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| 型號： | 376 |
| 廠商： | Intel Corp. |
| 英文描述： | DIODE SCHOTTKY DUAL-DUAL SERIES 30V 200mW 0.38V-vf 200mA-IFM 1mA-IF 0.2uA-IR SOT-363 3K/REEL |
| 中文描述： | 376TM，高性能32位嵌入式處理器 |
| 文件頁數(shù)： | 24/95頁 |
| 文件大小： | 1293K |
| 代理商： | 376 |
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376 EMBEDDED PROCESSOR
3.2 Segmentation
Segmentation is one method of memory manage-
ment and provides the basis for protection in the
80376. Segments are used to encapsulate regions
of memory which have common attributes. For ex-
ample, all of the code of a given program could be
contained in a segment, or an operating system ta-
ble may reside in a segment. All information about
each segment, is stored in an 8-byte data structure
called a descriptor. All of the descriptors in a system
are contained in tables recognized by hardware.
TERMINOLOGY
The following terms are used throughout the discus-
sion of descriptors, privilege levels and protection:
PL:
Privilege Level
DOne of the four hierarchical
privilege levels. Level 0 is the most privileged
level and level 3 is the least privileged.
RPL:
Requestor Privilege Level
DThe privilege
level of the original supplier of the selector.
RPL is determined by the least two significant
bits of a selector.
DPL:
Descriptor Privilege Level
DThis is the least
privileged level at which a task may access
that descriptor (and the segment associated
with that descriptor). Descriptor Privilege Lev-
el is determined by bits 6:5 in the Access
Right Byte of a descriptor.
CPL:
Current Privilege Level
DThe privilege level
at which a task is currently executing, which
equals the privilege level of the code seg-
ment being executed. CPL can also be deter-
mined by examining the lowest 2 bits of the
CS register, except for conforming code seg-
ments.
EPL:
Effective Privilege Level
DThe effective
privilege level is the least privileged of the
RPL and the DPL. EPL is the numerical maxi-
mum of RPL and DPL.
Task: One instance of the execution of a program.
Tasks are also referred to as processes.
DESCRIPTOR TABLES
The descriptor tables define all of the segments
which are used in an 80376 system. There are three
types of tables on the 80376 which hold descriptors:
the Global Descriptor Table, Local Descriptor Table,
and the Interrupt Decriptor Table. All of the tables
are variable length memory arrays, they can range in
size between 8 bytes and 64 Kbytes. Each table can
hold up to 8192 8-byte descriptors. The upper 13
bits of a selector are used as an index into the de-
scriptor table. The tables have registers associated
with them which hold the 32-bit linear base address,
and the 16-bit limit of each table.
Each of the tables have a register associated with it:
GDTR, LDTR and IDTR; see Figure 3.2. 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 these base and limit values. These are
privileged instructions.
240182–12
Figure 3.2. Descriptor Table Registers
Global Descriptor Table
The Global Descriptor Table (GDT) contains de-
scriptors which are possibly available to all of the
tasks in a system. The GDT can contain any type of
segment descriptor except for interrupt and trap de-
scriptors. Every 80376 system contains a GDT. A
simple 80376 system contains only 2 entries in the
GDT; a code and a data descriptor. For maximum
performance, descriptor tables should begin on
even addresses.
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-
24
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| 376 | 制造商:Pro-Signal 功能描述:; Supply Voltage:28VDC; Lamp Base Type:M |
| 37-6.8 | 制造商:Distributed By MCM 功能描述:Resistor 1/2W 6.8 Ohm Flameproof, Bag of 20 制造商:MCM 功能描述:1/2 Watt Flameproof Resistors, Resistance: 6.8 Ohm, Tolerance: 5 %, Quantity: 20 per bag |
| 37-6.8K | 制造商:Distributed By MCM 功能描述:Resistor 1/2W 6.8K Ohm Flameproof, Bag of 20 制造商:MCM 功能描述:RESISTOR 1/2W 6.8K OHM FLAMPRF20 PER BAG |
| 376/377-1ZM | 制造商:Rayovac 功能描述:SILVER ELECTRONIC BTRY 1PK MERCURY FREE |
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