PM0215 datasheet(27/91 Pages) STMICROELECTRONICS

27 / 91 page

PM0215

The STM32 Cortex-M0 processor

Doc ID 022979 Rev 1

27/91

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.

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:

●

a POP instruction that loads the PC

●

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.

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 not a normal branch

operation and, instead, that the exception is complete. Therefore, it starts the exception

return sequence. Bits[3:0] of the EXC_RETURN value indicate the required return stack and

processor mode as shown in Table 13.

Table 13.

Exception return behavior

EXC_RETURN[31:0]

Description

0xFFFFFFF1

Return to Handler mode.

Exception return gets state from the main stack.

Execution uses MSP after return.

0xFFFFFFF9

Return to Thread mode.

Exception return gets state from MSP.

Execution uses MSP after return.

0xFFFFFFFD

Return to Thread mode.

Exception return gets state from PSP.

Execution uses PSP after return.

All other values

Reserved

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