CRT
CONTROL
word
are
redundant.
The
graphics
pointer
is
used
to select
graphics cell images
programmed
into
the
system
RAM.
The
values of
the
II-bit
pointers
never
need
to
be changed
and
since
the
CRTC
start
address is also never changed,
the
pointers
always access
the
same
graphics cell image as
corresponding pixels are
mapped
onto
the
screen,
SYSTEM
RAM
UTILISATION
General
The
utilisation
of
the
System
RAM by
the
CRT
control
circuitry
is
dependent
on
the
selected
mode. In
text
mode,
the
BIOS allocates space
in
the
System
RAM for
three
character
fonts, a default
font
and
two
user
specified fonts.
Each font
has
space for 256 characters.
The
default font
loaded by
the
BIOS for UK use, is a slightly modified ASCII
SINTL
01
character
set
(strikethrough
bit
added).
This
occupies 8 Kbytes of sY$tem
memory
from
address
location
0800H
to
27FFH.
The
two
user
font
areas occupy
the
next
16
Kbytes of
system
memory
(2800H
to
47FFH
and
4800H
to
67FFH respectively).
In
the
graphics mode, 40 Kbytes of
the
system
memory
are required
to
map
every pixel
on
the
800 x 400
resolution
screen.
The
System
RAM allocated
to
the
graphics screen
image
is
from
2800H
to
C440H,
which
means
the
two
user
fonts are always
overwritten
when
graphics is selected.
The
CRT
control
circuitry
is
guaranteed
access to
the
System
RAM, once every
character
clock
period during
active
display
time
to
refresh
the
screen.
Contention
between
processor access
to
the
System
RAM
and
CRT
control
access is
managed
by
the
system
memory
control
circuits.
The
control
circuits
automatically
add
wait
state(sJ
to
a processor
memory
cycle if
the
processors
attempt
to
access
the
RAM, during
the
CRT
control
access period.
The
number
of
wait
states
inserted
for
text
is from 1
to
3, for
graphics 1
to
5.
The
address generated
by
the
CRT
control
circuit
to
access
the
System
RAM
is
produced
by
offsetting
the
Il-bit
pointer
from
the
Screen RAM
by
5 significant places
and
then
combining
the
result
with
the
4-bit raster address, as
To Next Page
Loading...
