Intel IA-32 - Manual (Page 386)

9-12 Vol. 3A
PROCESSOR MANAGEMENT AND INITIALIZATION
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If paging is to be used, at least one page directory and one page table.
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A code segment that contains the code to be executed when the processor switches to
protected mode.
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One or more code modules that contain the necessary interrupt and exception handlers.
Software initialization code must also initialize the following system registers before the
processor can be switched to protected mode:
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The GDTR.
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(Optional.) The IDTR. This register can also be initialized immediately after switching to
protected mode, prior to enabling interrupts.
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Control registers CR1 through CR4.
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(Pentium 4, Intel Xeon, and P6 family processors only.) The memory type range registers
(MTRRs).
With these data structures, code modules, and system registers initialized, the processor can be
switched to protected mode by loading control register CR0 with a value that sets the PE flag
(bit 0).
9.8.1 Protected-Mode System Data Structures
The contents of the protected-mode system data structures loaded into memory during software
initialization, depend largely on the type of memory management the protected-mode operating-
system or executive is going to support: flat, flat with paging, segmented, or segmented with
paging.
To implement a flat memory model without paging, software initialization code must at a
minimum load a GDT with one code and one data-segment descriptor. A null descriptor in the
first GDT entry is also required. The stack can be placed in a normal read/write data segment,
so no dedicated descriptor for the stack is required. A flat memory model with paging also
requires a page directory and at least one page table (unless all pages are 4 MBytes in which case
only a page directory is required). See Section 9.8.3, “Initializing Paging.”
Before the GDT can be used, the base address and limit for the GDT must be loaded into the
GDTR register using an LGDT instruction.
A multi-segmented model may require additional segments for the operating system, as well as
segments and LDTs for each application program. LDTs require segment descriptors in the
GDT. Some operating systems allocate new segments and LDTs as they are needed. This
provides maximum flexibility for handling a dynamic programming environment. However,
many operating systems use a single LDT for all tasks, allocating GDT entries in advance. An
embedded system, such as a process controller, might pre-allocate a fixed number of segments
and LDTs for a fixed number of application programs. This would be a simple and efficient way
to structure the software environment of a real-time system.