Atmel Xplained Pro Evaluation Kit ATSAM4E-XPRO ATSAM4E-XPRO Data Sheet
Product codes
ATSAM4E-XPRO
67
SAM4E [DATASHEET]
Atmel-11157D-ATARM-SAM4E16-SAM4E8-Datasheet_12-Jun-14
Figure 12-5.
Little-endian Format
12.4.2.7 Synchronization Primitives
The Cortex-M4 instruction set includes pairs of synchronization primitives. These provide a non-blocking
mechanism that a thread or process can use to obtain exclusive access to a memory location. The software can
use them to perform a guaranteed read-modify-write memory update sequence, or for a semaphore mechanism.
mechanism that a thread or process can use to obtain exclusive access to a memory location. The software can
use them to perform a guaranteed read-modify-write memory update sequence, or for a semaphore mechanism.
A pair of synchronization primitives comprises:
A Load-exclusive Instruction, used to read the value of a memory location, requesting exclusive access to that
location.
location.
A Store-Exclusive instruction, used to attempt to write to the same memory location, returning a status bit to a
register. If this bit is:
register. If this bit is:
0: It indicates that the thread or process gained exclusive access to the memory, and the write succeeds,
1: It indicates that the thread or process did not gain exclusive access to the memory, and no write is
performed.
performed.
The pairs of Load-Exclusive and Store-Exclusive instructions are:
The word instructions LDREX and STREX
The halfword instructions LDREXH and STREXH
The byte instructions LDREXB and STREXB.
The software must use a Load-Exclusive instruction with the corresponding Store-Exclusive instruction.
To perform an exclusive read-modify-write of a memory location, the 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
4.
Test 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 This indicates that the value returned at step 1 might be out of date.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 must retry the read-modify-write sequence.
The software can use the synchronization primitives to implement a semaphore 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 instruction to write the claim value to the semaphore
address.
address.
3.
If the returned status bit from step 2 indicates that the Store-Exclusive instruction succeeded then the
software has claimed the semaphore. However, if the Store-Exclusive instruction failed, another process
might have claimed the semaphore after the software performed the first step.
software has claimed the semaphore. However, if the Store-Exclusive instruction failed, another process
might have claimed the semaphore after the software performed the first step.
Memory
Register
Address A
A+1
lsbyte
msbyte
A+2
A+3
0
7
B0
B1
B3
B2
31
24 23
16 15
8 7
0
B0
B1
B2
B3