Re: Looking for Acorn Electron help

VinceH wrote:
40x25 Characters sounds very good, but how does it occupy 1 KB? is
it without colours?.
It has 8 colours (black red green blue yellow cyan magenta white),
plus 8 flashing combinations.

A teletext display consists of 25 lines of 40 7-bit characters.
The eigth bit is ignored. Each character can either be a displayable
character in the range &20 to &7F (or &A0 to &FF) or a control
character in the range &00 to &1F (or &80 to &9F). You will find
that BBC MODE 7 displays usually use the range &20 to &9F, ie b7
clear for displayable characters and b7 set for control characters.

Control characters take up a character cell on the screen, and are
usually displayed as a space.

Each line starts in a defined state, this being:
black background, white foreground, text, nonflashing, single height,
contiguous (joined) graphics.

Control characters change the state of the /following/ characters
on the same line. Control characters are:

&00/&80 - NULL
&01/&81 - Red alphanumeric
&02/&82 - Green alphanumeric
&03/&83 - Yellow alphanumeric
&04/&84 - Blue alphanumeric
&05/&85 - Magenta alphanumeric
&06/&86 - Cyan alphanumeric
&07/&87 - White alphanumeric
&08/&88 - Flash
&09/&89 - Steady
&0A/&9A - End Box
&0B/&9B - Start Box
&0C/&8C - Normal height
&0D/&8D - Double height
&0E/&8E - SO (NULL)
&0F/&8F - SI (NULL)
&10/&90 - DLE (NULL)
&11/&91 - Red graphics
&12/&92 - Green graphics
&13/&93 - Yellow graphics
&14/&94 - Blue graphics
&15/&95 - Magenta graphics
&16/&96 - Cyan graphics
&17/&97 - White graphics
&18/&98 - Conceal
&19/&99 - Contiguous graphics
&1A/&9A - Seperated graphics
&1B/&9B - Black background
&1C/&9C - New background - new background colour is current foreground
colour
&1D/&9D - ESC (NULL)
&1E/&9E - Hold graphics - last displayed graphics character is
displayed
instead of a space in a character cell occupied by a control code
&1F/&9F - Release graphics

After a alphanumeric colour code, all displayable characters are
displayed in the current background colour and the specified
foreground colour. After a graphics colour code, all upper case
letters are displayed, and other displayable characters are
displayed as a 6-pixel graphics character (see RTR later).

The £, #, _ characters are swapped around in the teletext character
set, so a memory dump will have &23 for £, &60 for _ and &5F for #.

The BBC Micro User Guide has a bitmap diagram of the teletext
character set in one of the appendixes.

To edit a teletext screen, you could try ABZTTxt at
<http://mdfs.net/System/Teletext/>, works on any system
capable of running BBC BASIC and with MODE 7 (BBC, Master,
RISC OS, Windows.....)

See also http://www.rtrussell.co.uk/bbcwin/manual/bbcwinh.html

--
JGH
.

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