U UNIVAC 1218 - Bitsavers
U UNIVAC 1218 - Bitsavers
U UNIVAC 1218 - Bitsavers
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The location (CAT) is specified by the contents of the address in the lower 12<br />
bits of the instruction. By using this method the programmer may load an<br />
interrupt processing routine at CAT+l and terminate this routine with an indirect<br />
jump on CAT to return program control to the interrupted program. Most<br />
interrupts are optional in the sense that they can be disabled or locked out.<br />
This is accomplished when a set interrupt lockout (SIL) instruction is executed.<br />
The SIL instruc'tion locks out all interrupts except fault interrupts. A set<br />
external interrupt lockout (SXL) instruction is also provided. This instruction<br />
locks out only external interrupts. Two corresponding instructions (RIL and RXL)<br />
are provided to remove these interrupt lockout conditions. A lockout condition<br />
prevents the transfer of program control; however, it does not inhibit the<br />
occurrence of the interrupt or the setting of, the interrupt flip-flop. Therefore,<br />
after a lockout condition is removed, interrupts which occurred but were<br />
not honored during the lockout are honored according to the priority sequence.<br />
If two interrupts of the same type occur during a lockout, the second is ignored.<br />
Interrupts can be classified as channel interrupts and special interrupts.<br />
Channel interrupts are associateq with a particular I/O channel. They are<br />
either external interrupts received on the input cable of an I/O channel or<br />
buffer monitor interrupts generated internally by the I/O section of the computerg<br />
Special interrupts are not associated with any particular channel.<br />
They provide special-purpose interrupt capability.<br />
2.2.1 CHANNEL INTERRUPTS<br />
1) External Interrupts.<br />
External interrupts originate in equipment outside of the computer and<br />
are transmitted to the computer through an I/O channel. Normally an<br />
interrupt code is associated with an external interrupt. This code can<br />
be used by the computer program to interpret the meaning of the interrupt.<br />
The external equipment places the interrupt code on the input data lines<br />
and sets the external interrupt request line. During the I/O priority<br />
scan, the computer senses the external interrupt request, stores the<br />
interrupt code at the external interrupt entrance address plus one, and<br />
transfers program control to the external interrupt entrance address.<br />
2) Buffer Monitor Interrupts.<br />
Input data, output data, and external function buffers can be originated<br />
with or without monitor. When a buffer is initiated with monitor, an<br />
interrupt is generated when the buffer terminates. The interrupt, which<br />
is generated internally by the I/O section of the computer, transfers<br />
program control to the interrupt entrance address for that channel and<br />
buffer type.<br />
2.2.2 SPECIAL INTERRUPTS<br />
1) Fault Interrupts.<br />
Fault interrupts indicate program faults. A fault interrupt is generated<br />
when an instruction with an illegal function code is encountered in the<br />
program. Illegal function codes are 00, 01, 77, 5000. 5001, and 5077.<br />
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