User's Manual

THE 80286 INSTRUCTION SET
#SS 12 Stack Fault (Selector or Zero Error Code)
This exception is generated when a limit violation is detected in addressing through the SS register. It
can occur on stack-oriented instructions such as PUSH or POP, as well as other types of memory
references using
SS such as MOY AX,[BP+28]. It also can occur on an ENTER instruction when
there
is not enough space on the stack for the indicated local variable space, even if the stack exception
is not triggered by pushing BP or copying the display stack. A stack exception can therefore indicate
a stack overflow, a stack underflow or a wild offset. The error code
will- be zero.
#SS is also generated on an attempt to load SS with a descriptor that is marked not present but is
otherwise valid. This can occur in a task switch, an inter-level call, an inter-level return, a move to the
SS instruction or a pop to the SS instruction. The error code will be non-zero.
#SS is never generated when addressing through the DS or ES registers even if the offending register
points to the same segment
as the SS register.
The
#SS exception handler must contain special code to complete the loading of segment registers.
The
DS and ES registers will not be fully loaded if a not-present condition is detected while loading
the
SS register. Therefore, the #SS exception handler should execute code such as the following to
insure full loading of the segment registers:
MOY AX,DS
MOYDS,AX
MOY AX,ES
MOYES,AX
Generally, the instruction causing #SS can be restarted, but there is one special case when it cannot:
when a
PUSHA orPOPA instruction attempts to wrap around the 64K boundary of a stack segment.
This condition
is identified by the value of the saved SP, which can be either OOOOH, OOOIH; OFFFEH,
or OFFFFH.
#TS 10 Invalid Task State Segment (Selector Error Code)
This exception is generated during a task switch when the new task state segment is invalid, that is,
when a task state segment
is too small; when the LDT indicated in a TSS is invalid or not present;
when the
SS, CS, DS, or ES indicated ina TSS are invalid (task switch); when the back link in a TSS
is invalid (inter-task IRET).
#TS
is not generated when the SS, CS, DS, or ES back link or privileged stack selectors point to a
descriptor that
is not present but otherwise is valid. #NP is generated in these cases.
The error code passed to the exception handler contains
the selector of the offending segment, which
caneitJIer be the Task State Segment itself, or it selector found within the Task State Segment.·
The instruction causing #TS can be restarted.
#TS must be handled through a task gate.
The exception handler must reset the busy bit
in the new TSS.
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