Intel IA-32 - Manual (Page 82)

3-2 Vol. 3A
PROTECTED-MODE MEMORY MANAGEMENT
If paging is not used, the linear address space of the processor is mapped directly into the phys-
ical address space of processor. The physical address space is defined as the range of addresses
that the processor can generate on its address bus.
Because multitasking computing systems commonly define a linear address space much larger
than it is economically feasible to contain all at once in physical memory, some method of
“virtualizing” the linear address space is needed. This virtualization of the linear address space
is handled through the processor’s paging mechanism.
Paging supports a “virtual memory” environment where a large linear address space is simulated
with a small amount of physical memory (RAM and ROM) and some disk storage. When using
paging, each segment is divided into pages (typically 4 KBytes each in size), which are stored
either in physical memory or on the disk. The operating system or executive maintains a page
directory and a set of page tables to keep track of the pages. When a program (or task) attempts
to access an address location in the linear address space, the processor uses the page directory
and page tables to translate the linear address into a physical address and then performs the
requested operation (read or write) on the memory location.
Figure 3-1. Segmentation and Paging
Global Descriptor
Table (GDT)
Linear Address
Space
Segment
Segment
Descriptor
Offset
Logical Address
Segment
Base Address
Page
Phy. Addr.
Lin. Addr.
Segment
Selector
Dir Table Offset
Linear Address
Page Table
Page Directory
Entry
Physical
Space
Entry
(or Far Pointer)
Paging Segmentation
Address
Page