PM0214 datasheet(43/245 Pages) STMICROELECTRONICS

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PM0214

The Cortex-M4 processor

244

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

•

an LDR instruction with 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 the return stack and

processor mode. Table 18 shows the EXC_RETURN values with a description of the

exception return behavior.

All EXC_RETURN values have bits[31:5] set to one. 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.

2.4

Fault handling

Faults are a subset of the exceptions, see Exception model on page 36. The following

generate a fault:

•

A bus error on:

–

An instruction fetch or vector table load

–

A data access

•

An internally-detected error such as an undefined instruction

•

Attempting to execute an instruction from a memory region marked as Non-Executable

(XN).

•

A privilege violation or an attempt to access an unmanaged region causing an MPU

fault.

Table 18. Exception return behavior

EXC_RETURN[31:0]

Description

0xFFFFFFF1

Return to Handler mode, exception return uses non-floating-point state from

the MSP and execution uses MSP after return.

0xFFFFFFF9

Return to Thread mode, exception return uses non-floating-point state from

MSP and execution uses MSP after return.

0xFFFFFFFD

Return to Thread mode, exception return uses non-floating-point state from

the PSP and execution uses PSP after return.

0xFFFFFFE1

Return to Handler mode, exception return uses floating-point-state from MSP

and execution uses MSP after return.

0xFFFFFFE9

Return to Thread mode, exception return uses floating-point state from MSP

and execution uses MSP after return.

0xFFFFFFED

Return to Thread mode, exception return uses floating-point state from PSP

and execution uses PSP after return.

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