User's Manual

THE 80286 INSTRUCTION SET
The offending floating point instruction cannot be restarted; the task which attempted to execute the
offending numeric instruction must be aborted. However, if exception 9 interrupted another
ta~k, the
interrupted task may be restarted.
The exception 9 handler
must execute FNINIT before executing any ESCAPE or WAIT instruction.
#NM 7 No Math Unit Available (No Error Code)
This exception occurs when any floating point instruction is executed while the EM bit or the TS bit
of the Machine Status Word
is 1. It also occurs when a WAIT instruction is encountered and both the
MP and TS bits of the Machine Status Word are 1.
Depending on the setting of the MSW bits that caused this exception, the exception handler could
provide emulation of the
80287, or it could perform a context switch of the math processor to prepare
it
for use by another task.
The instruction causing #NM can be restarted if the handler performs a numeric context switch.
If the
handler provided emulation of the math unit, it should advance the return pointer beyond the floating
point instruction that caused NM.
#NP 11 Not Present (Selector Error Code)
This exception occurs when CS, DS, ES, or the Task Register is loaded with a descriptor that is
marked not present but is otherwise valid. It can occur in an LLDT instruction, but the #NP exception
will not occur if the processor attempts to load the LDT register during a task switch. A not-present
LDT encountered during a task switch causes the
#TS exception.
The error code passed
is the selector of the descriptor that is marked not present.
Typically, the Not
Present exception handler is used to implement a virtual memory system. The
operating system can swap inactive memory segments to a mass-storage device such as a disk.
Appli-
cations programs need not be told about this; the next time they attempt to access the swapped-out
memory segment, the Not
Present handler will be invoked, the segment will be brought back into
memory, and the offending instruction within the applications program
will be restarted.
If #NP is detected on loading CS, DS, or ES in a task switch, the exception occurs in the new task,
and the IRET from the exception handler jumps directly
to the next instruction in the new task.
The Not
Present exception handler must contain special code to complete the loading of segment
!"egi~ter~ ~~lhe!! #NP i~ tl",te>cte>ci in loading the CS orDS registers in a task switch and a trap or inter-
rupt gate was used. T~e DS and ES registers have been loaded but their descriptors have not been
loaded. Any memory reference using the segment register may cause exception
13. The #NP exception
handler should execute code such as the following
to ensure full loading of the segment registers:
MOV AX,DS
MOVDS,AX
MOV AX,ES
MOVES,AX
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