Datasheet

Programmer’s Model
ARM7TDMI Data Sheet
ARM DDI 0029E
3-3
Open Access
3.3.1 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.
3.3.2 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.
3.4 Instruction Length
Instructions are either 32 bits long (in ARM state) or 16 bits long (in THUMB state).
3.5 Data Types
ARM7TDMI supports byte (8-bit), halfword (16-bit) and word (32-bit) data types.
Words must be aligned to four-byte boundaries and half words to two-byte boundaries.
Higher Address 31 24 23 16 15 8 7 0 Word Address
8 9 10 11 8
4 5 6 7 4
0 1 2 3 0
Lower Address • Most significant byte is at lowest address
• Word is addressed by byte address of most significant byte
Figure 3-1: Big endian addresses of bytes within words
Higher Address 31 24 23 16 15 8 7 0 Word Address
11 10 9 8 8
7 6 5 4 4
3 2 1 0 0
Lower Address • Least significant byte is at lowest address
• Word is addressed by byte address of least significant byte
Figure 3-2: Little endian addresses of bytes within words