Intel IA-32 - Manual (Page 385)

Vol. 3A 9-11
PROCESSOR MANAGEMENT AND INITIALIZATION
9.7.1 Real-Address Mode IDT
In real-address mode, the only system data structure that must be loaded into memory is the IDT
(also called the “interrupt vector table”). By default, the address of the base of the IDT is phys-
ical address 0H. This address can be changed by using the LIDT instruction to change the base
address value in the IDTR. Software initialization code needs to load interrupt- and exception-
handler pointers into the IDT before interrupts can be enabled.
The actual interrupt- and exception-handler code can be contained either in EPROM or RAM;
however, the code must be located within the 1-MByte addressable range of the processor in
real-address mode. If the handler code is to be stored in RAM, it must be loaded along with the
IDT.
9.7.2 NMI Interrupt Handling
The NMI interrupt is always enabled (except when multiple NMIs are nested). If the IDT and
the NMI interrupt handler need to be loaded into RAM, there will be a period of time following
hardware reset when an NMI interrupt cannot be handled. During this time, hardware must
provide a mechanism to prevent an NMI interrupt from halting code execution until the IDT and
the necessary NMI handler software is loaded. Here are two examples of how NMIs can be
handled during the initial states of processor initialization:
•
A simple IDT and NMI interrupt handler can be provided in EPROM. This allows an NMI
interrupt to be handled immediately after reset initialization.
•
The system hardware can provide a mechanism to enable and disable NMIs by passing the
NMI# signal through an AND gate controlled by a flag in an I/O port. Hardware can clear
the flag when the processor is reset, and software can set the flag when it is ready to handle
NMI interrupts.
9.8 SOFTWARE INITIALIZATION FOR PROTECTED-MODE
OPERATION
The processor is placed in real-address mode following a hardware reset. At this point in the
initialization process, some basic data structures and code modules must be loaded into physical
memory to support further initialization of the processor, as described in Section 9.7, “Software
Initialization for Real-Address Mode Operation.” Before the processor can be switched to
protected mode, the software initialization code must load a minimum number of protected
mode data structures and code modules into memory to support reliable operation of the
processor in protected mode. These data structures include the following:
•
A IDT.
•
A GDT.
•
A TSS.
•
(Optional) An LDT.