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參數(shù)資料
| 型號： | KU386 |
| 廠商： | Intel Corp. |
| 英文描述： | SX MICROPROCESSOR |
| 中文描述： | SX微處理器 |
| 文件頁數(shù)： | 26/102頁 |
| 文件大?。?/td> | 1268K |
| 代理商： | KU386 |
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Intel386
TM
SX MICROPROCESSOR
Each of the tables has a register associated with it:
GDTR, LDTR, and IDTR; see Figure 2.1. The LGDT,
LLDT, and LIDT instructions load the base and limit
of the Global, Local, and Interrupt Descriptor Tables
into the appropriate register. The SGDT, SLDT, and
SIDT store the base and limit values. These are priv-
ileged instructions.
Global Descriptor Table
The Global Descriptor Table (GDT) contains de-
scriptors which are available to all of the tasks in a
system. The GDT can contain any type of segment
descriptor except for interrupt and trap descriptors.
Every Intel386 SX CPU system contains a GDT.
The first slot of the Global Descriptor Table corre-
sponds to the null selector and is not used. The null
selector defines a null pointer value.
Local Descriptor Table
LDTs contain descriptors which are associated with
a given task. Generally, operating systems are de-
signed so that each task has a separate LDT. The
LDT may contain only code, data, stack, task gate,
and call gate descriptors. LDTs provide a mecha-
nism for isolating a given task’s code and data seg-
ments from the rest of the operating system, while
the GDT contains descriptors for segments which
are common to all tasks. A segment cannot be ac-
cessed by a task if its segment descriptor does not
exist in either the current LDT or the GDT. This pro-
vides both isolation and protection for a task’s seg-
ments while still allowing global data to be shared
among tasks.
Unlike the 6-byte GDT or IDT registers which contain
a base address and limit, the visible portion of the
LDT register contains only a 16-bit selector. This se-
lector refers to a Local Descriptor Table descriptor in
the GDT (see figure 2.1).
Interrupt Descriptor Table
The third table needed for Intel386 SX Microproces-
sor systems is the Interrupt Descriptor Table. The
IDT contains the descriptors which point to the loca-
tion of the up to 256 interrupt service routines. The
IDT may contain only task gates, interrupt gates, and
trap gates. The IDT should be at least 256 bytes in
size in order to hold the descriptors for the 32 Intel
Reserved Interrupts. Every interrupt used by a sys-
tem must have an entry in the IDT. The IDT entries
are referenced by INT instructions, external interrupt
vectors, and exceptions.
DESCRIPTORS
The object to which the segment selector points to
is called a descriptor. Descriptors are eight byte
quantities which contain attributes about a given re-
gion of linear address space. These attributes in-
clude the 32-bit base linear address of the segment,
the 20-bit length and granularity of the segment, the
protection level, read, write or execute privileges,
the default size of the operands (16-bit or 32-bit),
and the type of segment. All of the attribute informa-
tion about a segment is contained in 12 bits in the
segment descriptor. Figure 4.4 shows the general
format of a descriptor. All segments on the Intel386
SX Microprocessor have three attribute fields in
common: the P bit, the DPL bit, and the S bit. The P
31
0
BYTE
ADDRESS
SEGMENT BASE 15 . . . 0
SEGMENT LIMIT 15 . . . 0
0
BASE 31 . . . 24
G
D
0
AVL
LIMIT
19 . . . 16
P
DPL
S
TYPE
A
BASE
23 . . . 16
a
4
BASE
LIMIT
P
DPL
S
TYPE
A
G
D
0
AVL
Base Address of the segment
The length of the segment
Present Bit
1
e
Present
Descriptor Privilege Level 0–3
Segment Descriptor
Type of Segment
Accessed Bit
Granularity Bit
Default Operation Size (recognized in code segment descriptors only)
Bit must be zero (0) for compatibility with future processors
Available field for user or OS
0
e
Not Present
0
e
System Descriptor
1
e
Code or Data Segment Descriptor
1
e
Segment length is page granular
0
e
Segment length is byte granular
1
e
32-bit segment
0
e
16-bit segment
Figure 4.4. Segment Descriptors
26
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