Intel 80286, 80287 manual Gate Descriptor Format Call Gates

PROTECTION

D1

+7		INTEL RESERVED'	+6
+5	pi DPL \01	TYPE jx X Xl COUNTWORD ....,	+4
+3	DESTINATION SELECTOR'5-2 IX		X +2
+1	DESTINATION OFFSET,s-a
	IS	• 7

Gate Descriptor Fields

Name	Value		Description
	4	Call Gate.
TYPE	5	Task Gate.
	6	Interrupt Gate.
	7	Trap Gate.
P	0	Descriptor Contents are
		not valid.
	1	Descriptor Contents are
		valid.
DPL	0-3	Descriptor Privilege Level.
WORD		Number of words to copy
COUNT	0-31	from	caller's stack to
		called	procedure'sstack.

Only used with call gate.

'Mustbe set to 0 for compatibility

DESTINA-

Selector to the target code segment (Call, Interrupt or

with 80386 (X is don'tcare)

TION

16-bit

Trap Gate).

SELECTOR

selector

Selector to the target task state segment (Task Gate).

DESTINA- 16-bit

TION offset

OFFSET

Entry point within the target code segment.

G30108

Figure 7-10. Gate Descriptor Format

7.5.1.1 CALL GATES

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