Device Request/Release Logic - Tandy 600 Programmer's Reference Manual

Device Request/Release Logic
In
order
to
aid
the
Hand-Held operating
system
in
performing device
management
to
avoid
contention
between devices, the
BIOS
must support
a
device request/release system.
The
logic
of this
system
is
intended
to
be quite simple.
A
single
flag
for
each device
is all
that
is
required to
implement
it.
When
a
device request
function
call
is
received,
the
'in
use' flag for
that
device
is
tested.
If
the device
is
not
in
use, the
flag
is
set
to
indicate that
it
is
in
use,
and
the function returns with
an error
code
of
which
indicates
a successful
request.
If
the
'in
use'
flag is
set,
then the
function returns
a
'Device
In
Use' error.
When
a
device release function
call is
received, the
'in
use' flag for
the
device
is
reset.
It
is
not
an
error to
release
a device
which
is
not
'in
use'.
There
is
no record
of
who 'owns' the device, the
flag
merely indicates
that
the device
is
in
use.
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