Intel 80286, 80287 manual #GP 13 General Protection Selector or Zero Error Code

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, FNSAVE, 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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