Ziatech Corporation ZT 8809A, Руководство по эксплуатации

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Interrupt Controller (8259A)
Automatic Rotating Mode
In this mode, the interrupt priority rotates. Once an interrupt on a
given input is serviced, that interrupt assumes the lowest priority.
Thus, if a number of simultaneous interrupts occur, the priority rotates
among the interrupts in numerical order. For example, if interrupts
IR4 and IR6 request service simultaneously, IR4 receives the highest
priority. After service, the priority level rotates so that IR4 has the
lowest priority and IR5 assumes the highest priority. In the worst
case, seven other interrupts are serviced before IR4 again has the
highest priority. Of course, if IR4 is the only request, it is serviced
promptly. The priority shifts when the PIC receives an End-of-
Interrupt (EOI) command.
Specific Rotating Mode
In this mode, the software can change interrupt priority by specifying
the lowest priority, which automatically sets the highest priority. For
example, if IR5 is assigned the lowest priority, IR6 assumes the
highest priority. In specific rotating mode, the priority can be rotated
by writing a Specific Rotate at EOI (SEOI) command to the PIC. This
command contains the BCD code of the interrupt being serviced; that
interrupt is reset as the lowest priority. In addition, the lowest priority
interrupt can be fixed at any time by writing a Command Word to the
appropriate PIC.
Special Mask Mode
One or more of the eight interrupt request inputs can be individually
masked during the PIC initialization or at any subsequent time. If an
interrupt is masked while being serviced, lower priority interrupts are
inhibited. You can enable lower priority interrupts in two ways:
1. Write an End-Of-Interrupt (EOI) command.
2. Set the special mask mode.
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