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2: Programmer’s Model
2-2 EPSON ARM720T CORE CPU MANUAL
2.2 Memory formats
The ARM720T processor views memory as a linear collection of bytes numbered upwards from
zero, as follows:
Bytes 0 to 3 Hold the first stored word.
Bytes 4 to 7 Hold the second stored word.
Bytes 8 to 11 Hold the third stored word.
Words are stored in memory as big or little-endian, as described in the following sections:
•
Big-endian format
•
Little-endian format
on page 2-3.
The endianness used depends on the status of the B bit in the Control Register of the system
control coprocessor. See
Control Register
on page 3-4 for more information.
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.
Big-endian format is shown in Figure 2-1.
Figure 2-1 Big-endian addresses of bytes with words
Note:
• Most significant byte is at lowest address
• Word is addressed by byte address of most significant byte.
Higher address
8
4
0
31 24 23
Word
address
16 15 8 7 0
Lower address
4
9
5
10
6
11
7
8
0 1 2 3