http://www.triceraprog.fr/vg5000u-video-memory-layout.html

Every displayable unit is encoded into a pair of bytes. The first byte is mainly
the character code, the second byte contains mainly the attributes.
Character set is selected by a bit in each byte.

First byte of the pair has this layout:

+---------+-------------+---------------------------+
| Bit     | 7           | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
+---------+-------------+---------------------------+
| Content | N (0)/E (1) |     character number      |
+---------+-------------+---------------------------+

The second byte of the pair has this layout:

+---------+--------+---------+----------+-----------+-------+-----+------+-----+
| Bit     | 7      |    6    |     5    |     4     |   3   |  2  |   1  |  0  |
+---------+--------+---------+----------+-----------+-------+-----+------+-----+
| Content | TX (0) | Reverse | Width x2 | Height x2 | Blink |       Color      |
+---------+--------+---------+----------+-----------+-------+------------------+
| Content | GR (1) |         Background Color       | Blink | Foreground Color |
+---------+--------+--------------------------------+-------+------------------+

With N/E (Normal/Extended) and TX/GR, the selection of the character set is made.
This corresponds to the various graphic selections in BASIC.
Normal TX is TX, Extended TX is ET, Normal GR is GR, Extended GR is EG.
Extended mode allows character redefinition.
Normal text is ASCII, normal graphics is the Semi-Graphic character set.
