Intel IA-32 - Manual (Page 336)

8-12 Vol. 3A
ADVANCED PROGRAMMABLE INTERRUPT CONTROLLER (APIC)
8.4.6 Local APIC ID
At power up, system hardware assigns a unique APIC ID to each local APIC on the system bus
(for Pentium 4 and Intel Xeon processors) or on the APIC bus (for P6 family and Pentium
processors). The hardware assigned APIC ID is based on system topology and includes
encoding for socket position and cluster information (see Figure 7-2).
In MP systems, the local APIC ID is also used as a processor ID by the BIOS and the operating
system. Some processors permit software to modify the APIC ID. However, the ability of soft-
ware to modify the APIC ID is processor model specific. Because of this, operating system soft-
ware should avoid writing to the local APIC ID register. The value returned by bits 31-24 of the
EBX register (when the CPUID instruction is executed with a source operand value of 1 in the
EAX register) is always the Initial APIC ID (determined by the platform initialization). This is
true even if software has changed the value in the Local APIC ID register.
The processor receives the hardware assigned APIC ID (or Initial APIC ID) by sampling pins
A11# and A12# and pins BR0# through BR3# (for the Pentium 4, Intel Xeon, and P6 family
processors) and pins BE0# through BE3# (for the Pentium processor). The APIC ID latched
from these pins is stored in the APIC ID field of the local APIC ID register (see Figure 8-6), and
is used as the Initial APIC ID for the processor.
For the P6 family and Pentium processors, the local APIC ID field in the local APIC ID register
is 4 bits. Encodings 0H through EH can be used to uniquely identify 15 different processors
connected to the APIC bus. For the Pentium 4 and Intel Xeon processors, the xAPIC specifica-
tion extends the local APIC ID field to 8 bits. These can be used to identify up to 255 processors
in the system.
8.4.7 Local APIC State
The following sections describe the state of the local APIC and its registers following a power-
up or RESET, after the local APIC has been software disabled, following an INIT reset, and
following an INIT-deassert message.
Figure 8-6. Local APIC ID Register
31 0 23 24
Reserved APIC ID*
Address: 0FEE0 0020H
Value after reset: 0000 0000H
* For the P6 family and Pentium processors,
bits 28-31 are reserved. For Pentium 4
and Xeon processors, 21-31 are reserved.