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PROGRAMMER'S MODEL
2.3 MEMORY FORMATS
ARM9TDMI views memory as a linear collection of bytes numbered upwards from zero. Bytes 0 to 3 hold the first
stored word, bytes 4 to 7 the second and so on. ARM9TDMI can treat words in memory as being stored either in
Big-Endian or Little-Endian format.
2.3.1 BIG-ENDIAN FORMAT
In Big-Endian format, the most significant byte of a word is stored at the lowest numbered byte and the least
significant byte at the highest numbered byte. Byte 0 of the memory system is therefore connected to data lines
31 through 24.
Higher Address
Lower Address
The data locations in the external memory are different with Figure 2-1 in the S3C2501X. Please refer to
the chapter 4, system manager.
2.3.2 LITTLE-ENDIAN FORMAT
In Little-Endian format, the lowest numbered byte in a word is considered the word's least significant byte, and
the highest numbered byte the most significant. Byte 0 of the memory system is therefore connected to data lines
7 through 0.
Higher Address
Lower Address
2-2
31
24
23
8
9
4
5
0
1
Most significant byte is at lowest address.
Word is addressed by byte address of most significant byte.
Figure 2-1. Big-Endian Addresses of Bytes within Words
31
24
23
11
10
7
6
3
2
Least significant byte is at lowest address.
Word is addressed by byte address of least significant byte.
Figure 2-2. Little-Endian Addresses of Bytes Words
16
15
8
7
10
11
6
7
2
3
NOTE
16
15
8
7
9
8
5
4
1
0
Word Address
0
8
4
0
Word Address
0
8
4
0
S3C2501X
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