LM3S1637 Datasheet, PDF (86/696 Pages) Texas Instruments – ARM and Thumb are registered trademarks and Cortex is a trademark

The Cortex-M3 Processor
2.5.7.2
2.6
Exception Return
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:
■ An LDM or POP instruction that loads the PC
■ A BX instruction using any register
■ An LDR instruction with the PC as the destination
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 four
bits of this value provide information on the return stack and processor mode. Table 2-10 on page 86
shows the EXC_RETURN values with a description of the exception return behavior.
EXC_RETURN bits 31:4 are all set. When this value is loaded into the PC, it indicates to the processor
that the exception is complete, and the processor initiates the appropriate exception return sequence.
Table 2-10. Exception Return Behavior
EXC_RETURN[31:0]
0xFFFF.FFF0
0xFFFF.FFF1
0xFFFF.FFF2 - 0xFFFF.FFF8
0xFFFF.FFF9
0xFFFF.FFFA - 0xFFFF.FFFC
0xFFFF.FFFD
0xFFFF.FFFE - 0xFFFF.FFFF
Description
Reserved
Return to Handler mode.
Exception return uses state from MSP.
Execution uses MSP after return.
Reserved
Return to Thread mode.
Exception return uses state from MSP.
Execution uses MSP after return.
Reserved
Return to Thread mode.
Exception return uses state from PSP.
Execution uses PSP after return.
Reserved
Fault Handling
Faults are a subset of the exceptions (see “Exception Model” on page 78). The following conditions
generate a fault:
■ A bus error on an instruction fetch or vector table load or a data access.
■ An internally detected error such as an undefined instruction or an attempt to change state with
a BX instruction.
■ Attempting to execute an instruction from a memory region marked as Non-Executable (XN).
■ An MPU fault because of a privilege violation or an attempt to access an unmanaged region.
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June 18, 2012
Texas Instruments-Production Data