS CYAN}	0x96	Teletext: cyan block graphics from here.
{GRAPHICS WHITE}	0x97	Teletext: white block graphics from here.
{CONCEAL}	0x98	Teletext: conceal text until a colour change.
{CONTIGUOUS}	0x99	Teletext: contiguous block graphics.
{SEPARATED}	0x9A	Teletext: separated block graphics.
{BLACK BACKGROUND}	0x9C	Teletext: black background from here.
{NEW BACKGROUND}	0x9D	Teletext: current foreground colour becomes the background.
{HOLD GRAPHICS}	0x9E	Teletext: hold graphics (repeat the last graphics character under controls).
{RELEASE GRAPHICS}	0x9F	Teletext: release held graphics.

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