Screen (Lcd) Graphics Support - Tandy 600 Programmer's Reference Manual

Screen (LCD) Graphics Support
BIOS graphics support
is
fashioned
after
the
GW-BASIC
2.0
Graphics
Interface Specification.
A
thorough reading of that
document
is
recommended
prior to
implementing the
BIOS
graphics support described below.
The BIOS maintains a graphics cursor
which uniquely
addresses
a
pixel of
the display.
The
internal
representation of the
graphics cursor should
be chosen by
the
OEM
to
optimize the
speed
of
graphics operations.
The
only restriction
on
the internal
representation
is
that
it
must
not
exceed 6 bytes
in
length.
The BIOS
also maintains a graphics
attribute
which
specifies
what
effect
a
wet
pixel(s)
BIOS
call
has on the affected
pixel(s).
(The
LCDs
being
used
for
Hand-Held computers today
support only
two
states
for
a
pixel,
on
or
off.
Therefore
the graphics
attribute
simply
specifies
one
of
these two
states.)
As
with the
graphics cursor,
the internal
representation of
the
graphics
attribute
should
be chosen
to
optimize the
speed
of
graphics
operations.
There are
no
size restrictions
on
the
internal
representation
of
the graphics
attribute
since only the
external
form
is
used
when passing parameters
to
and from the
BIOS.
Because graphic
functions are
speed
critical
routines,
they
never
turn
the cursor
on
or off
even
if
the cursor
disturbs their
work. Turning the cursor
on
or
off
should be performed
before or
after
invocation
of
graphic functions
if it
is
necessary.
Graphics coordinates
are specified
such that
Y
runs
vertically
with
being the top
row
of
pixels
on the screen,
and
X
runs
horizontally with
being the leftmost
column
of pixels
on
the
screen.
The
following interrupts
support graphics
on the screen:
INT 52h, Function
-
Return Screen Graphics Resolution
Entry:
AH =
(function code)
Exit:
CX
=
highest
numbered
pixel
in
X
(number
of pixels
in
X
direction)-1
DX =
highest
numbered
pixel
in
Y
(number
of pixels
in
Y
direction)-
1
INT 52h, Function
1 -
Map
X,Y Coordinates to
Graphics Cursor
Entry:
AH =
1
(function
code)
CX
=
X
coordinate
DX
=
Y
coordinate
Exit:
No
registers specified
This routine
sets the graphics cursor
to
select the pixel
specified
by the
X
and
Y
coordinates
for
subsequent graphics
BIOS
calls.
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