Memory Endianness; Synchronization Primitives; Figure 10. Little-Endian Example - ST STM32F4 Series Programming Manual

PM0214
Reading a word in the alias region:
•
0x00000000 indicates that the targeted bit in the bit-band region is set to zero
•
0x00000001 indicates that the targeted bit in the bit-band region is set to 1
Directly accessing a bit-band region
Behavior of memory accesses on page 29
or word accesses to the bit-band regions.
2.2.6

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.
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. See
for an example.
2.2.7

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. 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:
•
Load-Exclusive instruction: used to read the value of a memory location, requesting
exclusive access to that location.
•
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: the thread or process gained exclusive access to memory, and the write succeeds.
1: the thread or process did not gain exclusive access to memory, and no write is
performed.

Figure 10. Little-endian example

Memory
7 0
Address A B0 lsbyte
A+1 B1
A+2 B2
A+3 B3 msbyte
DocID022708 Rev 6
The Cortex-M4 processor
describes the behavior of direct byte, halfword,
Register
31 24 23 16 15
B3 B2 B1
Figure 10
8 7 0
B0
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