Tandy 600 Programmer's Reference Manual page 11

INT
45h
-
File
System Moving
Hook
AX
BX
cx
DX
SP:
BP:
SI:
Dl:
IP:
FLG
CS
DS
SS
ES
Entry:
none
Exit:
none
The
internal
file
system under
HH
O/S
is
an in-memory system.
Each
file is
contained
in
a
single
block
of
memory
up
to
64k
bytes
in
size,
with the
files
stored contiguously
in
memory.
For
this
reason,
when one
file
grows
or shrinks,
it
causes other
files
in
the
system
to
be
moved
to either
make more room
for
a
file
that
is
expanding
or
to
occupy
the
space
left
by a
file
that
is
contracting.
Because
of this
dynamic
nature
of
the
file
system,
programs which
refer to
files
through
absolute
address
pointers
need
to
be kept
aware
of the
movement
of
files in
the
system.
Whenever
HH
O/S moves a
portion of the
file
system,
it
will
invoke the
File
System Moving
hook.
The
user provided
hook
routine
can
then
adjust
any
internal
pointers
necessary.
Any
program which
is
concerned about
the absolute
address
of
an
item
in
the
file
system can use
this
hook
to
keep
its file
pointers current.
The sequence
of
operations
will
be as
follows:
1.
The
affected portions
of
the
file
system
will
be moved.
2.
The
values
which
were
in
the
program's
segment
registers
at
the time
of
the current
function
call will
be adjusted
as appropriate
for
the
file
system
movement
taking place.
3.
The segment
portion
of
the vector
address
for
the
File
System Moving
interrupt
will
be
adjusted as appropriate
for
the
movement
taking place.