6-36 Vol. 3A

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.

•

A canonical violation is detected in 64-bit mode during an operation that reference memory using the stack 
pointer register containing a non-canonical memory address.

Recovery from this fault is possible by either extending the limit of the stack segment (in the case of a limit viola-
tion) or loading the missing stack segment into memory (in the case of a not-present violation. 
In the case of a canonical violation that was caused intentionally by software, recovery is possible by loading the 
correct canonical value into RSP. Otherwise, a canonical violation of the address in RSP likely reflects some register 
corruption in the software.

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 excep-
tion 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 gener-
ated 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 7.3, “Task 
Switching”). Here, the processor loads all the state information from the new TSS (without performing any addi-
tional limit, present, or type checks) before it generates the exception. The stack fault handler should thus not rely 
on being able to use the segment selectors found in the CS, SS, DS, ES, FS, and GS registers without causing 
another exception. The exception handler should check all segment registers before trying to resume the new 
task; otherwise, general protection faults may result later under conditions that are more difficult to diagnose. (See 
the Program State Change description for “Interrupt 10—Invalid TSS Exception (#TS)” in this chapter for additional 
information on how to handle this situation.)