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Selectors in the 80386

A segment selector is loaded into a segment register (cs, ds, etc.) to select one of the regular segments in the system as the one addressed via that segment register.

Segment selector Format:

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TI
Table indicator:
0 means selector indexes into GDT
1 means selector indexes into LDT
RPL
Privelege level. uses only two privelege levels.
0 means kernel
3 means user

Examples:

Kernel code segment

TI=0, index=1, RPL=0, therefore selector = 0x08 (GDT[1])
User data segment

TI=1, index=2, RPL=3, therefore selector = 0x17 (LDT[2])

Selectors used in :

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Selectors for system segments are not to be loaded directly into segment registers. Instead one must load the TR or LDTR.

On entry into syscall:



Converted on:
Mon Apr 1 10:20:16 EST 1996