INTEL386 Просмотр технического описания (PDF) - Intel

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INTEL386

Intel386 SX MICROPROCESSOR

Intel 

Intel386TM SX MICROPROCESSOR

Figure 4 7 Example Descriptor Selection

240187 – 12

4 3 Protection

The Intel386 SX Microprocessor has four levels of

protection which are optimized to support a multi-

tasking operating system and to isolate and protect

user programs from each other and the operating

system The privilege levels control the use of privi-

leged instructions I O instructions and access to

segments and segment descriptors The Intel386 SX

Microprocessor also offers an additional type of pro-

tection on a page basis when paging is enabled

The four-level hierarchical privilege system is an ex-

tension of the user supervisor privilege mode com-

monly used by minicomputers The user supervisor

mode is fully supported by the Intel386 SX Micro-

processor paging mechanism The privilege levels

(PL) are numbered 0 through 3 Level 0 is the most

privileged level

RULES OF PRIVILEGE

The Intel386 SX Microprocessor controls access to

both data and procedures between levels of a task

according to the following 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 segment procedure 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 Intel386 SX Micro-

processor always executes at one of the four privi-

lege levels The Current Privilege Level (CPL) speci-

fies what the task’s privilege level is A task’s CPL

may only be changed by control transfers through

gate descriptors to a code segment with a different

privilege level Thus an application program running

at PLe3 may call an operating system routine at

PLe1 (via a gate) which would cause the task’s CPL

to be set to 1 until the operating system routine was

finished

Selector Privilege (RPL)

The privilege level of a selector is specified by the

RPL field The selector’s RPL is only used to estab-

lish a less trusted privilege level than the current

privilege level of the task for the use of a segment

This level is called the task’s effective privilege level

(EPL) The EPL is defined as being the least privi-

leged (numerically larger) level of a task’s CPL and a

selector’s RPL The RPL is most commonly used to

verify that pointers passed to an operating system

procedure do not access data that is of higher privi-

lege than the procedure that originated the pointer

Since the originator of a selector can specify any

RPL value the Adjust RPL (ARPL) instruction is pro-

vided to force the RPL bits to the originator’s CPL

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