Intel IA-32 - Manual (Page 219)

Vol. 3A 5-43
INTERRUPT AND EXCEPTION HANDLING
Interrupt 11—Segment Not Present (#NP)
Exception Class Fault.
Description
Indicates that the present flag of a segment or gate descriptor is clear. The processor can generate
this exception during any of the following operations:
•
While attempting to load CS, DS, ES, FS, or GS registers. [Detection of a not-present
segment while loading the SS register causes a stack fault exception (#SS) to be
generated.] This situation can occur while performing a task switch.
•
While attempting to load the LDTR using an LLDT instruction. Detection of a not-present
LDT while loading the LDTR during a task switch operation causes an invalid-TSS
exception (#TS) to be generated.
•
When executing the LTR instruction and the TSS is marked not present.
•
While attempting to use a gate descriptor or TSS that is marked segment-not-present, but is
otherwise valid.
An operating system typically uses the segment-not-present exception to implement virtual
memory at the segment level. If the exception handler loads the segment and returns, the inter-
rupted program or task resumes execution.
A not-present indication in a gate descriptor, however, does not indicate that a segment is not
present (because gates do not correspond to segments). The operating system may use the
present flag for gate descriptors to trigger exceptions of special significance to the operating
system.
A contributory exception or page fault that subsequently referenced a not-present segment
would cause a double fault (#DF) to be generated instead of #NP.
Exception Error Code
An error code containing the segment selector index for the segment descriptor that caused the
violation is pushed onto the stack of the exception handler. If the EXT flag is set, it indicates that
the exception resulted from either:
•
an external event (NMI or INTR) that caused an interrupt, which subsequently referenced a
not-present segment
•
a benign exception that subsequently referenced a not-present segment
The IDT flag is set if the error code refers to an IDT entry. This occurs when the IDT entry for
an interrupt being serviced references a not-present gate. Such an event could be generated by
an INT instruction or a hardware interrupt.