UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
User manual Rev. 3 — 20 December 2013 743 of 841
NXP Semiconductors
UM10360
Chapter 34: Appendix: Cortex-M3 user guide
34.3.2.6 Memory endianness
The processor views memory as a linear collection of bytes numbered in ascending order
from zero. For example, bytes 0-3 hold the first stored word, and bytes 4-7 hold the
second stored word. Section 34.3.2.6.1
describes how words of data are stored in
memory.
34.3.2.6.1 Little-endian format
In little-endian format, the processor stores the least significant byte of a word at the
lowest-numbered byte, and the most significant byte at the highest-numbered byte. For
example:
34.3.2.7 Synchronization primitives
The Cortex-M3 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. 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.
• 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:
– 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,
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
.
Fig 147. Little-endian format
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