ARM ARM7TDMI User Manual download pdf (Page 50)

Programmer’s Model

2.3 Memory formats

The ARM7TDMI processor views memory as a linear collection of bytes numbered in

ascending order from zero. For example:

• bytes zero to three hold the first stored word

• bytes four to seven hold the second stored word.

The ARM7TDMI processor is bi-endian and can treat words in memory as being stored

in either:

• Little-endian on page 2-4.

• Big-Endian on page 2-5

Note

Little-endian is traditionally the default format for ARM processors.

The endian format of a CPU dictates where the most significant byte or digits must be

placed in a word. Because numbers are calculated by the CPU starting with the least

significant digits, little-endian numbers are already set up for the processing order.

Endian configuration has no relevance unless data is stored as words and then accessed

in smaller sized quantities (halfwords or bytes).

2.3.1 Little-endian

In little-endian format, the lowest addressed byte in a word is considered the

least-significant byte of the word and the highest addressed byte is the most significant.

So the byte at address 0 of the memory system connects to data lines 7 through 0.

For a word-aligned address A, Figure 2-1 shows how the word at address A, the

halfword at addresses A and A+2, and the bytes at addresses A, A+1, A+2, and A+3

map on to each other when the core is configured as little-endian.

Figure 2-1 LIttle-endian addresses of bytes and halfwords within words

31 24 23 16 15 8 7 0

Halfword at address A+2 Halfword at address A

Word at address A

Byte at address A+3 Byte at address A+2 Byte at address A+1 Byte at address A

1 2 ... 45 46 47 48 49 50 51 52 53 54 55 ... 283 284

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ARM ARM7TDMI User Manual Page 50