Intel
®
81341 and 81342—Interrupt Controller Unit
Intel
®
81341 and 81342 I/O Processors
Developer’s Manual
December 2007
742
Order Number: 315037-002US
11.3.2
The Exception Process
When an exception occurs, the Intel XScale
®
processor halts execution after the
current instruction and begins execution at a fixed address in low memory, known as
the exception vectors. There is a separate vector location for each exception (and two
for memory aborts to distinguish between data and instruction accesses).
An operating system installs a handler on every exception at initialization. Privileged
operating system tasks normally run in System mode to allow exceptions to occur
within the operating system without state loss (exceptions overwrite their R14 when an
exception occurs, and System mode is the only privileged mode that cannot be entered
by an exception).
11.3.3
Exception Priorities and Vectors
It is important to note that fast interrupt (FIQ) is higher priority than the normal
interrupt (IRQ). In addition, while an FIQ exception is executing, the IRQ exception is
masked out.
When an exception is taken by the processor, the Program Counter (PC) is loaded with
the vector associated with that exception as specified by
.
Generally, the instruction at this location is required to be a branch instruction to the
associated exception handler. However, in the case of an FIQ, this is not necessary
since the vector location is at the very bottom of all the defined exception vectors, thus
the entire FIQ exception handler can be placed at that vector location.
Table 451. Exception Priorities And Vectors
Exception
Priority
Vector
a
a. By enabling the Exception Vector Relocation mode (bit 13, CP15, Register 1), the Vectors (except Reset
Vector) can be relocated to be based at FFFF 0000H rather than 0000 0000H. (i.e., FIQ Vector located at
FFFF 001CH)
Reset
1 (Highest)
0000 0000H
Data Abort
2
0000 0010H
FIQ
3
0000 001CH
IRQ
4
0000 0018H
Prefetch Abort
5
0000 000CH
Undefined Instructions
6 (Lowest)
0000 0004H
Software Interrupt (SWI)
b
b. Undefined Instruction and SWI can not occur at the same time since SWI is a particular instruction decoding.
6 (Lowest)
0000 0008H