Intel IA-32 - Manual (Page 83)

Vol. 3A 3-3
PROTECTED-MODE MEMORY MANAGEMENT
If the page being accessed is not currently in physical memory, the processor interrupts execu-
tion of the program (by generating a page-fault exception). The operating system or executive
then reads the page into physical memory from the disk and continues executing the program.
When paging is implemented properly in the operating-system or executive, the swapping of
pages between physical memory and the disk is transparent to the correct execution of a
program. Even programs written for 16-bit IA-32 processors can be paged (transparently) when
they are run in virtual-8086 mode.
3.2 USING SEGMENTS
The segmentation mechanism supported by the IA-32 architecture can be used to implement a
wide variety of system designs. These designs range from flat models that make only minimal
use of segmentation to protect programs to multi-segmented models that employ segmentation
to create a robust operating environment in which multiple programs and tasks can be executed
reliably.
The following sections give several examples of how segmentation can be employed in a system
to improve memory management performance and reliability.
3.2.1 Basic Flat Model
The simplest memory model for a system is the basic “flat model,” in which the operating
system and application programs have access to a continuous, unsegmented address space. To
the greatest extent possible, this basic flat model hides the segmentation mechanism of the archi-
tecture from both the system designer and the application programmer.
To implement a basic flat memory model with the IA-32 architecture, at least two segment
descriptors must be created, one for referencing a code segment and one for referencing a data
segment (see Figure 3-2). Both of these segments, however, are mapped to the entire linear
address space: that is, both segment descriptors have the same base address value of 0 and the
same segment limit of 4 GBytes. By setting the segment limit to 4 GBytes, the segmentation
mechanism is kept from generating exceptions for out of limit memory references, even if no
physical memory resides at a particular address. ROM (EPROM) is generally located at the top
of the physical address space, because the processor begins execution at FFFF_FFF0H. RAM
(DRAM) is placed at the bottom of the address space because the initial base address for the DS
data segment after reset initialization is 0.
3.2.2 Protected Flat Model
The protected flat model is similar to the basic flat model, except the segment limits are set to
include only the range of addresses for which physical memory actually exists (see Figure 3-3).
A general-protection exception (#GP) is then generated on any attempt to access nonexistent
memory. This model provides a minimum level of hardware protection against some kinds of
program bugs.