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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
Memory Model
104
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Cortex
®
-M4F Processor
1.5.5.1
Directly Accessing an Alias Region
Writing to a word in the alias region updates one bit in the bit-band region.
Bit 0 of the value written to a word in the alias region determines the value written to the targeted bit in the
bit-band region. Writing a value with bit 0 set writes 1 to the bit-band bit, and writing a value with bit 0
clear writes 0 to the bit-band bit.
Bits 31:1 of the alias word have no effect on the bit-band bit. Writing 0x01 has the same effect as writing
0xFF. Writing 0x00 has the same effect as writing 0x0E.
When reading a word in the alias region, 0x0000.0000 indicates that the targeted bit in the bit-band region
is clear and 0x0000.0001 indicates that the targeted bit in the bit-band region is set.
1.5.5.2
Directly Accessing a Bit-Band Region
describes the behavior of direct byte, halfword, or word accesses to the bit-band regions.
1.5.6 Data Storage
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. Data is stored in
little-endian format, with the least significant byte (LSByte) of a word stored at the lowest-numbered byte,
and the most significant byte (MSByte) stored at the highest-numbered byte.
shows how data
is stored.
Figure 1-15. Data Storage
1.5.7 Synchronization Primitives
The Cortex-M4F instruction set includes pairs of synchronization primitives. These primitives provide a
nonblocking mechanism that a thread or process can use to obtain exclusive access to a memory
location. Software can use these primitives to perform an ensured read-modify-write memory update
sequence or for a semaphore mechanism.
NOTE:
The available pairs of synchronization primitives are available only for single processor use
and should not be used with multiprocessor systems.
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 try 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