UM10462 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2016. All rights reserved.
User manual Rev. 5.5 — 21 December 2016 449 of 523
NXP Semiconductors
UM10462
Chapter 24: LPC11U3x/2x/1x Appendix ARM Cortex-M0
Sufficient priority means the exception has greater priority than any limit set by the mask
register, see Section 24–24.3.1.3.6
. 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 to as stacking and the structure of eight data words is referred as a
stack frame. The stack frame contains the following information:
Immediately after stacking, the stack pointer indicates the lowest address in the stack
frame. The stack frame is aligned to a double-word address.
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.
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 exception entry, the processor starts
executing the exception handler and automatically changes the status of the
corresponding pending interrupt to active.
If another higher priority exception occurs during 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.
24.3.3.6.2 Exception return
Exception return occurs when the processor is in Handler mode and execution of one of
the following instructions attempts to set the PC to an EXC_RETURN value:
• a
POP
instruction that loads the PC
• a
BX
instruction using any register.
The processor saves an EXC_RETURN value to the LR on exception entry. The
exception mechanism relies on this value to detect when the processor has completed an
exception handler. Bits[31:4] of an EXC_RETURN value are
0xFFFFFFF
. When the
processor loads a value matching this pattern to the PC it detects that the operation is a
Fig 81. Exception entry stack contents
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