Clearing The In-Service Bits: Non-Specific End-Of-Interrupt - Intel 80C188EC User Manual

More than one In-Service bit can be set concurrently. Consider the case in which a low priority
interrupt handler is interrupted by a higher-priority interrupt request (the interrupts are nested).
The In-Service bits for both interrupt sources are set when the higher-priority interrupt is ac-
knowledged.
Setting the In-Service bit for an IR line inhibits (masks) further interrupts from that IR line and
all IR lines of a lower priority when the 8259A module is programmed for fully nested operation.
For example, if the 8259A module is programmed for default priority (IR0 highest) and the IR4
In-Service bit is set, then no interrupts are possible from IR4 through IR7 until the In-Service bit
is reset.
The default masking of interrupts by the In-Service Register can be circumvented by using either
Special Fully Nested Mode or Special Mask Mode (described below).
The In-Service bits are cleared by an End-of-Interrupt (EOI) command. The EOI command can
either be sent to the 8259A module by the CPU or be generated automatically by the 8259A mod-
ule itself.
8.3.4.1

Clearing the In-Service Bits: Non-Specific End-Of-Interrupt

The Non-Specific End-of-Interrupt (EOI) command instructs the 8259A module to reset the high-
est-priority In-Service bit. When the 8259A module is operating in Fully Nested Mode, the high-
est-priority In-Service bit always corresponds to the interrupt handler in progress; the 8259A
module does not need to be told explicitly which handler is ending. The Non-Specific EOI is a
shortcut for systems that use the fully nested interrupt structure.
8.3.4.2 Clearing the In-Service Bits: Specific End-Of-Interrupt
Some operating modes of the 8259A module do not use the fully nested interrupt structure. In
these alternate modes, a lower-priority interrupt request can interrupt a higher-priority handler. If
a Non-Specific EOI is issued in this case, the highest-priority In-Service bit is reset even though
the handler for that interrupt has not completed execution. The Specific End-of-Interrupt
(EOI) command instructs the 8259A module to reset a specific bit in the In-Service Register. Sys-
tems that are not using Fully Nested Mode must issue a Specific EOI command to ensure that the
proper In-Service bit is cleared.
8.3.4.3 Automatic End-Of-Interrupt Mode
The 8259A module can be programmed to clear the In-Service Bit for an IR line on the rising
edge of the second INTA pulse of the interrupt acknowledge cycle. When Automatic End-of-In-
terrupt (EOI) Mode is selected, the In-Service bit for any given IR line is set only between the
falling edge of the first INTA pulse and the rising edge of the second INTA pulse.
INTERRUPT CONTROL UNIT
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