Tandy 1000
Operating Modes
Technical Reference Manual
The
operating modes supported by the system software may be grouped into
two categories: alphanumeric. and graphic.
The alphanumeric mode has two basic types of operation: 80 character by
25
rows and
40
character by 25 rows. In both modes the characters are
generated by a 256 character
ROM.
The
character
ROM
is
divided into the
following groups:
96 Standard
ASCII
characters
48 Block Graphics characters
64 Foreign Language/Greek characters
16 Special Graphics characters
32 Word Processing/Scientific-Notation characters
In
both modes
all
characters are 7 dots wide by 7 dots high and
are
placed
in
an
8 dot wide by 8 or 9 dot high matrix.
In
both the 40 x
25
mode and the 80 x 25 mode, two bytes
of
data
are
used
to
define each character
on
the screen.
The
even address (0,2,4 etc.)
is
the
character code and
is
used
in
addressing the character generator
ROM.
The
odd address (1,3,5 etc.)
is
the attribute byte, that defines the foreground and
background color of
the
character.
The
following chart shows how the attribute
byte functions
to
control colors.
ATTRIBUTE BYTE
BIT
7
6 5 4 3
2
1 0
309
Bit
5 Background Foreground
= a
I
R
G
B
I R
G
B
309
Bit 5
Blink
I
R G B I
R
G
B
= 1
Blink
= A 1
in
bit
7 enables blinking of the foreground.
46
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