Tandy 600 Programmer's Reference Manual page 169

INT
57h,
Function
A
-
Release
Disk Device
Entry:
AH = A
(function
code)
Exit:
AL=
Standard
BIOS
error
code
This function
is
called
when
access
to
a
disk drive
is
to
be
terminated.
The
device should
be
marked
as
not busy.
This
function
will
always
turn
off
the
power
to
the
FDD
circuit
and
its
motor.
164