HP 3000 III Series Manual page 163

Machine Instructions and Stack Operations
Within the dispatcher, a PSEB which decrements (QI-18) to zero,
clears (QI) eliminating any pending Start Dispatcher requests.
PSDB and PSEB are used at the beginning of the Dispatcher to
prevent any interrupts which request a dispatch from causing the
first portion of the Dispatcher to be unnessarily repeated. PSEB
instructions which do not transfer to the Dispatcher set CCG in
the Sta tus Reg i ster.
Figure 4-13 is a simplified flowchart of IXIT operation. IXIT
operates in one of two manners. The first, (1)
~n
the figure, is
by the dispatcher to transfer to a process b~ing launched; the
second, (2) through (6), is to exit from ICS interrupt service
routines.
If an interrupt service routine is not in segment #1, it is as-
sumed to be an external interrupt routine and a "Reset Interrupt"
is sent to the device whose device number is at Q+3. (Q+3) is
assumed to be valid in memory, which is normally the case since
the device n umber supplied to external interrupt routines as a
parameter is written into memory.
I fbi tOo f (Q) i s ze r
0, (
Q( 0) ) = 0, the n i f Q=Q I , the ret urn is
to the interrupted process (2). Otherwise the return is to a
lower priority interrupt which was interrupted (3).
If (Q(O»= 1 and (QI(O»= 0, the return is to the Dispatcher
which was interrupted (4).
If (Q(O»= 1 and (QI(O»= 1, a DISP instruction has been executed
and the request to start the Dispatcher is still pending. If
(QI-18)= 0, the Dispatcher is not disabled, QI is cleared, and a
transfer is made to the Dispatcher's entry point (5) or (6). It
doesn't matter whether a process, Q=QI, or the Dispatcher, Q not
=QI was interrupted. If (QI-18) not 0, starting the Dispatcher is
disabled and the DISP request cannot be carried out at this time.
Instead IXIT returns to the interrupted Dispatcher,Q not =QI(4a),
or to the interrupted process, Q=QI (2a). The "Start Dispatcher
II
reques t
i
is still pend ing, (QI (0) ) = 1.
8. SIO. The I/O instructions in the HP 3000 Command System in-
struction set are as follows:
SIO Start I/O
RIO Read I/O
WIO Wr i te I/O
TIO Test I/O
CIO Control I/O
Additional information for these instructions is contained in
Section VII. The distinction to note here is that the SIO in-
struction is used in conjunction with an I/O program ~nd the
others are not. That is, the SIO instruction commands a device
controller to begin executing its associated I/O program, which
effects a block transfer of data between an I/O device and mem-
ory. This is termed as "SIO transfer" mode. The other instruc-
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