Tandy 600 Programmer's Reference Manual page 151

INT
52h,
Function
B
-
Store
Graphics Cursor
Entry;
AH = B
(function
code)
BX
=
word 2
of graphics cursor
CX =
word 3
of graphics cursor
DX = word
1
of
graphics cursor
Exit:
No
registers specified
Since
the internal
representation
of
the graphics cursor
is
unknown
outside
of
the BIOS,
the
fetch
and
store
graphics cursor
routines are not intended
to
allow the
caller to
operate
on
the
cursor. Instead, these routines are
provided
solely to
allow the
caller to
preserve and
later
restore
the contents
of
the graphics
cursor.
INT
52h,
Function
C
-
Return
Aspect
Ratio
Entry:
AH = C
(function
code)
Exit:
BX
=
256 *
(aspect
ratio)
DX =
256
/
(aspect
ratio)
The
aspect
ratio of
the
screen
is
defined
as
the
ratio of
the
Y
distance
between two
vertically
adjacent pixels
to
the
X
distance
between two
horizontally
adjacent
pixels,
where
the
distances are
measured
from the center
of the
first
pixel
to
the center
of
the
second
pixel.
Typically
this ratio
is
less than
1,
so
a
given
number
of
pixels
spans a
shorter distance
in
Y
than
in
X.
The
aspect
ratio
can be used by
callers to
compensate
for
unequal
X
and
Y
pixel
spacing
when
drawing graphics
on
the screen.
For example, the
CIRCLE
statement
in
GW-BASIC
uses
the
aspect
ratio
to
draw
spatially
round
circles.
The
aspect
ratio
can be
empirically
determined as
follows:
1
.
Turn
on
a
box
of pixels of
dimension
N
by
N,
where
N
is
the
minimum
of
the screen's
pixel
resolution
in
X
and
Y.
(In
other words,
draw
the
largest
box
the
screen can
accommodate
having the
same
number
of pixels
in
X
and
Y.
2.
Measure
the
Y
(vertical)
dimension
of
the box.
3.
Measure
the
X
(horizontal)
dimension
of
the box.
4.
Divide the
Y
dimension
of
the
box by
the
X
dimension
to
obtain the
aspect
ratio of
the
screen.
146