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Programmer’s Model
ARM DDI 0192A
Copyright © ARM Limited 1997, 1998, 2000. All rights reserved.
2-3
2.2
Memory formats
The bigend bit in the Control Register selects whether the ARM720T treats words in
memory as being stored in big-endian or little-endian format. See Chapter 3
Configuration for more information on the Control Register.
ARM720T 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 bytes 8 to 11 the
third. ARM720T can treat words in memory as being stored as follows:
•
Big-endian format
•
Little-endian format on page 2-4.
2.2.1
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 to 24. This is shown in Figure
2-1.
Figure 2-1 Big-endian addresses of bytes with words
Higher address
8
4
0
31
24 23
Word
address
16 15
8 7
0
· Most significant byte is at lowest address
· Word is addressed by byte address of most significant byte
Lower address
4
9
5
10
6
11
7
8
0
1
2
3
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