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System/CPU Overview
Condition code
pattern D (CCD) is used with some
(not all)
I/O'
instructions.
In
CCD,
the condition code
is set to 00 if the
external device is not ready.
(This condition is
usually caused
by the
device being busy.)
This .code is only used with instruc-
tions that
will (WIO and RIO) or could (SIO) require data to
be
moved.
The code is set to 01 if the
Device Controller
does not
respond.
(This condition
can be caused by loss of power
in the
external
device or
Device Controller,
a malfunction in the ex-
ternal device
or Device Controller or,
more
normally,
the ex-
ternal device
or Device Controller waiting
for a response to an
interrupt request.)
The condition code is set to 10
if the
ex-
ternal device or Device Controller has responded normally and the
instr uction has been completed properly.
2-47. OPERATING MODES
The computer system can be operated in either privileged or
user
mode
and has
the
capability of switching
from one mode to the
other
depending on
the type of
operation
being executed
at a
given
instant.
The operating mode currently
in effect is indi-
cated at
all times by the STA Register's bit 0
as discussed
in
paragraph 2-45.
Privileged mode operation is characterized by the ability to
ex-
ecute privileged instructions and to call segments that have been
declared uncallable. (The method of declaring a code segment
un-
callable
involves the
use of the uncallable bit
(bit 1) in the
associated STT
local program label shown in
figure 2-6.)
Priv-
ileged
operations such
as
I/O operations are
performed by the
operating system operating in privileged mode.
For an
non-priv-
ileged
user
to perform such operations,
it is necessary to call
one of the callable intrinsics of the operating system which,
in
turn,
will call the uncallablp intrinsics that
will perform the
operation on behalf of the nonprivileged user.
However, a
priv-
ileged mode user can use the computer as if he were the operating
sys tern.
I
CAUTION
I
The normal checks and limitations that apply to
the standard non-privileged users
in
MPE
are
bypassed
during privileged mode.
It is possi-
ble for a privileged mode
program
to
destroy
file integrity including the MPE operating sys-
tem software. Hewlett-Pac kard cannot be respon-
sible
for
system
integrity
when
programs
written
by
users
operate
in
the privileged
mode.
2-34
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