User's Manual

Vol. 3 19-31
ARCHITECTURE COMPATIBILITY
19.26.3 IDT Limit
The LIDT instruction can be used to set a limit on the size of the IDT. A double-fault
exception (#DF) is generated if an interrupt or exception attempts to read a vector
beyond the limit. Shutdown then occurs on the 32-bit IA-32 processors if the double-
fault handler vector is beyond the limit. (The 8086 processor does not have a shut
-
down mode nor a limit.)
19.27 ADVANCED PROGRAMMABLE INTERRUPT
CONTROLLER (APIC)
The Advanced Programmable Interrupt Controller (APIC), referred to in this book as
the local APIC, was introduced into the IA-32 processors with the Pentium
processor (beginning with the 735/90 and 815/100 models) and is included in the
Pentium 4, Intel Xeon, and P6 family processors. The features and functions of the
local APIC are derived from the Intel 82489DX external APIC, which was used with
the Intel486 and early Pentium processors. Additional refinements of the local APIC
architecture were incorporated in the Pentium 4 and Intel Xeon processors.
19.27.1 Software Visible Differences Between the Local APIC and
the 82489DX
The following features in the local APIC features differ from those found in the
82489DX external APIC:
• When the local APIC is disabled by clearing the APIC software enable/disable flag
in the spurious-interrupt vector MSR, the state of its internal registers are
unaffected, except that the mask bits in the LVT are all set to block local
interrupts to the processor. Also, the local APIC ceases accepting IPIs except for
INIT, SMI, NMI, and start-up IPIs. In the 82489DX, when the local unit is
disabled, all the internal registers including the IRR, ISR and TMR are cleared and
the mask bits in the LVT are set. In this state, the 82489DX local unit will accept
only the reset deassert message.
• In the local APIC, NMI and INIT (except for INIT deassert) are always treated as
edge triggered interrupts, even if programmed otherwise. In the 82489DX, these
interrupts are always level triggered.
• In the local APIC, IPIs generated through the ICR are always treated as edge
triggered (except INIT Deassert). In the 82489DX, the ICR can be used to
generate either edge or level triggered IPIs.
• In the local APIC, the logical destination register supports 8 bits; in the 82489DX,
it supports 32 bits.
• In the local APIC, the APIC ID register is 4 bits wide; in the 82489DX, it is 8 bits
wide.