Intel 64 and IA-32 Architectures Software Developers Manual Volume 3B, System Programming Guide Part 2

Vol. 3 F-1
APPENDIX F
APIC BUS MESSAGE FORMATS
This appendix describes the message formats used when transmitting messages on
the serial APIC bus. The information described here pertains only to the Pentium and
P6 family processors.
F.1 BUS MESSAGE FORMATS
The local and I/O APICs transmit three types of messages on the serial APIC bus: EOI
message, short message, and non-focused lowest priority message. The purpose of
each type of message and its format are described below.
F.2 EOI MESSAGE
Local APICs send 14-cycle EOI messages to the I/O APIC to indicate that a level trig-
gered interrupt has been accepted by the processor. This interrupt, in turn, is a result
of software writing into the EOI register of the local APIC. Table F-1 shows the cycles
in an EOI message.
Table F-1. EOI Message (14 Cycles)
Cycle Bit1 Bit0
11 111 = EOI
2 ArbID3 0 Arbitration ID bits 3 through 0
3ArbID2 0
4ArbID1 0
5ArbID0 0
6 V7 V6 Interrupt vector V7 - V0
7V5V4
8V3V2
9V1V0
10 C C Checksum for cycles 6 - 9
11 0 0
12 A A Status Cycle 0
13 A1 A1 Status Cycle 1
14 0 0 Idle