Segment Descriptors - Intel Quark SoC X1000 Core Developer's Manual

Protected Mode Architecture—Intel
granularity is unrelated to paging. A Intel
of segments with byte granularity and page granularity, whether or not paging is
enabled.
The executable (E) bit tells if a segment is a code or data segment. A code segment
(E=1, S=1) may be execute-only or execute/read as determined by the Read (R) bit.
Code segments are execute-only if R=0, and execute/read if R=1. Code segments may
never be written to.
Note: Code segments can be modified via aliases. Aliases are writeable data segments that
occupy the same range of linear address space as the code segment.
The D bit indicates the default length for operands and effective addresses. If D=1, 32-
bit operands and 32-bit addressing modes are assumed. When D=0, 16-bit operands
and 16-bit addressing modes are assumed.
Another attribute of code segments is determined by the conforming (C) bit.
Conforming segments, indicated when C=1, can be executed and shared by programs
at different privilege levels (see
Figure 28.

Segment Descriptors

31
Base 31...24
BASE
LIMIT
P
DPL
S
TYPE
A
G
D
0
AVL
Note:
October 2013
Order Number: 329679-001US
®
Quark Core
Section
Segment Base 15...0
Limit
G D 0 AVL
19...16
Base Address of the segment
The length of the segment
Present Bit 1=Present, 0=Not Present
Descriptor Privilege Level 0–3
Segment Descriptor 0=System Descriptor, 1=Code or Data Segment Descriptor
Type of Segment
Accessed Bit
Granularity Bit 1=Segment length is page granular, 0=Segment length is byte granular
Default Operation Size (recognized in code segment descriptors only)
1=32-bit segment, 0=16-bit segment
Bit must be zero (0) for compatibility with future processors
Available field for user or OS
In a maximum-size segment (i.e., a segment with G=1 and segment limit 19...0=FFFFFH), the
lowest 12 bits of the segment base should be zero (i.e., segment base 11...000=000H).
®
Quark SoC X1000 Core system can consist
6.3).
Segment Limit 15...0
DPL Type
P S A
0 Byte
Address
0
+4
Base 23...16
®
Intel Quark SoC X1000 Core
Developer's Manual
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