Intel
®
81341 and 81342 I/O Processors
December 2007
Developer’s Manual
Order Number: 315037-002US
741
Interrupt Controller Unit—Intel
®
81341 and 81342
11.3
The Intel XScale
®
Processor Exceptions Architecture
The Intel XScale
®
processor supports five types of exceptions
26
, and a privileged
processing mode for each type.
• IRQ and FIQ internal interrupt exceptions (normal and fast interrupts, respectively)
• memory aborts (used to implement memory protection or virtual memory)
• attempted execution of an undefined instruction
• software interrupts (SWIs) (used to make a call to an Operating System)
When an exception occurs, some of the standard registers are replaced with registers
specific to the exception mode. All exceptions have replacement (or banked) registers
for R14 and R13, and one interrupt mode has more registers for fast interrupt
processing.
After an exception, R14 holds the return address for exception processing, which is
used both to return after the exception is processed and to address the instruction that
caused the exception.
R13 is banked across exception modes to provide each exception handler with a private
stack pointer (SP). The fast interrupt mode also banks R8 to R12, so that interrupt
processing can begin without the need to save or restore these registers. There is a
seventh processing mode, System Mode, that does not have any banked registers (it
uses the User mode registers), which is used to run normal (non-exception) tasks that
require a privileged processor mode.
11.3.1
CPSR and SPSR
All other processor state is held in status registers. The current operating processor
status is in the Current Program Status Register or CPSR. The CPSR holds:
• Four condition code flags (Negative, Zero, Carry and Overflow)
• Two interrupt disable bits (one for each type of interrupt)
• Five bits which encode the current processor mode
All five exception modes also have a Saved Program Status Register (SPSR) which
holds the CPSR of the task immediately before the exception occurred. Both the CPSR
and SPSR are accessed with special instructions.
26.Exception Description from the ARM Architecture Reference Manual, p. 1-3, Copyright Advanced
RISC Machines Ltd. (ARM) 1996