82801DB Datasheet, PDF (116/587 Pages) Intel Corporation – Intel 82801DB I/O Controller Hub 4 (ICH4)

Functional Description
EOI Message for Level Triggered Interrupts
EOI messages are used by local APICs to send an EOI cycle occurring for a level-triggered
interrupt to an I/O APIC. This message is needed so that the I/O APIC can differentiate between a
new interrupt on the interrupt line versus the same interrupt on the interrupt line. The target of the
EOI is given by the local APIC through the transmission of the priority vector (V7 through V0) of
the interrupt. Upon receiving this message, the I/O APIC resets the Remote IRR bit for that
interrupt. If the interrupt signal is still active after the IRR bit is reset, the I/O APIC will treat it as a
new interrupt.
Table 5-20. EOI Message
Cycle
1
2–5
Bit 1
0
ARBID
6
NOT(V7)
7
NOT(V5)
8
NOT(V3)
9
NOT(V1)
10
NOT(C1)
11
1
12
NOT(A)
13
NOT(A1)
14
1
Bit 0
0
1
NOT(V6)
NOT(V4)
NOT(V2)
NOT(V0)
NOT(C0)
1
NOT(A)
NOT(A1)
1
Comments
EOI message
Arbitration ID
Interrupt vector bits V7 - V0 from redirection table
register
Check Sum from Cycles 6–9
Postamble
Status Cycle 0
Status Cycle 1
Idle
116
Intel^® 82801DB ICH4 Datasheet