Datasheet

SAM4S [DATASHEET]

11100E–ATARM–24-Jul-13

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 “LDREX and STREX” and “CLREX” .

12.4.2.8 Programming Hints for the Synchronization Primitives

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);

Table 12-8. CMSIS Functions for Exclusive Access Instructions

Instruction CMSIS Function

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)