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5-8 Vol. 3
PROTECTION
The processor examines type information at various times while operating on
segment selectors and segment descriptors. The following list gives examples of
typical operations where type checking is performed (this list is not exhaustive):
• When a segment selector is loaded into a segment register — Certain
segment registers can contain only certain descriptor types, for example:
— The CS register only can be loaded with a selector for a code segment.
— Segment selectors for code segments that are not readable or for system
segments cannot be loaded into data-segment registers (DS, ES, FS, and
GS).
— Only segment selectors of writable data segments can be loaded into the SS
register.
• When a segment selector is loaded into the LDTR or task register — For example:
— The LDTR can only be loaded with a selector for an LDT.
— The task register can only be loaded with a segment selector for a TSS.
• When instructions access segments whose descriptors are already
loaded into segment registers — Certain segments can be used by instruc
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tions only in certain predefined ways, for example:
— No instruction may write into an executable segment.
— No instruction may write into a data segment if it is not writable.
— No instruction may read an executable segment unless the readable flag is
set.
• When an instruction operand contains a segment selector — Certain
instructions can access segments or gates of only a particular type, for example:
— A far CALL or far JMP instruction can only access a segment descriptor for a
conforming code segment, nonconforming code segment, call gate, task
gate, or TSS.
— The LLDT instruction must reference a segment descriptor for an LDT.
— The LTR instruction must reference a segment descriptor for a TSS.
— The LAR instruction must reference a segment or gate descriptor for an LDT,
TSS, call gate, task gate, code segment, or data segment.
— The LSL instruction must reference a segment descriptor for a LDT, TSS, code
segment, or data segment.
— IDT entries must be interrupt, trap, or task gates.
• During certain internal operations — For example:
— On a far call or far jump (executed with a far CALL or far JMP instruction), the
processor determines the type of control transfer to be carried out (call or
jump to another code segment, a call or jump through a gate, or a task
switch) by checking the type field in the segment (or gate) descriptor pointed
to by the segment (or gate) selector given as an operand in the CALL or JMP