User Guide

Memory Model 11

24592—Rev. 3.15—November 2009 AMD64 Technology

Figure 2-2. Segment Registers

For details on segmentation and the segment registers, see “Segmented Virtual Memory” in Volume 2.

2.1.3 Physical Memory

Physical memory is the installed memory (excluding cache memory) in a particular computer system

that can be accessed through the processor’s bus interface. The maximum size of the physical memory

space is determined by the number of address bits on the bus interface. In a virtual-memory system, the

large virtual-address space (also called linear-address space) is translated to a smaller physical-

address space by a combination of segmentation and paging hardware and software.

Segmentation is illustrated in Figure 2-1 on page 10. Paging is a mechanism for translating linear

(virtual) addresses into fixed-size blocks called pages, which the operating system can move, as

needed, between memory and external storage media (typically disk). The AMD64 architecture

supports an expanded version of the legacy x86 paging mechanism, one that is able to translate the full

64-bit virtual-address space into the physical-address space supported by the particular

implementation.

2.1.4 Memory Management

Memory management strategies translate addresses generated by programs into addresses in physical

memory using segmentation and/or paging. Memory management is not visible to application

programs. It is handled by the operating system and processor hardware. The following description

gives a very brief overview of these functions. Details are given in “System-Management Instructions”

in Volume 2.

513-312.eps

15 0

(Base only)

(Attributes only)

Legacy Mode and

Compatibility Mode

64-Bit

Mode

ignored