Intel 80286, 80287 manual Return to Same Level, Return to Outer Privilege Level

THE 80286 INSTRUCTION SET

RETURN TO SAME LEVEL:

Return selector must be non-null else #GP(O)

Selector index must be within its descriptor table limits else #GP (selector) Descriptor AR byte must indicate code segment else #GP (selector)

If non-conforming then code segment DPL must equal CPL else #GP (selector) If conforming then code segment DPL must be :s; CPL else #GP (selector) Code segment must be PRESENT else #NP (selector)

Top word on stack must be within stack limits else #SS(O) IP must be in code segment limit else #GP(O)

Load CS:IP from stack

Load CS-cache with descriptor

Increment SP by 4 plus the immediate offset if it exists Else

RETURN TO OUTER PRIVILEGE LEVEL:

Top (8 + immeqiate) bytes on stack must be within stack limits else #SS(O) Examine return CS selector (at SP+2) and associated descriptor:

Selector must be non-null else #GP(O)

Selector index must be within its descriptor table limits else #GP (selector) Descriptor AR byte must indicate code segment else #GP (selector)

If non-cohforming then code segment DPL must equal return selector RPL else #GP (selector) If conforming then code segment DPL must be :s; return selector RPL else #GP (selector) Segment must be PRESENT else #NP (selector)

Examine return SS selector (at SP+6+imm) and associated descriptor: Selector must be non-null elSe #GP(O)

Selector index must be within its descriptor table limits else #GP (selector) Selector RPL must equal the RPL of the return CS selector else #GP (selector) Descriptor AR byte must indicate a writable data segment else #GP (selector) Descriptor DPL must equal the RPL of the return CS selector else #GP (selector) Segment must be PRESENT else #SS (selector)

IP must be in code segment limit else # GP(O) Set CPL to the RPL of the return CS selector Load CS:IP from stack

Set CS RPL to CPL

Increment SP by 4 plus the immediate offset if it exists Load SS:SP from stack

Load the CS-cache with the return CS descriptor Load the SS-cache with the return SS descriptor For each of ES and DS:

If the current register setting is not valid for the outer level, set the register to null (selector = AR = 0)

To be valid, the register setting must satisfy the following properties: Selector index must be within descriptor table limits Descriptor AR byte must indicate data or readable code segment If segment is data or non-conforming code, then:

DPL must be 2:: CPL, or

DPL must be 2:: RPL

PROTECTED MODE EXCEPTIONS

#GP, #NP, or #SS, as described in the above listing.

REAL ADDRESS MODE EXCEPTIONS

Interrupt 13 if the stack pop wraps around from OFFFFH to O.

8-95