Intel ARCHITECTURE IA-32 User Manual download pdf (Page 93)

Vol. 3A 3-13

PROTECTED-MODE MEMORY MANAGEMENT

segment limit has the reverse function; the offset can range from the segment

limit to FFFFFFFFH or FFFFH, depending on the setting of the B flag. Offsets

less than the segment limit generate general-protection exceptions. Decreasing

the value in the segment limit field for an expand-down segment allocates new

memory at the bottom of the segment's address space, rather than at the top.

IA-32 architecture stacks always grow downwards, making this mechanism

convenient for expandable stacks.

Base address fields

Defines the location of byte 0 of the segment within the 4-GByte linear address

space. The processor puts together the three base address fields to form a single

32-bit value. Segment base addresses should be aligned to 16-byte boundaries.

Although 16-byte alignment is not required, this alignment allows programs to

maximize performance by aligning code and data on 16-byte boundaries.

Type field Indicates the segment or gate type and specifies the kinds of access that can be

made to the segment and the direction of growth. The interpretation of this field

depends on whether the descriptor type flag specifies an application (code or

data) descriptor or a system descriptor. The encoding of the type field is

different for code, data, and system descriptors (see Figure 4-1). See Section

3.4.5.1, “Code- and Data-Segment Descriptor Types”, for a description of how

this field is used to specify code and data-segment types.

S (descriptor type) flag

Specifies whether the segment descriptor is for a system segment (S flag is

clear) or a code or data segment (S flag is set).

DPL (descriptor privilege level) field

Specifies the privilege level of the segment. The privilege level can range from

0 to 3, with 0 being the most privileged level. The DPL is used to control access

to the segment. See Section 4.5, “Privilege Levels”, for a description of the

relationship of the DPL to the CPL of the executing code segment and the RPL

of a segment selector.

P (segment-present) flag

Indicates whether the segment is present in memory (set) or not present (clear).

If this flag is clear, the processor generates a segment-not-present exception

(#NP) when a segment selector that points to the segment descriptor is loaded

into a segment register. Memory management software can use this flag to

control which segments are actually loaded into physical memory at a given

time. It offers a control in addition to paging for managing virtual memory.

Figure 3-9 shows the format of a segment descriptor when the segment-present

flag is clear. When this flag is clear, the operating system or executive is free

to use the locations marked “Available” to store its own data, such as informa-

tion regarding the whereabouts of the missing segment.

D/B (default operation size/default stack pointer size and/or upper bound) flag

Performs different functions depending on whether the segment descriptor is

an executable code segment, an expand-down data segment, or a stack

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