Intel architecture ia-32 User Manual

3-16 Vol. 3A

Stack segments are data segments which must be read/write segments. Loading the SS register
with a segment selector for a nonwritable data segment generates a general-protection exception
(#GP). If the size of a stack segment needs to be changed dynamically, the stack segment can be
an expand-down data segment (expansion-direction flag set). Here, dynamically changing the
segment limit causes stack space to be added to the bottom of the stack. If the size of a stack
segment is intended to remain static, the stack segment may be either an expand-up or expand-
down type.

The accessed bit indicates whether the segment has been accessed since the last time the oper-
ating-system or executive cleared the bit. The processor sets this bit whenever it loads a segment
selector for the segment into a segment register, assuming that the type of memory that contains
the segment descriptor supports processor writes. The bit remains set until explicitly cleared.
This bit can be used both for virtual memory management and for debugging. 

For code segments, the three low-order bits of the type field are interpreted as accessed (A), read
enable (R), and conforming (C). Code segments can be execute-only or execute/read, depending
on the setting of the read-enable bit. An execute/read segment might be used when constants or
other static data have been placed with instruction code in a ROM. Here, data can be read from
the code segment either by using an instruction with a CS override prefix or by loading a
segment selector for the code segment in a data-segment register (the DS, ES, FS, or GS regis-
ters). In protected mode, code segments are not writable.

Code segments can be either conforming or nonconforming. A transfer of execution into a more-
privileged conforming segment allows execution to continue at the current privilege level. A
transfer into a nonconforming segment at a different privilege level results in a general-protec-
tion exception (#GP), unless a call gate or task gate is used (see Section 4.8.1, “Direct Calls or
Jumps to Code Segments”, for more information on conforming and nonconforming code
segments). System utilities that do not access protected facilities and handlers for some types of
exceptions (such as, divide error or overflow) may be loaded in conforming code segments. Util-
ities that need to be protected from less privileged programs and procedures should be placed in
nonconforming code segments. 

Execution cannot be transferred by a call or a jump to a less-privileged
(numerically higher privilege level) code segment, regardless of whether the
target segment is a conforming or nonconforming code segment. Attempting
such an execution transfer will result in a general-protection exception.

All data segments are nonconforming, meaning that they cannot be accessed by less privileged
programs or procedures (code executing at numerically high privilege levels). Unlike code
segments, however, data segments can be accessed by more privileged programs or procedures
(code executing at numerically lower privilege levels) without using a special access gate.

If the segment descriptors in the GDT or an LDT are placed in ROM, the processor can enter an
indefinite loop if software or the processor attempts to update (write to) the ROM-based
segment descriptors. To prevent this problem, set the accessed bits for all segment descriptors
placed in a ROM. Also, remove operating-system or executive code that attempts to modify
segment descriptors located in ROM.