Page
3.2
General Theory of Operation
The
CPU addresses the
RAM
beginning
at
location
BOOOO.
The
AO
bit
is
used to enable either the display
RAM
or the attribute
RAM.
This causes
the
display to
be
addressed using even numbers
and
the attribute
RAM
to
be
addressed with odd numbers. Example addresses are shown
in
Table 3.1.
Table 3.1. Character/Attribute Addressing
DISPLAY LOCATION CHARACTER ADDRESS ATTRIBUTE ADDRESS
1
BOOOO
BOO01
2
BOO02
BOO03
3
BOO04
BOO05
4
BOO06
BOO07
80
(last in line 1)
BOOAO
BOOA1
81
(1
st in line 2)
BOOA2
BOOA3
1975
(1
st in line 25)
BOF5E
BOF5F
2000 (last on page)
BOFAO
BOFA1
Character ROM
The
character ROM installed on the card at the factory
is
an
8K x 8
ROM
which provides decoding of the ASCII code stored
in
the
RAM
into
pixel on/off instructions. The standard
ROM
provides decoding for two
256
character fonts. An optional 16K x 8
ROM
may be used to provide
up
to four fonts. The font
in
use
is
software selectable.
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