Processor Programming (Continued)
3.5.2.3 Segment Mechanism in Protected Mode
The segment mechanism in protected mode is more com-
plex. Basically as in real and virtual 8086 modes the offset
address is added to the segment base address to pro-
duce a linear address (Figure 3-5). However, the calcula-
tion of the segment base address is based on the
contents of descriptor tables.
divided in to three fields: the RPL, TI and INDEX fields as
shown in Figure 3-6 on page 68.
The segments are assigned permission levels to prevent
application program errors from disrupting operating pro-
grams. The Requested Privilege Level (RPL) determines
the effective privilege level of an instruction. RPL = 0 indi-
cates the most privileged level, and RPL = 3 indicates the
least privileged level. Refer to Section 3.9 “Protection” on
page 91.
If paging is enabled the linear address is further pro-
cessed by the paging mechanism.
Descriptor tables hold descriptors that allow management
of segments and tables in address space while in pro-
tected mode. The Table Indicator Bit (TI) in the selector
selects either the General Descriptor Table (GDT) or one
Local Descriptor Table (LDT). If TI = 0, GDT is selected; if
TI =1, LDT is selected. The 13-bit INDEX field in the seg-
ment selector is used to index a GDT or LDT.
A more detailed look at the segment mechanisms for real
and virtual 8086 modes and protected modes is illustrated
in Figure 3-6 on page 68. In protected mode, the segment
selector is cached. This is illustrated in Figure 3-7 on page
69.
3.5.2.4 Segment Selectors
The segment registers are used to store segment selec-
tors. In protected mode, the segment selectors are
32
Offset Address
Offset Mechanism
Linear
Address
Physical
Memory
Address
32
Optional
Paging Mechanism
32
Segment Base
Address
32
Selector Mechanism
Figure 3-5. Protected Mode Address Calculation
Revision 1.1
67
www.national.com
NSC [ National Semiconductor ]
