376 datasheet(27/95 Pages) INTEL

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376 EMBEDDED PROCESSOR

SEGMENT BASE 150

SEGMENT LIMIT 150

BASE

G000

LIMIT

DPL

TYPE

BASE

3124

1916

2316

Type

Defines

Type

Defines

Invalid

Reserved

Available 8037680386 TSS

LDT

Undefined (Intel Reserved)

Reserved

Busy 8037680386 TSS

Reserved

8037680386 Call Gate

Task Gate (8037680386 Task)

Undefined (Intel Reserved)

Reserved

8037680386 Interrupt Gate

Reserved

8037680386 Trap Gate

Figure 35 System Descriptors

240182 – 13

Figure 36 Example Descriptor Selection

33 Protection

The 80376 offers extensive protection features

These protection features are particularly useful in

sophisticated embedded applications which use

multitasking real-time operating systems For sim-

pler embedded applications these protection capa-

bilities can be easily bypassed by making all applica-

tions run at privilege level (PL) 0

RULES OF PRIVILEGE

The 80376 controls access to both data and proce-

dures between levels of a task according to the fol-

lowing rules

Data stored in a segment with privilege level p

can be accessed only by code executing at a

privilege level at least as privileged as p

A code segmentprocedure with privilege level p

can only be called by a task executing at the

same or a lesser privilege level than p

PRIVILEGE LEVELS

At any point in time a task on the 80376 always

executes at one of the four privilege levels The Cur-

rent Privilege Level (CPL) specifies what the task’s

privilege level is A task’s CPL may only be changed

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