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SYSTEM DESCRIPTION
24.7 -- Interrupt Handling
are inhibited throughout.
Therefore, an IRG cannot always be
serviced within a definite time window
if
diskette
accesses
can ·be
"in progress.
The
time is dependent on the length of
the diskette access.
Another potential problem exists
if
NMI
is
to
be
used
while
diskette functions are
in
progress.
The NMI vector is
taken over by the
diskette
controller
while
the
diskette
access
is
in
progre~s.
The
NMI
is
uS2d
as
a timeout
con d
i
t ion.
T
h
us,
i
f a user's s y stem
i
s g en era tin
9 NM I
s
til
h i 1
e
diskette
accesses
are
going
on,
a timeout condition will
result and the user will not be able to process the NMI.
I t
is
for
this
reason
that
no
user-defined
NMI
vector is
provided by MDOS.
The system functions provided
by
MDOS
are
accessible
th~ough
use
Qf the software interrupt or SWI instruction. A
full explanation regarding the MOOS SWIs is given in the next
section;
howeverl
like
the
user-defined
IRG vectoT"J
MDOS
allows a user-defined SWI vector to be configured through the
va ria
b 1
e
SW I
$UV .
L
ike
the
use r
-d
e fin e d
I R G
han
d I
er
I
the
user-defined SWI handler is only accessible as
long
as
the
MOOS
command interpreter is not reentered.
Whenever control
is returned to the MOOS command interpreter,
the user-defined
SWI
vector
will
be
changed to point back int·o
MDOS.
Thus,
user-defined SWls cannot be processed after the user
program
has
terminated.
This is to be expected,
since MDOS commands
and user programs all load into one area
of
memory.
Thus,
the
user-defined
SWI handler is not resident after the MDOS
command interpreter regains control.
24.8
System Function Calls
All of the system functions that MOOS commands
use
are
also
available
to the user and can be incorporated into his
program development.
All MDOS system functions are
accessed
via the software interrupt or
SWI
instruction.
Each
SUI
must
be followed by
a
byte
that
contains
the
number
of
the
function
to be executed.
MDOS's resident
soft~are
interrupt
handler can access up to
128
(decimal)
functions;
however,
not
all
of
these
functions are defined.
An error message
will
be
printed
if
the
software
interrupt
handler
is
activated and the function number is not defined.
A speCial convention is used to allow the user to define
a maximum
o~
128
~unctions
also
(to
be
processed
b~
the
user's
software
interrupt
handler
that
is configured via
SWISUV).
If the sign bit of the function number byte (bit 7)
is
set
to
one,
a
user-defined
software
interrupt
is
indicated.
All MOOS software interrupts have function number
b y
t e s
wit h . the
s i
9
n bit set to
Z
e r o.
The use r-
d
e fin
e
d . SW I
handler gets control with the registers on the stack as if it
intercepted
the
SWI
directly.
The
B
accumulator will have
Page
24-23
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