Cortex-A8 Technical Reference Manual - ARM Information Center

2.10 Memory formats
2.10.1 Byte-invariant big-endian format
2.10.2 Little-endian format
Programmers Model
The processor views memory as a linear collection of bytes numbered in ascending order from
zero. For example, bytes 0-3 in memory hold the first stored word, and bytes 4-7 hold the second
stored word.
The processor can treat words in memory as either:
• Byte-invariant big-endian format
• Little-endian format.
Additionally, the processor supports mixed-endian and unaligned data accesses. See Chapter 4
Unaligned Data and Mixed-endian Data Support for details.
Note
Instructions are always treated as little-endian.
In byte-invariant big-endian format, the processor stores the most significant byte of a word at
the lowest-numbered byte, and the least significant byte at the highest-numbered byte.
Therefore, byte 0 of the memory system connects to data lines 31-24 as Figure 2-8 shows.
Bit
Higher address
Lower address
31 24 23 16 15 8 7 0 Word address
Figure 2-8 Big-endian addresses of bytes within words
In little-endian format, the lowest-numbered byte in a word is the least significant byte of the
word and the highest-numbered byte is the most significant. Therefore, byte 0 of the memory
system connects to data lines 7-0. This is shown in Figure 2-9.
Bit
Higher address
Lower address
8
4
0
• Most significant byte is at lowest address
• Word is addressed by byte address of most significant byte
Figure 2-9 Little-endian addresses of bytes within words
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9
5
1
10
6
2
31 24 23 16 15 8 7 0 Word address
11
7
3
10
6
2
• Least significant byte is at lowest address
• Word is addressed by byte address of least significant byte
9
5
1
11
7
3
8
4
0
8
4
0
8
4
0