Datasheet

77
SAM4S Series [DATASHEET]
11100F–ATARM–29-Jan-14
An Exception entry occurs when there is a pending exception with sufficient priority and either the processor is in Thread
mode, or the new exception is of a higher priority than the exception being handled, in which case the new exception
preempts the original exception.
When one exception preempts another, the exceptions are nested.
Sufficient priority means that the exception has more priority than any limits set by the mask registers, see “Exception
Mask Registers” . An exception with less priority than this is pending but is not handled by the processor.
When the processor takes an exception, unless the exception is a tail-chained or a late-arriving exception, the processor
pushes information onto the current stack. This operation is referred as stacking and the structure of eight data words is
referred to as stack frame.
Figure 12-7. Exception Stack Frame
Immediately after stacking, the stack pointer indicates the lowest address in the stack frame. The alignment of the stack
frame is controlled via the STKALIGN bit of the Configuration Control Register (CCR).
The stack frame includes the return address. This is the address of the next instruction in the interrupted program. This
value is restored to the PC at exception return so that the interrupted program resumes.
In parallel to the stacking operation, the processor performs a vector fetch that reads the exception handler start address
from the vector table. When stacking is complete, the processor starts executing the exception handler. At the same
time, the processor writes an EXC_RETURN value to the LR. This indicates which stack pointer corresponds to the stack
frame and what operation mode the processor was in before the entry occurred.
If no higher priority exception occurs during the exception entry, the processor starts executing the exception handler and
automatically changes the status of the corresponding pending interrupt to active.
Pre-IRQ top of stack
xPSR
PC
LR
R12
R3
R2
R1
R0
{aligner}
IRQ top of stack
Decreasing
memory
address
xPSR
PC
LR
R12
R3
R2
R1
R0
S7
S6
S5
S4
S3
S2
S1
S0
S9
S8
FPSCR
S15
S14
S13
S12
S11
S10
{aligner}
IRQ top of stack
...
Exception frame with
floating-point storage
Exception frame without
floating-point storage
Pre-IRQ top of stack
...