Interrupt; Interrupt Modes Of Operation - IBM 1710 Manual

Power Failure Protection. If a power failure occurs,
the spp's are inoperative, except upon manual inter-
vention. Thus, instrumentation set-points cannot be
falsely altered as a result of power failures.
Interrupt
The Interrupt feature gives the 1710 the ability to
recognize conditions demanding immediate attention.
The disturbance (interrupting condition) causes the
computer to suspend routine operation. In an open-
loop system, the control room operator is notified of the
disturbance by the console typewriter. A closed-loop
system can actually initiate corrective action as well
as notify the control room operator or the process
operator.
Interrupt Modes
of Operation
The 1710 normally operates in the interruptible mode,
that is, it interrupts the main program when an inter-
rupt condition occurs. However, once the interrupt is
recognized, the 1710 is in the noninterruptible mode
until the interrupt subroutine has been completed. In
other words, an interrupt cannot interrupt another
interrupt until the appropriate action required by the
first interrupt has been completed. Interrupts are serv-
iced in the program in a sequence that corresponds to
the priority established for them by the process engi-
neer. Four interrupt instructions, Branch Out (of non-
interruptible mode), Branch Out (of noninterruptible
mode) and Load, Mask, and Unmask, are provided
with this feature for interrupt control. The Branch Out
and Branch Out and Load instructions are used to end
an interrupt subroutine and to place the 1710 in the
interruptible mode. The Mask instruction is used to
mask all interrupts, i.e., it places the 1710 in the non-
interruptible mode, delaying all interrupts until the
interruptible mode is restored. The Unmask instruc-
tion is used to restore the interruptible mode when the
noninterruptible mode exists as a result of a Mask
instruction.
Figure 17 shows the relationship between the four
interrupt instructions and the two interrupt modes.
Starting at the top (progress in time is shown from
top to bottom) interrupt 1 causes the 1710 program to
branch to interrupt subroutine 1 and to enter the non-
interruptible mode. A Branch Out or Branch Out and
Load instruction returns the 1710 to the main program
and the interruptible mode. A Mask instruction places
the 1710 in the noninterruptible mode, also causing
interrupt 2 to be delayed (not lost) until the Unmask
instruction is executed. The 1710 then enters the inter-
ruptible mode long enough to recognize interrupt 2;
this causes an immediate return to the noninterruptible
mode and the interrupt 2 subroutine. With the 1710
in the noninterruptible mode, as a result of interrupt 2,
the Unmask has no effect, and the Branch Out. or
Branch Out and Load returns the 1710 to the main pro-
gram and the interruptible mode. Interrupt 3 causes
the 1710 to branch to interrupt subroutine 3 and to
enter the noninterruptible mode. Execution of the
Mask instruction during interrupt subroutine 3 keeps
the 1710 in the noninterruptible mode even after the
Branch Out or Branch Out and Load has returned
the computer to the main program. Any interrupts
occurring after the Mask instruction are delayed, not
only until a Branch Out or Branch Out and Load
instruction is executed but until an Unmask restores
the computer to the interruptible mode.
Interrupt 1
Int~r:r:oo~
__
~n:~=t;ble
_-------J-Branch Out or
J
Branch Out
Mask _ _ ....
~,
________
~
and Load
Interrupt 2
~-
....
~
No Effect
I
No effect
I
at this time
I
L-__
~~z~~:=:-=.;:!~unmask
~Unmask
(no effect)
~---
- - - -
~BranchOutor
1
Branch Out
and Load
Interrupt3--...
~
- - - - - - -
~ ~Mask
~
Branch Out or Branch
Out
and
Load (1710
returns to main
program
but
stays
in Noninterruptible
Mode because of mask)
r--------
.... Unmask
Figure 17. Interrupt Modes
25