User's Manual

THE 80286 INSTRUCTION SET
If another exception is detected while attempting to perform the double fault exception, the 80286 will
enter shutdown (see section 11.5).
#GP 13 General Protection (Selector or Zero Error Code)
This exception is generated for all protection violations not covered by the other exceptions in this
section. Examples of this include:
1. An attempt to address a memory location by using an offset that exceeds the limit for the segment
involved.
2. An attempt to jump to a data segment.
3. An attempt to load SS with a selector for a read-only segment.
4. An attempt to write to a read-only segment.
5. Exceeding the maximum instruction length of 10 bytes.
If #GP occurred while loading a descriptor, the error code passed contains the selector involved. Other-
wise, the error code
is zero.
If the error code is not zero, the instruction can be restarted if the erroneous condition is rectified. If
the error code is zero either a limit violation, a write protect violation, or an illegal usc of invalid
segment register occurred. An invalid segment register
contains the values 0-3. A write protect fault
on ADC, SBB, RCL, RCR, or XCHG is not restartable.
#MF 16 Math Fault (No Error Code)
This exception is generated when the numeric processor extension (the 80287) detects an error signalled
by the
ERROR input pin leading from the 80287 to the 80286. The ERROR pin is tested at the
beginning of most floating point instructions, and when a WAIT instruction
is executed with the EM
bit of the Machine Status Word set to 0 (Le., no emulation of the math unit). The floating point
instructions that
do not cause the ERROR pin to be tested are FNCLEX, FNINIT, FSETPM,
FNSTCW, FNSTSW, FNSA VE, and FNSTENV.
If the handler corrects the error condition causing the exception, the floating point instruction that
caused #MF can be restarted. This is not accomplished by IRET, however, since the fault occurs at
the floating point instruction that follows the offending instruction. Before restarting the numeric
instruction, the handler must obtain from the
80287 the address of the offending instruction and the
address of the optional numeric operand.
#MP 9 Math Unit Protection Fault (No Error Code)
This exception is generated if the numeric operand is larger than one word and has the second or
subsequent words outside the segment's limit. Not all math addressing errors cause exception 9.
If the
effective address of an
ESCAPE instruction is not in the segment's limit, or if a write is attempted on
a read-only segment, or if a one-word operand violates a segment limit, exception 13 will occur.
The #MP exception occurs during the execution of the numeric instruction by the
80287. Thus, the
80286 may be in an unrelated instruction stream at the time. Exception 9 may occur in a task unrelated
to the task that executed the
ESC instruction. The operating system should keep track of which task
last used the
NPX (see section 11.4).
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