U UNIVAC 1218 - Bitsavers

The location (CAT) is specified by the contents of the address in the lower 12
bits of the instruction. By using this method the programmer may load an
interrupt processing routine at CAT+l and terminate this routine with an indirect
jump on CAT to return program control to the interrupted program. Most
interrupts are optional in the sense that they can be disabled or locked out.
This is accomplished when a set interrupt lockout (SIL) instruction is executed.
The SIL instruc'tion locks out all interrupts except fault interrupts. A set
external interrupt lockout (SXL) instruction is also provided. This instruction
locks out only external interrupts. Two corresponding instructions (RIL and RXL)
are provided to remove these interrupt lockout conditions. A lockout condition
prevents the transfer of program control; however, it does not inhibit the
occurrence of the interrupt or the setting of, the interrupt flip-flop. Therefore,
after a lockout condition is removed, interrupts which occurred but were
not honored during the lockout are honored according to the priority sequence.
If two interrupts of the same type occur during a lockout, the second is ignored.
Interrupts can be classified as channel interrupts and special interrupts.
Channel interrupts are associateq with a particular I/O channel. They are
either external interrupts received on the input cable of an I/O channel or
buffer monitor interrupts generated internally by the I/O section of the computerg
Special interrupts are not associated with any particular channel.
They provide special-purpose interrupt capability.
2.2.1 CHANNEL INTERRUPTS
1) External Interrupts.
External interrupts originate in equipment outside of the computer and
are transmitted to the computer through an I/O channel. Normally an
interrupt code is associated with an external interrupt. This code can
be used by the computer program to interpret the meaning of the interrupt.
The external equipment places the interrupt code on the input data lines
and sets the external interrupt request line. During the I/O priority
scan, the computer senses the external interrupt request, stores the
interrupt code at the external interrupt entrance address plus one, and
transfers program control to the external interrupt entrance address.
2) Buffer Monitor Interrupts.
Input data, output data, and external function buffers can be originated
with or without monitor. When a buffer is initiated with monitor, an
interrupt is generated when the buffer terminates. The interrupt, which
is generated internally by the I/O section of the computer, transfers
program control to the interrupt entrance address for that channel and
buffer type.
2.2.2 SPECIAL INTERRUPTS
1) Fault Interrupts.
Fault interrupts indicate program faults. A fault interrupt is generated
when an instruction with an illegal function code is encountered in the
program. Illegal function codes are 00, 01, 77, 5000. 5001, and 5077.
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