Processor Programming (Continued)
Gate Descriptors
Four kinds of gate descriptors are used to provide protec-
tion during control transfers:
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
•
•
•
•
Call gates
Trap gates
Interrupt gates
Task gates
•
DPL = Descriptor Privilege Level in the destination
code segment
(For more information on protection refer to Section 3.9
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 than or equal to the CPL.
“Protection” on page 91.)
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 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
destination code segment.
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
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
9
8
7
6
5
4
3
2
1
0
OFFSET[31:16]
P
DPL
0
TYPE
0
0
0
PARAMETERS
Memory Offset +0
SELECTOR[15:0]
OFFSET[15:0]
Table 3-25. Gate Descriptors Bit Definitions
Memory
Bit
Offset
Name
Description
31:16
15:0
31:16
15
+4
+0
+0
+4
+4
+4
OFFSET
Offset: Offset used during a call gate to calculate the branch target.
SELECTOR
P
Segment Selector
Segment Present
14:13
11:8
DPL
Descriptor Privilege Level
Segment Type:
TYPE
0100 = 16-bit call gate
0101 = Task gate
0110 = 16-bit interrupt gate
0111 = 16-bit trap gate
1100 = 32-bit call gate
1110 = 32-bit interrupt gate
1111 = 32-bit trap gate
4:0
+4
PARAMETERS Parameters: Number of parameters to copy from the caller’s stack to the called proce-
dure’s stack.
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