GXM datasheet(69/244 Pages) NSC

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Processor Programming (Continued)

3.7.5

Gate Descriptors

Four kinds of gate descriptors are used to provide protec-

tion during control transfers: call gates, trap gates, inter-

rupt gates and task gates. (For more information on

protection refer to Section 3.13 “Protection” on page 86.)

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

define legal entry points to a procedure with a higher priv-

ilege level. The call gates are used by CALL and JUMP

instructions in much the same manner as code segment

descriptors. When the CPU decodes an instruction and

sees it refers to a call gate descriptor in the GDT table or a

LDT table, the call gate is used to point to another

descriptor in the table that defines the destination 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 desti-

nation DPL equals the CPL. For a CALL instruction the

destination DPL is less or equals 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 proce-

dure is to be executed in a conforming code segment.

Conforming code segments have the C bit set in the

TYPE field in their descriptor.

The bit structure and definitions for gate descriptors are

showninTables3-24 and Table3-25onpage 69.

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

8765

43210

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 proce-

dure’s stack.

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