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Exception Model
112
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Cortex
®
-M4F Processor
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.
1.6.7.2
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 5 bits of this
value provide information on the return stack and processor mode.
shows the EXC_RETURN
values with a description of the exception return behavior.
EXC_RETURN bits 31:5 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 1-20. Exception Return Behavior
EXC_RETURN[31:0]
Description
0xFFFF.FFE0
Reserved
0xFFFF.FFE1
Return to handler mode.
Exception return uses floating-point state from MSP.
Execution uses MSP after return.
0xFFFF.FFE2 to 0xFFFF.FFE8
Reserved
0xFFFF.FFE9
Return to thread mode.
Exception return uses floating-point state from MSP.
Execution uses MSP after return.
0xFFFF.FFEA to 0xFFFF.FFEC
Reserved
0xFFFF.FFED
Return to thread mode.
Exception return uses floating-point state from PSP.
Execution uses PSP after return.
0xFFFF.FFEE to 0xFFFF.FFF0
Reserved
0xFFFF.FFF1
Return to handler mode.
Exception return uses nonfloating-point state from MSP.
Execution uses MSP after return.
0xFFFF.FFF2 to 0xFFFF.FFF8
Reserved
0xFFFF.FFF9
Return to thread mode.
Exception return uses nonfloating-point state from MSP.
Execution uses MSP after return.
0xFFFF.FFFA to 0xFFFF.FFFC
Reserved
0xFFFF.FFFD
Return to thread mode.
Exception return uses nonfloating-point state from PSP.
Execution uses PSP after return.
0xFFFF.FFFE to 0xFFFF.FFFF
Reserved