UM10360 Datasheet, PDF (741/835 Pages) NXP Semiconductors – LPC17xx User manual

NXP Semiconductors
UM10360
Chapter 34: Appendix: Cortex-M3 User Guide
To perform a guaranteed read-modify-write of a memory location, software must:
1. Use a Load-Exclusive instruction to read the value of the location.
2. Update the value, as required.
3. Use a Store-Exclusive instruction to attempt to write the new value back to the
memory location, and tests the returned status bit. If this bit is:
– 0: The read-modify-write completed successfully,
– 1: No write was performed. This indicates that the value returned at step 1 might
be out of date. The software must retry the read-modify-write sequence,
Software can use the synchronization primitives to implement a semaphores as follows:
1. Use a Load-Exclusive instruction to read from the semaphore address to check
whether the semaphore is free.
2. If the semaphore is free, use a Store-Exclusive to write the claim value to the
semaphore address.
3. If the returned status bit from step 2 indicates that the Store-Exclusive succeeded
then the software has claimed the semaphore. However, if the Store-Exclusive failed,
another process might have claimed the semaphore after the software performed
step 1.
The Cortex-M3 includes an exclusive access monitor, that tags the fact that the processor
has executed a Load-Exclusive instruction.
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 the processor can resolve semaphore conflicts
between different threads.
For more information about the synchronization primitive instructions, see Section
34–2.4.8 “LDREX and STREX” and Section 34–2.4.9.
3.2.8 Programming hints for the synchronization primitives
ANSI C cannot directly generate the exclusive access instructions. Some C compilers
provide intrinsic functions for generation of these instructions:
Table 639. C compiler intrinsic functions for exclusive access instructions
Instruction
Intrinsic function
LDREX, LDREXH, or LDREXB
unsigned int __ldrex(volatile void *ptr)
STREX, STREXH, or STREXB
int __strex(unsigned int val, volatile void *ptr)
CLREX
void __clrex(void)
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:
__ldrex((volatile char *) 0xFF);
UM10360_1
User manual
Rev. 01 — 4 January 2010
© NXP B.V. 2010. All rights reserved.
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