LM3S6100 Datasheet, PDF (75/559 Pages) Texas Instruments – Stellaris® LM3S6100 Microcontroller

Stellaris^® LM3S6100 Microcontroller
2.5.7.2
2.6
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,
indicating 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, known as late arrival, the processor
starts executing the exception handler for this exception and does not change the pending status
of the earlier exception.
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 75
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 68). The following conditions
generate a fault:
June 18, 2012
75
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