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Programmer’s Model

2.6Memory formats

The processor views memory as a linear collection of bytes numbered in ascending order from zero. Bytes 0-3 hold the first stored word, and bytes 4-7 hold the second stored word, for example.

The processor can treat words in memory as being stored in either:

Legacy big-endian format

Little-endian format.

Additionally, the processor supports mixed-endian and unaligned data accesses. For details see Chapter 4 Unaligned and Mixed-endian Data Access Support.

2.6.1Legacy big-endian format

In legacy 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. Figure 2-4 shows this.

Bit

31

24 23

16 15

8

7

0

Word address

Higher address

 

8

 

9

 

10

11

 

8

 

 

 

 

 

 

 

 

 

 

 

 

4

 

5

 

6

7

 

4

 

 

 

 

 

 

 

 

 

 

Lower address

 

0

 

1

 

2

3

 

0

 

 

 

 

 

 

 

 

 

 

Most significant byte is at lowest address

Word is addressed by byte address of most significant byte

Figure 2-4 Big-endian addresses of bytes within words

2.6.2Little-endian format

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. Figure 2-5 shows this.

Bit

31

24 23

16 15

8 7

0

Word address

Higher address

11

 

10

 

9

 

8

 

8

 

 

 

 

 

 

 

 

 

4

 

7

 

6

 

5

 

4

 

 

 

 

 

 

 

 

 

 

 

Lower address

3

 

2

 

1

 

0

 

0

 

 

 

 

 

 

 

 

 

 

Least significant byte is at lowest address

Word is addressed by byte address of least significant byte

Figure 2-5 Little-endian addresses of bytes within words

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