GX1 datasheet(74/247 Pages) NSC

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Processor Programming (Continued)

Gate Descriptors

Four kinds of gate descriptors are used to provide protec-

tion during control transfers:

• Call gates

• Trap gates

• Interrupt gates

• Task gates

(For more information on protection refer to Section 3.9

“Protection” on page 91.)

Call Gate Descriptor (CGD). Call gates are used to define

legal entry points to a procedure with a higher privilege

level. Thecallgates areusedbyCALLand JUMP instruc-

tions in much the same manner as code segment descrip-

tors. When a decoded instruction refers to a call gate

descriptor in the GDT or LDT, the call gate is used to point

to another descriptor in the table that defines the destina-

tion code segment.

The following privilege levels are tested during the transfer

through the call gate:

• CPL = Current Privilege Level

• RPL = Segment Selector Field

• DPL = Descriptor Privilege Level in the call gate

descriptor

• DPL = Descriptor Privilege Level in the destination code

segment

The maximum value of the CPL and RPL must be equal or

less than the gate DPL. For a JMP instruction the destina-

tion DPL equals the CPL. For a CALL instruction the desti-

nation DPL is less than or equal to the CPL.

Conforming Code Segments. Transfer to a procedure

with a higher privilege level can also be accomplished by

bypassing the use of call gates, if the requested procedure

is to be executed in a conforming code segment. Conform-

ing code segments have the C bit set in the TYPE field in

their descriptor.

The bit structure and definitions for gate descriptors are

shown in Tables 3-24 and 3-25.

Table 3-24. Gate Descriptors

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10

Memory Offset +4

OFFSET[31:16]

DPL

TYPE

PARAMETERS

Memory Offset +0

SELECTOR[15:0]

OFFSET[15:0]

Table 3-25. Gate Descriptors Bit Definitions

Bit

Memory

Offset

Name

Description

31:16

OFFSET

Offset: Offset used during a call gate to calculate the branch target.

15:0

31:16

SELECTOR

Segment Selector

Segment Present

14:13

DPL

Descriptor Privilege Level

11:8

TYPE

Segment Type:

0100 = 16-bit call gate

1100 = 32-bit call gate

0101 = Task gate

1110 = 32-bit interrupt gate

0110 = 16-bit interrupt gate

1111 = 32-bit trap gate

0111 = 16-bit trap gate

4:0

PARAMETERS

Parameters: Number of parameters to copy from the caller’s stack to the called procedure’s

stack.

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