PM0214 datasheet(33/245 Pages) STMICROELECTRONICS

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PM0214

The Cortex-M4 processor

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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 describes the behavior of direct byte, halfword,

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 Figure 10

for an example.

Figure 10. Little-endian 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:

•

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: 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.

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