82801CA Datasheet, PDF (116/521 Pages) Intel Corporation – I/O Controller Hub 3-S (ICH3-S)

Functional Description
5.8.3.3 Bus Message Formats
After bus arbitration, the winner is granted exclusive use of the bus and will drive its message.
APIC messages come in four formats, determined by the delivery mode bits. These four messages
are of different length, and are known by all APICs on the bus through the transmission of the
delivery mode bits:
Table 5-19. APIC Message Formats
Message
EOI
Short
Lowest Priority
Remote Read
# of
Cycles
14
21
33
39
Delivery Mode
Bits
Comments
xxx
001, 010, 100,
101, 111
001
011
End of Interrupt transmission from Local APIC to I/O APIC
on Level interrupts. EOI is known by the EOI bit at the start
of arbitration
I/O APIC delivery on Fixed, NMI, SMI, Reset, ExtINT, and
Lowest Priority with focus processor messages
Transmission of Lowest Priority interrupts when the status
field indicates that the processor does not have focus
Message from one Local APIC to another to read registers.
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
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Intel^® 82801CA ICH3-S Datasheet