Processor Programming (Continued)
3.10.4 Interrupt and Exception Priorities
generated upon each attempt to execute the instruction.
Each exception service routine should make the appropri-
ate corrections to the instruction and then restart the
instruction. In this way, exceptions can be serviced until
the instruction executes properly.
As the CPU executes instructions, it follows a consistent
policy for prioritizing exceptions and hardware interrupts.
The priorities for competing interrupts and exceptions are
listed in Table 3-30. SMM interrupts always take prece-
dence. Debug traps for the previous instruction and next
instructions are handled as the next priority. When NMI
and maskable INTR interrupts are both detected at the
same instruction boundary, the GXm processor services
the NMI interrupt first.
The CPU supports instruction restart after all faults,
except when an instruction causes a task switch to a task
whose task state segment (TSS) is partially not present. A
TSS can be partially not present if the TSS is not page
aligned and one of the pages where the TSS resides is
not currently in memory.
The CPU checks for exceptions in parallel with instruction
decoding and execution. Several exceptions can result
from a single instruction. However, only one exception is
Table 3-30. Interrupt and Exception Priorities
Priority
Description
Notes
0
1
Warm Reset.
Caused by the assertion of WM_RST.
SMM hardware interrupt.
SMM interrupts are caused by SMI# asserted and always have
highest priority.
2
3
Debug traps and faults from previous instruction.
Debug traps for next instruction.
Includes single-step trap and data breakpoints specified in the
debug registers.
Includes instruction execution breakpoints specified in the debug
registers.
4
5
6
Non-maskable hardware interrupt.
Maskable hardware interrupt.
Caused by NMI asserted.
Caused by INTR asserted and IF = 1.
Faults resulting from fetching the next instruction.
Includes segment not present, general protection fault and page
fault.
7
8
Faults resulting from instruction decoding.
WAIT instruction and TS = 1 and MP = 1.
ESC instruction and EM = 1 or TS = 1.
Floating point error exception.
Includes illegal opcode, instruction too long, or privilege violation.
Device not available exception generated.
9
Device not available exception generated.
10
11
Caused by unmasked floating point exception with NE = 1.
Includes segment not present, stack fault, and general protection
Segmentation faults (for each memory reference
required by the instruction) that prevent transferring fault.
the entire memory operand.
12
13
Page Faults that prevent transferring the entire
memory operand.
Alignment check fault.
www.national.com
76
Revision 3.1
NSC [ National Semiconductor ]
