SAM3X8C Atmel Corporation, SAM3X8C Datasheet - Page 75

• Manufacturer Part Number: SAM3X8C
• Manufacturer: Atmel Corporation

11.5.8
11057A–ATARM–17-Feb-12
11057A–ATARM–17-Feb-12
Programming hints for the synchronization primitives
Software can use the synchronization primitives to implement a semaphores as follows:
The Cortex-M3 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:
In a multiprocessor implementation:
For more information about the synchronization primitive instructions, see
on page 108
ANSI C cannot directly generate the exclusive access instructions. Some C compilers provide
intrinsic functions for generation of these instructions:
Table 11-8.
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 require
LDREXB operation:
Instruction
LDREX, LDREXH, or
LDREXB
STREX, STREXH, or
STREXB
CLREX
• Use a Load-Exclusive instruction to read from the semaphore address to check whether the
• If the semaphore is free, use a Store-Exclusive to write the claim value to the semaphore
• If the returned status bit from the second step indicates that the Store-Exclusive succeeded
• It executes a CLREX instruction
• It executes a Store-Exclusive instruction, regardless of whether the write succeeds.
• An exception occurs. This means the processor can resolve semaphore conflicts between
• 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
semaphore is free.
address.
then the software has claimed the semaphore. However, if the Store-Exclusive failed, another
process might have claimed the semaphore after the software performed the first step.
different threads.
tags, and all global exclusive access tags for the processor.
1: No write was performed. This indicates that the value returned the first step might be out
of date. The software must retry the read-modify-write sequence,
__ldrex((volatile char *) 0xFF);
and
C compiler intrinsic functions for exclusive access instructions
“CLREX” on page
unsigned int __ldrex(volatile void *ptr)
int __strex(unsigned int val, volatile void *ptr)
Intrinsic function
void __clrex(void)
110.
“LDREX and STREX”
SAM3X/A
SAM3X/A
75
75