ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

H.3.2 Endian support
ARMv4 and ARMv5 Differences
ARMv4 and ARMv5 support big and little endian operation. Little endian support is consistent with
ARMv7. Big endian control, configuration, and the connectivity of data bytes between the ARM register
file and memory is different. However, the difference is only visible when communicating between big
endian and little endian agents using memory. The agents can be different processors or programs running
with different endianness settings on the same processor.
For ARMv4 and ARMv5, the distinction between big endian memory and little endian memory is managed
by changing the addresses of the bytes in a word. For ARMv7, the distinction between big endian memory
and little endian memory is managed by keeping the byte addresses the same, and reordering the bytes in
the halfword or word. The endian formats are:
LE Little endian format used by ARMv4, ARMv5, ARMv6, and ARMv7
BE Big endian format used by ARMv6 (endianness controlled by the SETEND instruction) and
ARMv7
BE-32 Big endian format used by ARMv4, ARMv5, and ARMv6 (legacy format, endianness
controlled by the SCTLR.B bit).
Table H-1 shows how the addresses of bytes are changed in the BE-32 endian format. In this table, A is a
doubleword-aligned address and S, T, U, V, W, X, Y, Z are the bytes at addresses A to A+7 in the ARMv7
memory map.
Table H-1 Addresses of bytes in endian formats
Byte Address in format BE or LE Address in format BE-32
S A A+3
T A+1 A+2
U A+2 A+1
V A+3 A
W A+4 A+7
X A+5 A+6
Y A+6 A+5
Z A+7 A+4
Aligned memory accesses are performed using these byte addresses as shown in Table A3-4 on page A3-8
for the LE endian format and in Table A3-3 on page A3-7 for the BE and BE-32 formats, in each case
extended consistently to doubleword accesses. Table H-2 on page AppxH-8 shows which bytes are accessed
by each type of aligned memory access and the significance order in which they are accessed.
ARM DDI 0406B Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. AppxH-7