Tandy 600 Programmer's Reference Manual page 116

Application Programs Under
HH
O/S
Introduction
Applications
programs
in
the
HH
O/S
system
reside
in
internal
or
memory
files in
the
HH
O/S
file
system.
Unlike
a
disk operating
system which
copies
the
program
from
disk
into
memory
in
order
to run,
a
program under
HH
O/S
is
always
in
memory, and
runs
directly
from the
location
it
occupies
in
the
file
system. This achieves space
reductions,
as
there are not
two
copies
of the
program
file
in
the system,
but
complicates
the rules
that
the
program must obey
in
order
to
operate
successfully.
An
important point
to
note
is
that
because
the
HH
O/S
internal
file
system
is
an in-memory
file
system,
and because
there
is
no
memory
protection
hardware
available
on
the
8086
family
microprocessors,
the
internal
file
system
is
quite
fragile.
It
is
very
easy
for
an
ill
behaved
program
to
damage
the
internal
file
system.
In
addition
to
the
file
containing the
program code
to
be executed, an
application also
needs
memory
for
a work space
to
contain
variables,
and
working
data.
This work space
is
also
contained
in
a
file
within the
internal
file
system. Application
work space
files
are
called
AMI
(Application
Memory
Image)
files.
An AMI
file
contains
the
working
variables, stack space,
and
user data associated with an
invocation
of
an
application
program.
Because
the
entire state of
the running
program
is
contained
within
this
one
file,
it
is
possible
for
the application
to
suspend
execution,
and
then
resume where
it
left
off at
a
later time.
This allows there
to
be
multiple
invocations
of
a given
application
in
existence
in
the system
at
the
same
time,
each
AMI
file
corresponding
to
an
application
is
a separate invocation
of that application.
It
is
the operational philosophy
of the Microsoft
Hand-Held
applications software
that
a user
can
exit
an
application
at
any
time,
execute a second
application,
and
return to the
first
application
at
a
later
time
with exactly the
same
context as
when
he
exited.
This
is
accomplished through the use
of the
AMI.
The AMI
file is
a
program
'state'
file
which
contains
the
complete
operating
state of
an
invocation of the application program.
When
the application
terminates,
its
complete
state
is
preserved
in
the
AMI
file.
Because
of
this,
when
the
application
is
re-executed
it
is
possible
to return to
the exact place
where
it
left
off.
A
single
application
will
have as
many
'states'
as there are
AMI
files for
that application. For
this
reason,
no
variable
data should be
stored
in
the
application
code
file.
Because
the
program's work space
(AMI
file) is
actually
a
file
within the
file
system,
it
is
necessary
to
understand something
of
how
the
file
system
works.
A
file is
a contiguous block
of
memory
up
to
64k
bytes
in
size that
has a
name
and
a size associated
with
it.
Because
all
of
the
files in
the
system
are stored contiguously,
when
a
file
grows
or shrinks,
all
of
the
files
above
it
in
the
system must move
up
or
down.
This
means
that
the
file
system
is
a very
dynamic,
and
the actual
memory
addresses
where
a
file is
located
can be changing
frequently.
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