Tandy 600 Programmer's Reference Manual page 146

INT
51
h,
Function
B
-
Reverse Video of Specified Block
Entry:
AH = B
(function
code)
AL =
Video
state to set
=
Normal Video
1
=
Reverse Video
CX =
Number
of
characters
Exit:
No
registers specified
Starting at
the current cursor
position,
the specified
number
of
characters are
set to
the
specified
video
state.
If
the
number
of
characters
specified
goes past the
end
of
a
line,
output
should
wrap
to
the beginning
of
the next
line.
If
the
specified
number
of
characters
would
cause the output
to
go beyond
the
end
of the
last line
on
the screen, the extra characters are
ignored.
In
other
words,
wrap
to
the next
line
on any
line
but the
last.
On
the
last line,
ignore
any extra characters.
After
this
function
is
completed, the cursor should be
returned
to
its
original location.
This
function
does
not affect
the setting of
the current reverse video
state.
This function
will
never
cause
the
screen
to scroll.
INT 51
h,
Function
C
-
Enable Reverse Video
Entry:
AH
= C
(function
code)
Exit:
No
registers specified
INT 51
h,
Function
D
-
Disable
Reverse Video
Entry:
AH = D
(function code)
Exit:
No
registers specified
When
reverse video
is
disabled,
BIOS
puts the characters
on
the display
in
normal
video,
clearing
the
corresponding reverse video
bits.
When
reverse video
is
enabled, the characters
are put
on
the display
in
reverse video with
the corresponding reverse video
bits
set
to 1.
The
BIOS
initialization
sequence should
disable
reverse video.
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