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FUNCl'IONS
Successful
execution
of
a
function
typically
causes
the
alteration
of
a
destination
operand
(that
is,
an
actual
operand~
the
operands
field
of
the
function
request
area
is
never changed
by
function
execution).
In
addition,
or
alternatively,
same
functions
produce
a
return-value.
For example,
the
READ
PR:CESSOR
STATUS
AND
CLOCK
function
returns
the
current
values
of
the
IP'
s
system
clock
and
status.
The
IP
writes
return-values
into
the
results
field
of
the
function
request
area,
where
they
may
be
inspected
through
window
4.
The
low-order
byte
of
any
return-value
is
stored
in
the
lowest-addressed
location
of
the
field
and any
additional
bytes
are
stored
in
consecutively
higher
locations.
When
the
length
of
the
return-value
is
less
than
the
length
of
the
return-value
field,
the
content
of
excess
high-order
locations
is
undefined.
Appendix B
provides
the
format
and
interpretation
of
the
return-values
produced
by
all
functions.
Several
functions
produce
a
standard
type
of
return-value
called
a
boolean.
This
is
a
one-byte
value
that
indicates
"true"
or
"false."
The
low-order
bit
of
the
value
"true"
is
1 and
the
low-order
bit
of
the
value
"false"
is
O.
In
either
case
the
value
of
the
upper
seven
bits
of
a
boolean
is
undefined.
If
a
function
faults,
the
contents
of
the
return-value
field
is
undefined.
If
a
function
canpletes
successfully,
but
it
does
not
produce a
return-value,
then
the
IP
does
rot
alter
the
content
of
the
return-value
field.
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