User's Manual

PROTECTION
Gate Descriptor Fields
D1
+7
INTEL RESERVED'
pi DPL \01
TYPE jx X Xl
WORD
COUNT....,
+5
+3
DESTINATION SELECTOR'5-2
IX X
+1
DESTINATION OFFSET,s-a
IS
• 7
'Must be set to 0 for compatibility
with
80386 (X is don't care)
+6
+4
+2
Name
Value
4
TYPE
5
6
7
P 0
1
DPL
0-3
WORD
COUNT
0-31
DESTINA-
16-bit
TION
selector
SELECTOR
DESTINA-
16-bit
TION
offset
OFFSET
Figure 7-10. Gate Descriptor Format
7.5.1.1 CALL GATES
Description
Call Gate.
Task Gate.
Interrupt Gate.
Trap Gate.
Descriptor Contents are
not valid.
Descriptor Contents are
valid.
Descriptor Privilege
Level.
Number of words to copy
from
caller's
stack to
called procedure's stack.
Only used with call gate.
Selector to the target code
segment
(Call, Interrupt or
Trap Gate).
Selector to the target task
state segment (Task
Gate).
Entry point within the
target code segment.
G30108
Call gate descriptors are used by call and jump instructions in the same manner as a code segment
descriptor. The hardware automatically recognizes that the destination selector refers to a gate
descrip-
tor. Then, the operation of the instruction is expanded as determined by the contents of the call gate.
A jump instruction can access a call gate
only if the target code segment is at the same privilege level.
A call instruction uses a call gate for the same or more privileged access.
A call gate descriptor may reside
in either the GDT or the LDT, but not in the IDT. Figure 7-10 gives
the complete layout of a call gate descriptor.
A call gate can
be referred to by either the long JMP or CALL instructions. From the viewpoint of
the program executing a
JMP or CALL instruction, the fact that the destination was reached via a
call gate and not directly from the destination address of the instruction
is not apparent.
7-17