Atmel ARM-Based Evaluation Kit for SAM4S16C, 32-Bit ARM® Cortex® Microcontroller ATSAM4S-WPIR-RD ATSAM4S-WPIR-RD Data Sheet

SAM4S Series [DATASHEET]

Atmel-11100G-ATARM-SAM4S-Datasheet_27-May-14

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The Cortex-M4 includes an exclusive access monitor, that tags the fact that the processor has executed a Load-

Exclusive instruction. If the processor is part of a multiprocessor system, the system also globally tags the memory 

locations addressed by exclusive accesses by each processor.
The processor removes its exclusive access tag if:



It executes a CLREX instruction



It executes a Store-Exclusive instruction, regardless of whether the write succeeds.



An exception occurs. This means that the processor can resolve semaphore conflicts between different 

threads.

In a multiprocessor implementation:



Executing a CLREX instruction removes only the local exclusive access tag for the processor



Executing a Store-Exclusive instruction, or an exception, removes the local exclusive access tags, and all 

global exclusive access tags for the processor.

For more information about the synchronization primitive instructions, see 

 and 

.

ISO/IEC C cannot directly generate the exclusive access instructions. CMSIS provides intrinsic functions for 

generation of these instructions:

The actual exclusive access instruction generated depends on the data type of the pointer passed to the intrinsic 

function. For example, the following C code generates the required LDREXB operation:

__ldrex((volatile char *) 0xFF);

This section describes the exception model.

Each exception is in one of the following states:

The exception is not active and not pending.

The exception is waiting to be serviced by the processor.
An interrupt request from a peripheral or from software can change the state of the corresponding interrupt to 

pending.

LDREX

uint32_t __LDREXW (uint32_t *addr)

LDREXH 

uint16_t __LDREXH (uint16_t *addr)

LDREXB

uint8_t __LDREXB (uint8_t *addr)

STREX

uint32_t __STREXW (uint32_t value, uint32_t *addr)

STREXH

uint32_t __STREXH (uint16_t value, uint16_t *addr)

STREXB

uint32_t __STREXB (uint8_t value, uint8_t *addr)

CLREX

void __CLREX (void)