Intel 64 and IA-32 Architectures Software Developers Manual Volume 1, Basic Architecture

Vol. 1 3-11

BASIC EXECUTION ENVIRONMENT

• Real-address mode — When in real-address mode, the processor only supports

the real-address mode memory model.

• System management mode — When in SMM, the processor switches to a

separate address space, called the system management RAM (SMRAM). The

memory model used to address bytes in this address space is similar to the real-

address mode model. See Chapter 24, “System Management,” in the Intel® 64

and IA-32 Architectures Software Developer’s Manual, Volume 3B, for more

information on the memory model used in SMM.

• Compatibility mode — Software that needs to run in compatibility mode should

observe the same memory model as those targeted to run in 32-bit protected

mode. The effect of segmentation is the same as it is in 32-bit protected mode

semantics.

• 64-bit mode — Segmentation is generally (but not completely) disabled,

creating a flat 64-bit linear-address space. Specifically, the processor treats the

segment base of CS, DS, ES, and SS as zero in 64-bit mode (this makes a linear

address equal an effective address). Segmented and real address modes are not

available in 64-bit mode.

3.3.5 32-Bit and 16-Bit Address and Operand Sizes

IA-32 processors in protected mode can be configured for 32-bit or 16-bit address

and operand sizes. With 32-bit address and operand sizes, the maximum linear

address or segment offset is FFFFFFFFH (2

-1); operand sizes are typically 8 bits or

32 bits. With 16-bit address and operand sizes, the maximum linear address or

segment offset is FFFFH (2

-1); operand sizes are typically 8 bits or 16 bits.

When using 32-bit addressing, a logical address (or far pointer) consists of a 16-bit

segment selector and a 32-bit offset; when using 16-bit addressing, an address

consists of a 16-bit segment selector and a 16-bit offset.

Instruction prefixes allow temporary overrides of the default address and/or operand

sizes from within a program.

When operating in protected mode, the segment descriptor for the currently

executing code segment defines the default address and operand size. A segment

descriptor is a system data structure not normally visible to application code. Assem-

bler directives allow the default addressing and operand size to be chosen for a

program. The assembler and other tools then set up the segment descriptor for the

code segment appropriately.

When operating in real-address mode, the default addressing and operand size is 16

bits. An address-size override can be used in real-address mode to enable 32-bit

addressing. However, the maximum allowable 32-bit linear address is still 000FFFFFH

-1).