Intel 80386 manual Got/Lot, Ret,Iret, Call, Jmp

80386

Table 4-3. Descriptor Types Used for Control Transfer

Control Transfer Types

Intersegment within the same privilege level

Intersegment to the same or higher privilege level Interrupt within task may change CPL

Intersegment to a lower privilege level (changes task CPL)

Operation Types	Descriptor	Descriptor
	Referenced	Table
JMP, CALL, RET, IRET', Code Segment		GOT/LOT
CALL	Call Gate	GOT/LOT
Interrupt Instruction,	Trap or	lOT
Exception, External	Interrupt
Interrupt	Gate
RET,IRET'	Code Segment	GOT/LOT

	CALL, JMP	Task State	GOT
		Segment
Task Switch	CALL, JMP	Task Gate	GOT/LOT
	IRET"	Task Gate	lOT
	Interrupt Instruction,
	Exception, External
	Interrupt

'NT(Nested Task bit of flag register) ~ 0 "NT (Nested Task bit of flag register) ~ 1

or a jump to another routine. There are five types of control transfers which are summarized in Table 4-3. Many of these transfers result in a privilege level transfer. Changing privilege levels is done only via control transfers, by using gates, task switches, and interrupt or trap gates.

Control transfers can only occur if the operation which loaded the selector references the correct de- scriptor type. Any violation of these descriptor usage rules will cause an exception 13 (e.g. JMP through a call gate, or IRET from a normal subroutine call).

In order to provide further system security, all control transfers are also subject to the privilege rules.

The privilege rules require that:

-Privilege level transitions can only occur via gates.

-JMPs can be made to a non-conforming code segment with the same privilege or to a conform- ing code segment with greater or equal privilege.

-CALLs can be made to a non-conforming code segment with the same privilege or via a gate to a more privileged level.

-Interrupts handled within the task obey the same privilege rules as CALLs.

-Conforming Code segments are accessible by privilege levels which are the same or less privi- leged than the conforming-code segment'sOPL.

-Both the requested privilege level (RPL) in the selector pOinting to the gate and the task'sCPL

must be of equal or greater privilege than the gate'sOPL.

-The code segment selected in the gate must be the same or more privileged than the task'sCPL.

-Return instructions that do not switch tasks can only return control to a code segment with same or less privilege.

-Task switches can be performed by a CALL, JMP, or INT which references either a task gate or task state segment who'sOPL is less privi- leged or the same privilege as the old task'sCPL.

Any control transfer that changes CPL within a task causes a change of stacks as a result of the privi- lege level change. The initial values of SS:ESP for privilege levels 0, 1, and 2 are retained in the task state segment (see section 4.4.6 Task Switching). During a JMP or CALL control transfer, the new stack pointer is loaded into the SS and ESP regis- ters and the previous stack pointer is pushed onto the new stack.

When RETurning to the original privilege level, use of the lower-privileged stack is restored as part of the RET or IRET instruction operation. For subrou- tine calls that pass parameters on the stack and cross privilege levels, a fixed number of words (as specified in the gate'sword count field) are copied from the previous stack to the current stack. The inter-segment RET instruction with a stack adjust- ment value will correctly restore the previous stack pointer upon return.

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