Vol. 3A 6-21

INTERRUPT AND EXCEPTION HANDLING

Interrupt 1—Debug Exception (#DB)

Exception Class

Trap or Fault. The exception handler can distinguish between traps or faults by exam-

ining the contents of DR6 and the other debug registers.

Description

Indicates that one or more of several debug-exception conditions has been detected. Whether the exception is a 
fault or a trap depends on the condition (see Table 6-3). See Chapter 17, “Debug, Branch Profile, TSC, and 
Resource Monitoring Features,” for detailed information about the debug exceptions.

Exception Error Code

None. An exception handler can examine the debug registers to determine which condition caused the exception.

Saved Instruction Pointer

Fault — Saved contents of CS and EIP registers point to the instruction that generated the exception.
Trap — Saved contents of CS and EIP registers point to the instruction following the instruction that generated the 
exception.

Program State Change

Fault — A program-state change does not accompany the debug exception, because the exception occurs before 
the faulting instruction is executed. The program can resume normal execution upon returning from the debug 
exception handler.
Trap — A program-state change does accompany the debug exception, because the instruction or task switch being 
executed is allowed to complete before the exception is generated. However, the new state of the program is not 
corrupted and execution of the program can continue reliably.
Any debug exception inside an RTM region causes a transactional abort and, by default, redirects control flow to the 
fallback instruction address. If advanced debugging of RTM transactional regions has been enabled, any transac-
tional abort due to a debug exception instead causes execution to roll back to just before the XBEGIN instruction 
and then delivers a #DB. See Section 16.3.7, “RTM-Enabled Debugger Support,” of Intel® 64 and IA-32 Architec-
tures Software Developer’s Manual, Volume 1.

Table 6-3.  Debug Exception Conditions and Corresponding Exception Classes

Exception Condition

Exception Class

Instruction fetch breakpoint 

Fault

Data read or write breakpoint

Trap

I/O read or write breakpoint 

Trap

General detect condition (in conjunction with in-circuit emulation)

Fault

Single-step 

Trap

Task-switch

Trap