11.9. Big-Endian / Little-Endian Operation

The ARM720T treats words in memory as being stored in big-endian or little-endian format depending on the value of the bigend bit in the control register, see Memory Formats.

Load and store are the only instructions affected by the endianness. Refer to the ARM Architecture Reference Manual for details of the LDR and STR instructions.

Little‑endian format

In little-endian format:

  • the lowest‑numbered byte in a word is considered to be the least significant byte of the word.

  • the highest‑numbered byte is the most significant.

Byte 0 of the memory system should be connected to data lines 7 through 0 (BD[7:0]) in this format.

Figure 11.5. Little‑endian addresses of bytes within word

Big‑endian format

In big-endian format:

  • the most significant byte of a word is stored at the lowest‑numbered byte.

  • the least significant byte is stored at the highest‑numbered byte.

Byte 0 of the memory system should therefore be connected to data lines 31 through 24 (BD[31:24]).

Figure 11.6. Big-endian addresses of bytes within word

Copyright © 1997, 1998 ARM Limited. All rights reserved.DDI 0087E
Non-Confidential