LM3S6G11 Datasheet, PDF (74/827 Pages) Texas Instruments – Stellaris® LM3S6G11 Microcontroller

OBSOLETE: TI has discontinued production of this device.
The Cortex-M3 Processor
lowest-numbered byte, and the most-significant byte (msbyte) stored at the highest-numbered byte.
Figure 2-5 on page 74 illustrates how data is stored.
Figure 2-5. Data Storage
Memory
7
0
Register
Address A B0 lsbyte
31 24 23 16 15 8 7
0
B3
B2
B1
B0
A+1 B1
A+2 B2
A+3 B3 msbyte
2.4.7
Synchronization Primitives
The Cortex-M3 instruction set includes pairs of synchronization primitives which provide a
non-blocking mechanism that a thread or process can use to obtain exclusive access to a memory
location. Software can use these primitives to perform a guaranteed read-modify-write memory
update sequence or for a semaphore mechanism.
A pair of synchronization primitives consists of:
■ A Load-Exclusive instruction, which is used to read the value of a memory location and requests
exclusive access to that location.
■ A Store-Exclusive instruction, which is used to attempt to write to the same memory location and
returns a status bit to a register. If this status bit is clear, it indicates that the thread or process
gained exclusive access to the memory and the write succeeds; if this status bit is set, it indicates
that the thread or process did not gain exclusive access to the memory and no write was
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
Software must use a Load-Exclusive instruction with the corresponding Store-Exclusive instruction.
To perform an exclusive read-modify-write of a memory location, software must:
1. Use a Load-Exclusive instruction to read the value of the location.
2. Modify 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.
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July 24, 2012
Texas Instruments-Production Data