Intel ARCHITECTURE IA-32 User Manual download pdf (Page 221)

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Vol. 3A 5-45

INTERRUPT AND EXCEPTION HANDLING

Interrupt 12—Stack Fault Exception (#SS)

Exception Class Fault.

Description

Indicates that one of the following stack related conditions was detected:

• A limit violation is detected during an operation that refers to the SS register. Operations

that can cause a limit violation include stack-oriented instructions such as POP, PUSH,

CALL, RET, IRET, ENTER, and LEAVE, as well as other memory references which

implicitly or explicitly use the SS register (for example, MOV AX, [BP+6] or MOV AX,

SS:[EAX+6]). The ENTER instruction generates this exception when there is not enough

stack space for allocating local variables.

• A not-present stack segment is detected when attempting to load the SS register. This

violation can occur during the execution of a task switch, a CALL instruction to a different

privilege level, a return to a different privilege level, an LSS instruction, or a MOV or POP

instruction to the SS register.

Recovery from this fault is possible by either extending the limit of the stack segment (in the

case of a limit violation) or loading the missing stack segment into memory (in the case of a not-

present violation.

Exception Error Code

If the exception is caused by a not-present stack segment or by overflow of the new stack during

an inter-privilege-level call, the error code contains a segment selector for the segment that

caused the exception. Here, the exception handler can test the present flag in the segment

descriptor pointed to by the segment selector to determine the cause of the exception. For a

normal limit violation (on a stack segment already in use) the error code is set to 0.

Saved Instruction Pointer

The saved contents of CS and EIP registers generally point to the instruction that generated the

exception. However, when the exception results from attempting to load a not-present stack

segment during a task switch, the CS and EIP registers point to the first instruction of the new

task.

Program State Change

A program-state change does not generally accompany a stack-fault exception, because the

instruction that generated the fault is not executed. Here, the instruction can be restarted after

the exception handler has corrected the stack fault condition.

If a stack fault occurs during a task switch, it occurs after the commit-to-new-task point (see

Section 6.3, “Task Switching”). Here, the processor loads all the state information from the new

TSS (without performing any additional limit, present, or type checks) before it generates the

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