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ATSAM4S-XPLD
74
SAM4S [DATASHEET]
11100E–ATARM–24-Jul-13
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).
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.
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.
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.
automatically changes the status of the corresponding pending interrupt to active.
If another higher priority exception occurs during the exception entry, the processor starts executing the exception
handler for this exception and does not change the pending status of the earlier exception. This is the late arrival case.
handler for this exception and does not change the pending status of the earlier exception. This is the late arrival case.
Exception Return
An Exception return occurs when the processor is in Handler mode and executes one of the following instructions to load
the EXC_RETURN value into the PC:
the EXC_RETURN value into the PC:
An LDM or POP instruction that loads the PC
An LDR instruction with the PC as the destination.
A BX instruction using any register.
EXC_RETURN is the value loaded into the LR on exception entry. The exception mechanism relies on this value to
detect when the processor has completed an exception handler. The lowest five bits of this value provide information on
detect when the processor has completed an exception handler. The lowest five bits of this value provide information on
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
floating-point storage
Exception frame without
floating-point storage
Pre-IRQ top of stack
...