User's Manual

THE 80286 INSTRUCTION SET
2. Call gate-The offset part of the destination pointer is ignored. After checking for validity, the
processor jumps
to the location stored in the call gate descriptor.
3. Task gate-The current task's state is saved in its Task State Segment (TSS), and the TSS named
in the task gate is used to load a new context. The outgoing task is marked not busy, the new TSS
is marked busy, and execution resumes at the point at which the new task was last suspended.
4. TSS- The current task is suspended and the new task is initiated as in 3 above except that there
is no intervening gate.
Following
is the list of checks and actions taken for long jumps in protected mode:
JUMP FAR:
If indirect then check access
of EA doubleword #GP(O) or #SS(O) if limit violation
Destination selector is not
null else #GP(O)
Destination selector index is within its descriptor table limits else
#GP (selector)
Examine AR byte
of destination selector for legal values:
JUMP
CONFORMING CODE SEGMENT:
Descriptor DPL must be
:s CPL else #GP (selector)
Segment must be PRESENT else
#NP (selector)
IP must be in code segment limit else #GP(O)
Load
CS:IP from destination pOinter
Load CS-cache with new segment descriptor
JUMP
NONCONFORMING CODE SEGMENT:
RPL
of destination selector must be :s CPL else #GP (selector)
Descriptor DPL must
= CPL else #GP (selector)
Segment must be PRESENT else
#NP (selector)
IP must be in code segment limit else #GP(O)
Load
CS:IP from destination pOinter
Load CS-cache with new segment descriptor
Set RPL field
of CS register to CPL
JUMP
TO CALL GATE:
Descriptor DPL must be
:::: CPL else #GP (gate selector)
Descriptor DPL must be
:::: gate selector RPL else #GP (gate selector)
Gate must be PRESENT else
#NP (gate selector)
Examine selector to code segment given
in call gate descriptor:
Selector must not
be null else #GP(O)
Selector must be within its descriptor table limits else
#GP (CS selector)
Descriptor AR byte must indicate code segment else
#GP (CS selector)
If non-conforming, code segment descriptor DPL must = CPL else #GP (CS selector)
If conforming, then code segment descriptor DPL must be :s CPL else #GP (CS selector)
Code Segment must be PRESENT else
#NP (CS selector)
IP must be in code segment limit else #GP(O)
Load
CS:IP from call gate
Load CS-cache with new code segment
Set RPL
of CS to CPL
JUMP TASK GATE:
Gate descriptor DPL must be
:::: CPL else #GP (gate selector)
Gate descriptor DPL must be
:::: gate selector RPL else #GP (gate selector)
Task Gate must be PRESENT else
#NP (gate selector)
Examine selector to
TSS, given in Task Gate descriptor:
Must specify
global in the local/global bit else #GP (TSS selector)
Index must be within GDT limits else
#GP (TSS selector)
Descriptor AR byte must specify available
TSS (bottom bits 00001) else #GP (TSS selector)
Task State Segment must be PRESENT else
#NP (TSS selector)
SWITCH_TASKS without nesting
to TSS
IP must be in code segment limit else #GP(O)
8-57